The male and female GONADS, testes and ovaries, are derived from the

coelomic epithelium near the inner or median aspect of the nephrotomes

(Fig. 229, B). Here the epithelium remains columnar, and soon projects into

the ventral coelom as a continuous longitudinal ridge. It is probable that

at first the modified epithelium is segmented as a series of "gonotomes,"

but if so, the latter must soon coalesce into a continuous ridge. Some of

the epithelial cells enlarge to form the primitive sex-cells. In the

development of an ovary, portions of the epithelium sink inwards, carrying

with them the primitive ova. Certain of the cells form the epithelial walls

of a number of ovisacs, each of which encloses an ovum. As the ovisacs

increase in number and size the germinal ridges project more and more into

the coelom until, as ripe ovaries, they become suspended from its dorsal

wall by a double peritoneal fold, the "mesovarium" (Fig. 156). The testes

develop in a similar fashion except that the primitive sex-cells, which

later give rise to spermatozoa, form the lining of a number of simple or

ampulla-like tubules, the seminiferous tubules, and the suspensory fold is

termed the "mesorchium."

The Cyclostomes have gonads in the shape of unpaired organs extending

nearly the whole length of the coelom, but in all Fishes the organs are

primarily paired, although by fusion, or by the absorption of one gonad,

the ovaries or the testes sometimes appear as if single. The ovaries may

either be naked, as in Elasmobranchs, Dipnoi, Crossopterygii, and

Chondrostei, and in _Amia_ amongst the Holostei; or, as in _Lepidosteus_

and most Teleosts, they become enclosed in coelomic sacs. The {403}former,

or "gymnoarian," condition is primitive; the latter, or "cystoarian," is

secondary, and is brought about by the growth of two peritoneal folds round

the ovary and the union of their margins. Into these coelomic sacs the

egg-bearing or real ovarian tissue projects either in the form of processes

or of transversely- or longitudinally-arranged plates or folds (Fig. 232,

B). The testes are composed of seminal ampullae, as in Elasmobranchs, or of

radially-arranged and sometimes plexiform tubules opening into the

gonoduct, as in nearly all other Fishes (Fig. 232, A).

[Illustration: FIG. 232.—Diagrams to show the structure of the testes (A)

and of the ovaries (B) in a Herring. (From Cunningham.)]

In the Cyclostomes (e.g. _Petromyzon_) the eggs and spermatozoa are

discharged from the gonads into the coelom, whence they reach the exterior

through a pair of "genital pores" leading from the hinder end of the coelom

into a urinogenital sinus formed by the united extremities of the two

archinephric ducts.[479] _Myxine_ has, however, but a single median pore,

opening into an integumentary cloaca, which also receives the rectal and

urinary orifices. _Bdellostoma_ has two such pores communicating with a

similar cloaca.[480]

{404}The nature and homologies of the genital ducts in the different groups

of Fishes are amongst the most puzzling of the many problems which vex the

soul of the Vertebrate morphologist, and although there is a fairly general

agreement on some points, there are others of great importance of which it

may be said _quot homines, tot sententiae_.

Broadly speaking, there are two types of genital ducts in Fishes: (1) those

which are obviously derived from some part of the kidney system; and (2)

those which are special ducts and appear to have no connexion with

kidney-ducts.

The Elasmobranchs offer a typical example of gonoducts of the first kind.

At an early embryonic period in both sexes each archinephric duct becomes

longitudinally split into two ducts, of which one continues to receive the

openings of the mesonephric tubules and remains as a mesonephric duct (Fig.

229, B).[481] The other, which has no connexion with the mesonephros, opens

anteriorly into the coelom by means of the united nephrostomes of the

pronephros, and is known as the "Müllerian duct" (Fig. 230, C and D). In

the adult male the Müllerian ducts are useless vestiges, but in the female

they persist and act as oviducts, receiving the eggs set free from the

ovarian ovisacs through their coelomic apertures, and thence conveying them

to the cloaca. In the male, certain of the anterior mesonephric tubules

become connected with the testicular ampullae by means of a network of

slender tubules, the "vasa efferentia" or testicular network, and through

the latter the spermatozoa pass from the testes to the mesonephric duct

(Fig. 230, C). Consequently, the mesonephric duct conveys both spermatozoa

and the kidney excretion to the cloaca. It is obvious, therefore, that

both the male and female gonoducts are derived from kidney-ducts.

The Teleostei afford an equally typical illustration of the second type.

Each female gonoduct (oviduct) is formed by a backward growth of the same

two peritoneal folds which enclose the ovary; these are converted into a

"peritoneal tube" or canal by the union of their margins. The male

gonoducts are also formed in continuity with the testes, that is, as

backward prolongations from the latter. Each duct, male or {405}female,

seems to be a duct _sui generis_ and to have no connexion whatever with the

kidney system (Fig. 230, E). In the Salmonidae, Anguillidae, Galaxiidae,

Hyodontidae, Notopteridae, and Osteoglossidae, and also in _Misgurnus_, the

oviducts lose their continuity with ovaries and degenerate to an extent

which differs greatly in different families. Thus in some Salmonidae, as in

the Smelt (_Osmerus eperlanus_),[482] the oviducts end anteriorly in wide

funnel-like coelomic apertures after the fashion of Müllerian ducts, and do

not embrace the ovaries: hence the ovaries are naked and not cystoarian,

and their ducts are not peritoneal tubes but "peritoneal funnels" (Fig.

230, F). In other Salmonidae and in the Anguillidae the oviducts appear to

have so far degenerated that they are represented either by a pair of very

short funnels or by a pair of genital pores, which, as in the Salmon, have

a common external aperture behind the anus and in front of the single

orifice of the united archinephric ducts (Fig. 233, A). In all such

instances the eggs are set free from the ovaries into the coelom, from

whence they escape through the peritoneal funnels or genital pores. In the

Eels the male gonoducts also degenerate, and, losing all connexion with the

testes, they become reduced to genital pores as in the female.

The Holocephali and probably the Dipnoi conform to the Elasmobranch type in

the nature of their male and female gonoducts. In the Crossopterygii[483]

each testis has its own proper duct, which has no connexion with the kidney

system and apparently belongs to the Teleostean type, while the oviduct,

which is almost certainly not a Müllerian duct, is probably a peritoneal

funnel. On the other hand, the Chondrostei and the Holostei are in the

interesting transitional condition of possessing male ducts of the

Elasmobranch type and female ducts of the Teleostean type, the latter being

either ducts directly continuous with the ovaries, as in _Lepidosteus_, or

of the nature of peritoneal funnels, as in _Acipenser_, _Polyodon_, and

_Amia_ (Fig. 230, E and F).

How far the distinction between the two types of gonoduct holds good in the

case of the male is not quite clear, and it has recently been argued that

the Dipnoi offer a connecting link between the two.[484]

{406}[Illustration: FIG. 233.—Diagram to show the kidneys and gonoducts of

a female Salmon (A), and of a male _Protopterus_ (B). _md_^1 and _md_^2,

Anterior and posterior vestiges of the Müllerian duct; _t.t_, tubular

posterior portion of the testis (_t_). Other reference letters as in Fig.

230. (B, after Graham Kerr.)]

In _Protopterus_ each testis is divided into an anterior sperm-producing

part and a posterior tubular portion which has lost the capacity of

producing sex-cells. The testicular network is greatly reduced, and forms

but a limited connexion between the tubular portion of the testes and the

mesonephric duct (Fig. 233, B). If it be supposed that the testicular

network became still further reduced so that the connexion between the

testes and the kidney-duct took place directly through a single channel

instead of through several, the result would be a gonoduct essentially

similar to the male duct of an ordinary Teleost. Should this view prove to

be correct, it will follow that the male gonoducts of _all_ Fishes are

differently-modified examples of the Elasmobranch type. But there will

still remain the female gonoducts of Ganoids and Teleosts, which must be

regarded as distinct from Müllerian ducts unless it can be shown that their

different methods of development are not necessarily fatal to their

homology with Müllerian ducts, or that both types of gonoduct can be

derived from some intermediate type. Assuming that some Fishes do possess

male or female ducts which have not been derived from the kidney system,

but have been independently acquired, there is still the question, which of

the two types is the more primitive, or, in other words, has the

Elasmobranch type superseded the Teleostean, or _vice {407}versa?_ To this

question no decisive answer can at present be given.

The terminal relations of the kidney-ducts and the gonoducts, and the

presence of accessory or of vestigial organs in connexion with them, will

now be briefly dealt with. In the males of the Elasmobranchs the

mesonephric ducts which, as already pointed out, act both as kidney-ducts

and gonoducts, dilate posteriorly to form a pair of vesiculae seminales,

and then unite to form a urinogenital sinus, opening into the cloaca at the

extremity of a median papilla (Fig. 230, C). The sinus also receives ducts

from the hinder part of the mesonephros, either separately, as in the

female, or by a common duct on each side—the so-called metanephric duct—as

in the male. Two tubular caecal outgrowths from the sinus form two sperm

sacs. Only the anterior portions of the Müllerian ducts with their coelomic

apertures are retained in the adult. In the female the mesonephric ducts

are purely excretory, but otherwise they are similar, and the oviducts

(Müllerian ducts) open into the cloaca separately or by a common orifice

(Fig. 230, D). A glandular dilatation of each oviduct forms the oviducal or

shell gland by which the horny egg-cases are secreted. In the males of the

Holocephali the gonoducts open into a urinogenital sinus with an external

orifice distinct from and behind the anus; but the female has separate

apertures for the rectum, the conjoined oviducts, and the united

mesonephric ducts. Both sexes have complete Müllerian ducts communicating

with the coelom in front, and behind with the exterior. The Dipnoi of both

sexes essentially resemble the Elasmobranchs in the general relations of

their ducts, but the Müllerian ducts of the male exhibit marked differences

in the three genera.[485] In _Neoceratodus_ the ducts are as complete as

their functional representatives in the female. _Protopterus_ retains

anterior vestiges and the coelomic apertures, and also vestiges of the

hinder portions which unite and end blindly in the urinogenital papilla,

but the middle sections of the two ducts are suppressed (Fig. 233, B). In

the Teleostomi there is a general similarity in the terminal relations of

the gonoducts and kidney-ducts. In the Ganoids the archinephric ducts unite

and then expand into a urinary sinus or bladder, and the gonoducts of the

female, or of both sexes in _Lepidosteus_, open either into the

archinephric {408}ducts or into the common sinus, and therefore both ducts

communicate with the exterior by a urinogenital orifice behind the anus.

Peritoneal funnels, similar to the functional oviducts of the female, are

present in the males of the Chrondrostei and of _Amia_. In Teleosts the

terminal connexions of the ducts tend to become less intimate. The

archinephric ducts often dilate into a urinary bladder either before or

after their union, and the common duct joins the united gonoducts to form a

short urinogenital sinus which opens externally, or the confluent gonoducts

have an independent genital orifice between the anus and the urinary

aperture. Not rarely the genital or the urinogenital orifice is prolonged

into a tubular papilla, which in the male acts as an intromittent organ,

or, as in the females of the Cyprinoid _Rhodeus amarus_, the long oviducal

tube serves the purpose of an ovipositor. The males and females of the

Siluroid _Plotosus_ have a remarkable vascular and glandular arborescent

appendage just behind the urinogenital papilla, the use of which is

unknown.[486]

The EGGS of different Fishes[487] exhibit considerable diversity in size

and shape as well as in the nature of their external coverings and their

mode of deposition.[488] The size of the eggs largely depends on the

quantity of food-yolk stored up in their substance for the nutrition of the

embryo: hence the eggs of Elasmobranchs, which resemble Fowls' eggs in the

superabundance of their yolk, are by far the largest. Teleostomi have much

smaller eggs. The largest Teleostean ova are those which are heavy and sink

(demersal ova); the smallest, those which are buoyant and float (pelagic

ova). Of the former, the eggs of _Gymnarchus_ are about 10 mm. in diameter;

those of the Salmon about 5 mm.; and those of some species of _Arius_, 5 to

10 mm. The eggs of the Wolf-Fish (_Anarrhichas lupus_) are about 6 mm.

Smaller demersal ova are those of the Lump-sucker (_Cyclopterus_) and

_Heterotis_, which are 2.6 and 2.5 mm. respectively. Pelagic {409}eggs are

very small, those of the Plaice, which are exceptionally large, varying

from 1.65 to 1.95 mm.

[Illustration: FIG. 234.—Different types of egg-segmentation in Fishes. A,

a typical telolecithal egg. Holoblastic and unequal segmentation in _Amia_

(B) and in _Lepidosteus_ (C). D, the meroblastic segmentation of a Teleost.

_a.p_, Animal pole; _e.m_, egg-membrane; _ma_, macromeres; _mi_,

micromeres; _n_, nucleus; _o.g_, oil globule; _p_, protoplasm; _v.p_,

vegetative pole; _y_, yolk. (From Ziegler: A, after Hertwig; B, after

Whitman and Eycleshymer; C, after Eycleshymer.)]

An egg-cell consists of living protoplasm and a nucleus, a variable

quantity of non-living food-yolk, and of certain enveloping and protective

egg-membranes. The ova of Fishes differ principally in the amount and

disposition of the food-yolk, in the character of the egg-membranes, and in

the presence or absence of special perforations in the egg-membranes for

the entrance of spermatozoa into the eggs. In the small ova of some of the

lower Chordata (_e.g._ Amphioxus), where the very small quantity of

food-yolk is uniformly distributed, and its presence affects all parts of

the egg alike, the process of segmentation which follows fertilisation

results in the transformation of the entire egg into a mass of

approximately equal-sized cells or blastomeres (Fig. 82). {410}The eggs are

therefore described as "alecithal," and the segmentation as being

"holoblastic" and "equal." On the other hand, all Fishes possess

"telolecithal" eggs, that is, ova in which the food-yolk is more or less

abundant, and tends to accumulate at one pole of the egg ("vegetative

pole"), while the opposite or "animal pole" consists of protoplasm,

comparatively free from yolk granules and containing the nucleus (Fig. 234,

A). The term telolecithal is, however, a somewhat comprehensive one, and

covers important variations in the relations of the inert food-yolk and the

living protoplasm in different Fishes, which greatly modify the process of

segmentation. Thus there are some Fishes in which the amount of food-yolk

at the vegetative pole is sufficient to retard segmentation in that part of

the egg without actually preventing it, and consequently segmentation

begins in the animal pole, and takes place more rapidly there than it does

when it extends into the vegetative pole. Hence it follows that although

the entire egg is segmented the blastomeres are of unequal size, the animal

pole giving rise to a large number of small cells or micromeres, and the

vegetative pole to a smaller number of much larger cells or macromeres. The

segmentation of such an egg is said to be holoblastic but unequal (Fig.

234, B and C). This type of egg is characteristic of the Chondrostei, the

Holostei, and the Dipnoi. In other Fishes, like the Elasmobranchs and the

Teleostei, the food-yolk so greatly preponderates that it entirely prevents

segmentation in the vegetative part of the egg, and segmentation is

restricted to the small mass of protoplasm (germinal disc) at the animal

pole, in which the nucleus is situated (Fig. 234, D). Eggs undergoing

partial segmentation in this way are termed "meroblastic." No hard and fast

line can be drawn between the two types, and in the Chondrostei and

Holostei an interesting transition between the holoblastic and meroblastic

ova may be observed. The egg-membranes are formed either by the egg itself

or by the epithelium of the ovarian ovisacs, and, as will shortly be seen,

the character of the outer egg-membrane greatly influences the mode of

deposition of the eggs and their location afterwards. In Elasmobranchs the

egg is enclosed in a stout horny egg-shell, secreted by the oviducal shell

gland.[489] In many Fishes, as in the Chondrostei, Holostei, and Teleostei,

the egg-membranes {411}are perforated at the animal pole of the egg by a

small aperture or "micropyle," which is only large enough to admit of the

entrance of a single spermatozoon at a time (Fig. 235). Generally, there is

only a single micropyle, but, according to Salensky, the Sturgeon (_A.

sturio_) has from 3 to 9, and the Sterlet (_A. ruthenus_) from 5 to 13.

An important distinction may be made between the ova of different

Teleostomi as regards their location after extrusion from the female. From

this point of view two types of ova can be distinguished, _demersal_ and

_pelagic_ ova. Demersal eggs are characterised by their larger size and

greater weight, so that they always sink after extrusion; and by their

opacity. They may either have an outer egg-membrane which is viscid and

adhesive, so that the eggs readily adhere to one another or to foreign

objects, or the membrane is smooth and non-adhesive. The Salmonidae, for

example, produce non-adhesive demersal eggs, which remain separate after

being deposited on the gravelly bed of a stream. Most freshwater and many

marine shore Fishes have adhesive demersal eggs, which are deposited at the

bottom of the water, generally adhering to one another in larger or smaller

clumps, masses, or sheets, and attached to rocks, stones, or empty shells,

like the eggs of many shore Fishes, or to aquatic plants after the fashion

of the eggs of the Carp, Perch, and Pike, or even to branching zoophytes,

as is the case with the eggs of the Sea-snail (_Liparis_). In some adhesive

eggs the external egg-membrane forms threads for their attachment. The eggs

of the Gar-Fish (_Belone_), and those of the Saury Pike (_Scombresox_) and

of the Flying Fishes (_Exocoetus_), have viscid threads developed from

opposite points on the surface, which are either attached to foreign

objects or they become entangled with those of other eggs of the same

species. The oval eggs of some of the Gobies have a bunch of fibres at one

pole which serves to attach them. In the Smelt (_Osmerus eperlanus_) a

portion of the outer egg-membrane breaks away from the rest and becomes

turned back, inside out, but remains attached to the egg at one point. By

means of this membrane the egg is attached to rocks or stones. Pelagic eggs

are distinguished by their lightness and buoyancy, so that they always

float near the surface of the water, and by their smaller size and

remarkable transparency (Fig. 235). A conspicuous feature in many of them

is the presence of a single {412}large oil globule on the surface of the

yolk, and not infrequently the yolk becomes partially or completely broken

up into small masses. Pelagic eggs are always non-adhesive and free, and

they invariably belong to marine Fishes. Amongst the British food Fishes

which produce pelagic ova may be mentioned the Gadidae (_e.g._ Cod,

Whiting, Hake, Ling), the Pleuronectidae (_e.g._ Turbot, Brill, Sole,

Plaice), Scombridae (_e.g._ Mackerel), Triglidae (_e.g._ the Gurnards),

Percidae (_e.g._ the Bass), and Clupeidae like the Pilchard and Sprat, but

not the Herring, whose adhesive demersal eggs are deposited in clumps on

shingly banks in the sea at varying distances from the shore.

[Illustration: FIG. 235.—Diagrams of the pelagic ova of a Cod or a Plaice

(A) and of a Ling (_Molva_). _G_, Germinal disc; _M_, micropyle; _O.G_, oil

globule; _Y_, yolk. (From Cunningham.)]

The eggs of Elasmobranchs are deposited singly or in pairs at considerable

intervals, and the period of egg-laying is prolonged over a considerable

part of the year. In most other Fishes, as in Teleosts, the period of

spawning is limited to a few months, usually in the spring and summer in

temperate latitudes; and in the case of a single Fish it may last only a

few days or weeks, but the number of eggs produced is often enormous. Thus,

in a Ling 61 inches long and weighing 54 pounds the ovaries contained

28,361,000 eggs. A Turbot, 17 pounds in weight, had 9,161,000 eggs; and a

Cod of 21½ pounds 6,652,000. The least prolific of the British food Fishes

is the Herring, in which the number of ovarian eggs varied from 21,000 to

47,000 in four specimens examined.[490] The extraordinary fecundity of many

Fishes seems to bear no relation to the relative abundance of the Fishes

themselves, but rather it is to be associated with certain

{413}disadvantages attendant on the sexual relations of Fishes, involving a

considerable waste of the sex-cells, while in many Fishes it no doubt helps

to compensate for any subsequent mortality among the larvae, which may

result from an uncertain and precarious food supply and from the attacks of

enemies. Whenever internal fertilisation is the rule, or when, as in

nest-building and marsupial Fishes, the propinquity of the sexes in the

breeding season ensures the fertilisation of a larger proportion of the

eggs and the protection of the young, the number of eggs produced is small.

The male sex-cells or spermatozoa are essentially similar to those of other

Vertebrates, although in different Fishes they may vary in such details as

length, and the shape and size of the head, which may be rod-like and wavy,

elliptical or globular.

As a rule, in Fishes females are more numerous than males, and generally

they are larger, but to both statements there are notable exceptions. The

relations of the sexes in the breeding season are usually very promiscuous,

especially in those Teleosts which discharge their sex-cells while swimming

together in shoals. A female may, however, consort with several males

(_polyandry_), or a male with several females (_polygamy_); or, as in some

of the nest-building Fishes (e.g. _Gastrosteus_), there are not wanting

examples of the pairing of one male with one female (_monogamy_).

Fishes often migrate at the commencement of the breeding-season to

localities most suitable for the deposition of the eggs. Many marine

species seek banks or shallower water near the shore, and some, like the

Salmon and the Sturgeon, are _anadromous_, and ascend rivers for long

distances to deposit their spawn.

In all Fishes except the Elasmobranchs and a few Teleosts the fertilisation

of the eggs takes place in the water after their extrusion, the male

depositing its seminal fluid over the eggs or in their neighbourhood. The

waste of the sex-cells is often, no doubt, very considerable, especially

when the eggs are adhesive and fixed, and the seminal fluid is liable to

drift at the mercy of tides and currents. With pelagic ova the waste is

perhaps not so great, inasmuch as the eggs as well as the spermatozoa would

probably drift at the same rate and in the same direction. Liability to

waste must also be greatly diminished in many Fishes by their habit of

living in shoals, or of congregating {414}together in the breeding season,

in which they are sometimes aided by their power of emitting characteristic

sounds, and in the case of nest-building Fishes by the still more intimate

relations of the sexes. Even when the liability to waste is very great,

compensation may be afforded by exceptional fecundity. The copulation of

the sexes and the internal fertilisation of the eggs occur only in

Elasmobranchs and some Teleosts. The copulatory organs of Elasmobranchs are

the so-called "claspers" with which the males are provided. Some form of

copulation is probably the rule in the viviparous Teleosts, where the eggs

are fertilised in the oviducts, or even while they are still in the

ovaries, and the young are born alive. As mentioned above, an intromittent

organ is often formed by the prolongation of the genital or the

urinogenital orifice into a papilla, or a longer or shorter tube.[491] Some

Cyprinodontidae[492] (e.g. _Anableps_) have the anterior part of the anal

fin modified in the male to form an intromittent organ along which the

urinogenital canal runs (Fig. 374). In the females the genital aperture is

covered by a special scale, which is free on one side and not on the other.

"The male organ in some individuals is turned to the right, in others to

the left, and in some females the opening beneath the special scale is to

the right, in others to the left. Copulation thus takes place sideways, a

left-sided male pairing with a right-sided female, and _vice versa_."[493]

The anal fin also forms an intromittent organ in the "Half-beak"

(_Hemirhamphus_). In a genus (_Girardinus_) of the same family the anal fin

is modified to form an apparatus for holding the female during sexual

congress.[494] The singular method of fertilisation practised by the males

and females of _Callichthys paleatus_ is referred to elsewhere.[495]

With the exception of the pelagic _Antennarius_, which builds its nest in

the Sargasso weed in mid-ocean, nest-building and parental solicitude for

the young are confined to freshwater Fishes and to marine forms with

demersal ova. Pelagic ova must necessarily be beyond the scope of parental

care. As a rule it is the male which acts as guardian nurse, the female

troubling herself but little about the fate of her eggs or her offspring.

{415}[Illustration: FIG. 236.—The Butter-Fish (_Pholis gunnellus_) coiling

round a mass of eggs. (From Cunningham, after Holt.)]

Perhaps the more primitive form of parental foresight is exhibited by

those Fishes which, like the females of the Salmonidae, make a furrow in

the gravelly bottom of a running stream for the reception of the eggs, and

then cover them over with a layer of gravel, or like the Siluroid _Arius

australis_, of the Burnett river in Queensland, which deposits its eggs in

circular excavations in the sandy bed of the river and covers them with

layers of large stones. But in neither case does it appear that either the

male or the female takes any further interest in the eggs or in the young

when hatched. Without actual nest-building, or even the preparation of a

place for their reception, the eggs may be protected in various ways by the

male. The common British Gunnel or Butter-Fish (_Pholis gunnellus_) rolls

its eggs into a rounded mass by coiling its body round them, the male and

female taking possession of them alternately. The little clumps of eggs are

then deposited in holes made by the boring Mollusc, _Pholas_. Some British

Blennies attach their eggs in a single layer to the sides of cavities in

rocks, or between stones, where they are watched over by the male parent.

The eggs of the Lump-Sucker (_Cyclopterus lumpus_) are attached in masses

to rocks or to piles and guarded by the male, who aerates them by keeping

up a flow of water over the spawn through the action of his pectoral fins.

When hatched, the young fry cling to the body of their watchful parent by

their suckers. A more decided approach to nest-building is exhibited by the

Sand Goby (_Gobius minutus_). In this species the male scoops out the sand

from beneath an empty shell, generally that of a Pecten, and the female

deposits her adhesive eggs on the under surface of the shell.

{416}[Illustration: FIG. 237.—Showing the embryos of _Rhodeus amarus_ in

the gill-cavities of _Unio_. _e_, Embryos; _g_, inter-lamellar cavities;

_i.l.j_, an inter-lamellar junction. (From Olt.)]

The male remains on guard, and by the movements of its pectoral fins

promotes the aeration of this rude form of nest. References to some of the

more striking examples of true nest-building in Fishes will be found in the

systematic part of this volume, especially in those chapters treating of

the Dipnoi and Amiidae, and such Teleosts as the Mormyridae,

Osteoglossidae, Siluridae, Gastrosteidae, Centrarchidae, Osphromenidae,

Labridae, and Antennariidae. Other illustrations of parental care are to be

found in the development of marsupial pouches or grooves for the reception

of the eggs in the males of the Syngnathidae (Fig. 387) and the females of

the Solenostomidae, and the use of the oral cavity for a similar purpose in

the males, rarely in the females, of some Siluridae, and the males or

females, according to the species, of the Cichlidae. The singular method by

which the female _Aspredo_ safeguards both her eggs and her progeny is

referred to on p. 596. The Cyprinoid, _Rhodeus amarus_ (the "Bitterling" of

Central Europe), is unique in the means which it adopts to {417}secure the

same result.[496] By means of its long ovipositor the female Fish deposits

its eggs in the mantle cavity of a _Unio_, or of an Anodon. Here they are

fertilised by spermatozoa carried in through the inhalent siphon of the

Mollusc with the inspiratory water current, and they complete their

development in the gill-cavities (Fig. 237).[497]

The time which elapses between the fertilisation of the egg and the

hatching out of the young Fish varies greatly in different Teleosts. The

eggs of some Clupeidae hatch in a very short time, two to three days in the

Anchovy, and three to four days in the Sprat. In most of the British marine

food Fishes the period rarely exceeds twelve to fourteen days. The larger

demersal eggs with much food-yolk are longer in hatching; in the Salmon the

time ranging from thirty-five to one hundred and forty-eight days. A low

temperature lengthens the time. The eggs of the Herring which hatched in

eight to nine days at a temperature of 52° to 58° F. took forty-seven days

in water at 32° F.

The extent to which the development of the embryo proceeds while it is

still enclosed in the egg-membranes, and consequently the condition of the

embryo when hatched, depends largely but not exclusively on the quantity of

food-yolk which is present in the egg and available for the nutrition of

the embryo during its earlier stages. Embryos hatched from pelagic ova are

very small and imperfectly developed. The mouth is usually not yet formed.

The median fins, which later become isolated, are continuous, and the

caudal fin is diphycercal, although it subsequently becomes homocercal

after passing through a heterocercal stage. The blood is colourless, and

even the gill-clefts may at first be lacking. In this condition the

newly-hatched Fish is nourished at the expense of the residual food-yolk,

which is enclosed in a yolk-sac projecting from the ventral surface of the

body (Fig. 238). As the yolk is gradually used up the mouth is formed, and

the young Fish feeds on the minute organisms of various kinds living in the

sea, and by degrees the form, proportions, and structure of the more mature

Fish are acquired. In the case of the larger demersal eggs the young are

not only longer in hatching, but when hatched they are larger and more

advanced in development. The young of many Fishes are {418}provided with

larval or provisional organs, and they may be so unlike the adult in other

respects that their subsequent development assumes the form of a more or

less striking metamorphosis. As examples of larval organs, mention may be

made of the adhesive or cement organs of the larval Chondrostei and

Holostei, and of the Dipnoi (e.g. _Protopterus_ and _Lepidosiren_), and

also of a Teleost, probably the Mormyrid (_Hyperopisus bebe_, Lacép);[498]

the cutaneous gills of the Crossopterygii and some Dipnoi; the so-called

external gills of such Teleosts as _Cobitis_, _Gymnarchus_ (Fig. 239), and

_Heterotis_, which are singularly like those of Elasmobranchs; and the

defensive spines which are developed on the scales or scutes of the trunk,

and the dermal bones of the skull, in the young of some Plectognathi. The

most striking metamorphosis to be found in Fishes occurs in the Flat-Fishes

and in the Eels, an account of which will be found in other parts of this

volume (pp. 685, 602).

[Illustration: FIG. 238.—Newly-hatched embryo Teleost from a pelagic egg.

_A_, Auditory organ; _E_, eye; _FM_, continuous median fin; _Ht_, heart;

_I_, intestine; _N_, nostril; _Yk_, yolk-sac. (From Cunningham.)]

The only examples of viviparous Fishes occur in certain families of

Elasmobranchs,[499] and in five families of Teleosts, viz. the Blenniidae,

the Cyprinodontidae, the Scorpaenidae, the Comephoridae, and the

Embiotocidae.[500] In the Teleosts mentioned the eggs are fertilised while

they are still either in the ovarian ovisacs or in the cavity of the ovary,

and their development may take place in either position. In such

Cyprinodonts as _Gambusia_ and _Anableps_ the embryos are developed in the

ovisacs, but as a rule both fertilisation and development occur in the

ovarian cavity. During a prolonged gestation the young are nourished partly

by the {419}food-yolk present in the eggs, and partly by a nutritive

secretion derived from the ovarian walls or from the epithelial wall of the

ovisacs as the case may be. In _Anableps_ the secretion of the walls of the

ovisacs is absorbed by papillae developed on the surface of the yolk-sac of

the embryo along the course of its blood-vessels. The eggs of the

Embiotocidae have little food-yolk, and the embryos are mainly nourished by

the secretion of the ovarian walls, which is swallowed by the embryo and

absorbed by villi on the inner surface of the intestine. The number of

young produced varies considerably. In the Embiotocidae the ovarian cavity

contains 40 to 50 young. The viviparous Scorpaenid, _Sebastes norvegicus_

of Northern Europe, produces, it has been estimated, about 1000 young,

while the Blenny (_Zoarces viviparus_), the only other European viviparous

Teleost, produces from 20 to 300 or more, according to the size of the

female. In the Blenny the eggs are hatched in about twenty days, but the

young are not born until about four months after fertilisation, when they

are about an inch and a half long, and in every outward respect similar to

the adult Fish.

[Illustration: FIG. 239.—Young _Gymnarchus niloticus_, with its large

yolk-sac (_y.s_) and its long external gills (_e.g_). (From Budgett.)]

Besides the distinction between the sexes resulting from the different

nature of their gonads and sex-cells, the males and females are often

distinguished by secondary sexual characters {420}("sexual

dimorphism"[501]). As mentioned above, females are usually larger as well

as more numerous than the males, although in one or both respects the

reverse may be the case. Secondary sexual characters are best marked in

Teleosts, where they are generally related to the special rôle which each

sex takes in the deposition and fertilisation of the eggs, and the nurture

and protection of the young, of which examples have already been given. To

a more limited extent they may be associated with the struggle of the males

for the females, and in at least a few Teleosts the exuberant coloration of

the males in the breeding season suggests that instances of courtship and

sexual selection are not altogether wanting.[502]

Although the vast majority of Fishes are dioecious, instances of functional

hermaphroditism are not unknown in a few Teleosts.[503] Species of the

Percoid genus _Serranus_ (e.g. _S. cabrilla_, _S. hepatus_, and _S.

scriba_) are invariably hermaphrodite and self-fertilising. _Chrysophrys

auratus_ is an example of successive hermaphroditism, the male and female

sex-cells ripening alternately. As an occasional variation hermaphroditism

has been recorded in several other Teleosts, including amongst others such

well-known Fishes as the Cod, the Mackerel, and the Herring. The relations

of the gonads in hermaphrodites is subject to much variation. In the Cod,

for example, the testes may be double, each being continuous with the

hinder end of the ovary of its side, or there may be only a single testis

confluent with the anterior or the posterior portion, or with some other

part of the surface, of either the right or left ovary. In other Teleosts

individuals occasionally present themselves with a testis and an ovary on

opposite sides.

{421}CHAPTER XVI

CYCLOSTOMATA (SYSTEMATIC)

CLASS I. CYCLOSTOMATA

The Cyclostomata, or, as they are sometimes called, the Marsipobranchii,

from the pouch-like, or rather sac-like, shape of their branchial clefts,

are divided into two orders, the first comprising the "Hag-Fishes" or

"Borers," and the second the Lampreys.

ORDER I. MYXINOIDES.

The Hag-Fishes are probably the most primitive of all existing Craniates.

The mouth is nearly terminal, and there is no buccal funnel. The

naso-pituitary involution communicates behind with the oral cavity and

functions as a channel for the in-streaming water-current to the gills.

Four pairs of short tentacles, supported by a special tentacular skeleton,

are present in relation with the mouth and the terminally-placed

naso-pituitary orifice. The gill-sacs open directly into the pharynx. The

branchial basket is but feebly developed, and at the most it is only

represented by small isolated cartilages in relation with the external

branchial apertures. The lingual apparatus is remarkably developed. Besides

the lingual teeth there is only a single dorsal tooth in the roof of the

mouth. The dorsal arcualia are restricted to the tail, or they extend for a

short distance only into the trunk. A spiral valve is absent. There is a

row of mucus-secreting sacs along each side of the body. The brain has no

obvious cerebral hemispheres, nor a cerebellum. Only one semicircular canal

is present in the auditory organ. The eyes are degenerate, and the usual

eye-muscles with the cranial nerves {422}supplying them have atrophied. The

embryonic pronephros is retained in the adult. The eggs are large;

segmentation is meroblastic; and development is direct, without a larval

metamorphosis. Two families can be distinguished.

FAM. 1. MYXINIDAE.—Gill-sacs not exceeding six pairs, with a common

external aperture on each side of the body.

[Illustration: FIG. 240.—_Myxine glutinosa_. A, lateral view; B, view of

the ventral surface of the head, showing the mouth and tentacles. _l.l.p_,

Lateral pore-like apertures of the mucus-sacs; _v_, anus.]

The family includes a single genus, _Myxine_, of which the common Hag (_M.

glutinosa_) from the North Atlantic is the best known species (Figs. 92, A,

and 240). This Hag-Fish occurs off the coasts of Northern Europe, including

the British Isles, as well as on the Atlantic sea-board of North

America,[504] southwards to Cape Cod. Other species are found off the

coasts of Chili and Japan. _Myxine_ is quasi-parasitic in its habits,

boring its way into the bodies of large Fishes. By means of its rasping

"tongue" it devours all the soft parts of its prey, leaving little more

than a mere shell of skin and bones. The Fishes usually attacked are the

Cod and other Gadoids, but the Sturgeon is not immune, and the presence of

a Hag in the abdominal cavity of a Shark (_Lamna cornubica_) has been

recorded. _Myxine_ has the reputation of being very destructive to Fishes

caught on lines, and it is said that whole "catches" have been destroyed by

its depredations, so that North Sea fishermen have been forced to change

their fishing-ground. To what extent the Hags attack Fishes which are

living and free is somewhat uncertain, but the little evidence obtainable

seems to point to the conclusion that, as a rule, they only prey on Fishes

when the latter are hooked or netted, or injured or dead. When not seeking

food the Hag lives {423}in the mud of the sea-bottom at depths ranging to

nearly 350 fathoms. They are able to swim very rapidly in an undulatory

eel-like fashion. _M. glutinosa_ may grow to a length of nearly two feet.

The Hag has been described as a protandrous hermaphrodite, that is, it is

first a male and then a female, the gonad of the young first producing

spermatozoa, and at a later period becoming an ovary and giving rise to

eggs. This view has hitherto met with general acceptance, but it has

recently been urged with some force that the presence of the two kinds of

sex-cells in a young animal is no proof of functional hermaphroditism,

since it is not uncommon "to find immature eggs in the testis of many

Vertebrates (Teleosts, _Petromyzon_, Amphibia), where the assumption of

hermaphroditism, to say nothing of its protandric form, is entirely

unwarranted."[505] _Myxine_ produces eggs similar to those of

_Bdellostoma_. Nothing is known of its breeding habits, or of its

embryology.

FAM. 2. BDELLOSTOMATIDAE.—Gill-sacs 6-14 pairs, all with separate external

orifices. _Bdellostoma_ (Fig. 92, B) is found on the Pacific sea-board of

both North and South America, at the Cape of Good Hope, and on the coasts

of New Zealand. The numerical variation of the gill-sacs in different

species, and in different individuals of the same species, and even on

opposite sides of the same individual, is very remarkable. Out of 354

examples of the Californian species (_B. stouti_) examined by Dr.

Ayres,[506] 101 had 11 gill-sacs on each side; 26 had 11 on one side and 12

on the other; 208 had 12 on each side; 11 had 12 on one side and 13 on the

other; and 8 had 13 on each side. Occasional specimens may have 14

gill-sacs on each side. The variations are apparently quite independent of

size, age, or sex; and when the gill-sacs are asymmetrically developed, the

additional sac may be either on the right side or on the left. In the

Chilian species there are 10 gill-sacs on each side, but in the species

from the Cape of Good Hope the number is reduced to 6 or 7. _Bdellostoma_

closely resembles _Myxine_ in its habits and mode of feeding. The

Californian species attaches itself to the gills or to the isthmus of large

Fishes, and then rapidly bores its way into the body, devouring the viscera

and muscles but leaving the skin intact. It usually attacks large

{424}Flounders and species of _Sebastodes_, and it is especially

destructive to Fishes taken in gill-nets. At Monterey every net in the

summer contains the empty shells of eviscerated Fishes, and when these are

taken out of the water the Hag scrambles out with great alacrity. Large

fishes of even 30 pounds weight are often captured without either flesh or

viscera, and it cannot be supposed that they entered the net in this

condition.[507] The species lives on the sea-bottom most abundantly at a

depth of 10-20 fathoms, but becomes rarer as the water deepens or becomes

shallower.

[Illustration: FIG. 241.—A, Cluster of the eggs of _Bdellostoma stouti_,

connected by the interlocking of their anchor-shaped filaments; B, the

animal pole of an egg, showing the polar "anchors" and the opercular ring.

(From Bashford Dean.)]

The eggs of the Californian _Bdellostoma_ are large, varying in size from

14.3-29 mm. in length, and from 6.8-10.5 mm. in width, and each egg is

enclosed in a horny egg-case secreted by the epithelium of its ovarian

ovisac[508] (Fig. 241). At each pole of the egg-case there is a tuft of

numerous horny filaments which end in 2- 3- or 4-hooked, anchor-like

extremities. In the centre of the tuft of filaments at the animal pole of

the egg the egg-case is perforated by a micropyle, and a little below this

{425}point the case is encircled by an opercular groove, which enables the

polar portion to be thrown off like a cap at the time of hatching, so as to

allow the young _Bdellostoma_ to make its escape. The large size of the

egg, which almost completely fills the cavity of the egg-case, is due to

the fact that it consists mainly of food yolk, the germinal protoplasm

containing the nucleus forming only a small hillock near the inner

extremity of the micropyle. _Bdellostoma_ spawns during the greater part of

the year, but chiefly in the early summer, and probably about 20 eggs are

deposited at one time, generally on a shelly or rocky bottom. After

deposition the eggs become connected together in long chains or clusters by

the interlocking of their polar hooks. Fertilisation takes place after

extrusion, and the segmentation is meroblastic and discoidal, much as in

Teleosts. The embryo completes its development within the egg, and when

hatched it is a miniature of the adult.

[Illustration: FIG. 242.—Embryo of _Bdellostoma stouti_ near the time of

hatching. (From Bashford Dean.)]

ORDER II. PETROMYZONTES.

In the Lampreys there is a large suctorial buccal funnel leading behind and

above into the mouth, which is supported by special cartilages, and

furnished with a marginal fringe of small cirri. Numerous horny teeth are

present on the inner surface of the funnel as well as on the tongue. The

naso-pituitary involution forms a caecum and does not communicate with the

mouth. The gill-sacs, seven in number, open externally by separate

orifices, but internally they open into a median branchial canal, situated

below the oesophagus and opening into the mouth in front. There is a

well-developed branchial basket. Dorsal arcualia are present throughout the

precaudal as well as in the caudal region. A rudimentary spiral valve is

present. The brain consists of parts usually present in other Craniates,

including cerebral hemispheres and a cerebellum. The auditory organ

{426}has two semicircular canals, and the eyes are not degenerate. The

pronephros is suppressed in the adult. The eggs are small; the segmentation

is holoblastic; and there is a larval metamorphosis. There is but one

family.

FAM. 1. PETROMYZONTIDAE.—The family has a nearly world-wide distribution.

Most Lampreys are marine, although to a greater extent in some species than

in others, but all of them seem to ascend rivers for spawning. The genus

_Petromyzon_ is characteristic of the northern hemisphere, where it is

represented by various species on the coasts and in the rivers of Europe,

West Africa, Japan, and North America. Three species, widely distributed in

Europe, occur in the British Isles, viz.:—the Sea-Lamprey (_Petromyzon

marinus_), which may reach or even exceed three feet in length, and is also

found on the west coast of Africa and on the Atlantic coast of North

America; the "Lampern" or fresh-water Lamprey (_P. fluviatilis_), about 18

inches long; and the Sand-Pride, Sand-Piper, or lesser freshwater Lamprey

(_P. planeri_), usually less than a foot in length. _Ichthyomyzon_,

_Bathymyzon_, _Entersphenus_, and _Lampetra_ are also northern forms,

collectively distributed along the Atlantic and Pacific coasts and in the

rivers and great lakes of North America.[509] Other Lampreys occur only in

the southern hemisphere. _Geotria_ is common in the rivers of Chili,

Australia, and New Zealand; and another genus, _Mordacia_, has a parallel

distribution, being found on the coasts of Chili and Tasmania. A new genus

and species from Chili has been recently described under the name of

_Macrophthalmia chilensis_.[510] This Lamprey, which is only 107 mm. in

length, has remarkably large eyes (2.5 mm. in diameter), vertically

compressed gill-clefts, and a simple dentition resembling that of _Myxine_.

All Lampreys are carnivorous. They feed by attaching themselves to the

bodies of Fishes by their suctoral buccal funnels, and then rasping off the

flesh with their lingual teeth. While thus engaged they are carried about

by their victims. Salmon have been captured in the Rhone with the marine

Lamprey attached to them. The Lamprey usually keeps near the bottom, either

swimming with a graceful serpentine movement, or attached to stones by the

buccal funnel.

{427}[Illustration: FIG. 243.—Spawning of the Brook-Lamprey (_P. wilderi_).

On the right side of the figure a male is attached to the head of a female.

(From Bashford Dean and F. B. Sumner.)]

In the spring the Sea-Lamprey ascends the rivers to spawn, and, after

depositing its eggs in furrows which it excavates in the river-bottom, it

returns to the sea. The river-Lampreys spawn in the smaller streams and

brooks. The North American Brook-Lamprey, _Petromyzon_ (_Lampetra_)

_wilderi_, which is found in the neighbourhood of New York, deposits its

eggs on the gravelly bottom of a brook, in a small gravel-filled hole lying

between a number of large rounded stones[511] (Fig. 243). In the vicinity

of the "nest" some ten to twelve Lampreys congregate, the males, however,

being much more numerous (five to one) than the females.

{428}[Illustration: FIG. 244.—Head of the Ammocoetes of _P. fluviatilis_.

A, ventral view; B, side view. _br.1_, First branchial aperture; _eye_,

eye; _l.l_, lower lip; _na.ap_, naso-pituitary aperture; _u.l_, upper lip.

(From Parker and Haswell, after W. K. Parker.)]

Much energy is spent by both sexes in moving stones by lifting them with

the buccal funnel, but it is not always clear that this is done to

circumscribe the nest, or to remove impeding obstacles. Eventually, a male

attaches himself to the back of the head of a female, who at the same time

is holding fast to a stone. The male then rotates its body so that the

urino-genital papilla is brought near the genital orifice of the female,

and the simultaneous extrusion of eggs and spermatozoa at once follows.

Owing to the small amount of food-yolk which they contain the eggs of the

Lamprey (e.g. _P. planeri_) are small, measuring about 1.1-1.2 mm. in

length, and from 0.9-1.0 mm. in width. There is a micropyle at the animal

pole of the egg, but the characteristic horny egg-case and the polar hooks

of the Myxinoids are both wanting. The embryo hatches out as a larva known

as the "Ammocoetes." At this stage of its development the larva lacks

several of the most striking features which characterise the adult, and it

is highly probable that the Ammocoetes represents a stage in the evolution

of Vertebrates in some respects intermediate between _Amphioxus_ and a very

primitive Craniate. The mouth of Ammocoetes is bounded laterally and in

front by a curious hood-like upper lip, and behind by a short transverse

lower lip (Fig. 244). The eyes are deeply seated and rudimentary, and as

visual organs they are useless, but the parietal eye is well developed. As

in the adult, there are seven pairs of gill-sacs, but they open internally

into a pharynx, directly continuous behind with the rest of the alimentary

canal, and there is no dorsal oesophagus. Like the skull, the branchial

basket is still very rudimentary. The dorsal and caudal fins are

{429}continuous. A gall-bladder is present, and also a bile duct opening

into the gut. In its mode of life, and especially in the manner in which it

obtains its food, Ammocoetes presents a most remarkable resemblance to

_Amphioxus_ and the Ascidians. In the median line of the pharyngeal floor

there is an open groove, the hypopharyngeal groove or endostyle, and a

tract of ciliated cells along the dorsal wall represents a hyperpharyngeal

groove: connecting the two in front there is a peripharyngeal ciliated

groove.[512] The Ammocoetes feeds on small food particles carried through

the mouth into the pharynx by currents of water produced by ciliary action.

The food becomes entangled in strings of mucus probably secreted by the

cells lining the endostylar groove. The mucus is then swept upwards in the

pharyngeal groove, and finally wafted backwards to the stomach and

intestine by the cilia of the hyperpharyngeal band. The skin exhibits the

remarkable peculiarity of containing a peptic ferment capable of digesting

proteids in a .2 per cent solution of hydrochloric acid. As the larva lives

buried in the mud, the epidermic secretion probably helps to keep the skin

free from bacteria, microscopic spores, and fungoid, or other parasitic

growths.[513] The young Lamprey lives as an Ammocoetes from 3-4 years, and

then in the course of a few weeks in the winter it undergoes a

metamorphosis, losing its larval characters and acquiring the structure and

habits of the adult. During this period the buccal funnel is completed and

teeth are developed. The eyes approach the surface and become functional.

The continuity of the median fins becomes interrupted. The endostylar

groove becomes transformed into a thyroid gland, the gall-bladder

disappears, and the bile duct becomes obliterated and changed into a mass

of small follicles. The skull and branchial basket complete their

development. At the same time the pharynx loses its connection with the

rest of the alimentary canal and remains as the branchial canal. The

so-called oesophagus of the adult is apparently a new formation which grows

forwards and acquires a connection with the mouth. It is probable that it

represents a hyperpharyngeal groove constricted off from the dorsal wall of

the pharynx.

Both the marine Lamprey and the "Lampern" are captured {430}for food,

either by nets or wicker traps. Formerly the Lampern was taken in enormous

numbers in several British and Irish rivers, especially in the Severn from

February to May, and in the Thames during May and June, but for various

reasons the supply has much diminished in recent years. The Lampern makes

excellent bait for Cod and Turbot, and for this purpose large numbers used

to be taken in the Trent and Thames for despatch to Grimsby and other

fishing ports.[514]

{431}CHAPTER XVII

ELASMOBRANCHII: GENERAL CHARACTERS—PLEUROPTERYGII—ICHTHYOTOMI—ACANTHODEI—

PLAGIOSTOMI—SELACHII—BATOIDEI—HOLOCEPHALI

CLASS II. PISCES.

SUB-CLASS I. ELASMOBRANCHII.

In both the ancient and the modern Sharks, Dog-Fishes, and Rays, the

exoskeleton takes the form of a more or less uniform investment of dermal

denticles or "shagreen." The endoskeleton is wholly cartilaginous or

partially calcified, and there are neither cartilage- nor membrane-bones.

The vertebral column is acentrous or chordacentrous, generally with

alternating basi- and inter-dorsal elements, and terminating in a

heterocercal tail. The skull is usually hyostylic, very rarely amphistylic

or autostylic, and the lateral halves of the primary upper jaw

(palato-quadrate cartilages) usually meet in a highly characteristic median

symphysis beneath the base of the skull. Branchial arches and clefts are

five to seven in number, and the clefts are separated by complete

interbranchial septa, which, as a rule, are continuous externally with the

skin. An operculum is developed only in the Holocephali. A pelvic girdle is

present. With rare exceptions the pectoral fin is uniserial. The pelvic fin

is invariably uniserial. The exoskeletal supports of all the fins consist

of ceratotrichia, and, when present, the fin-spines are invested by enamel.

Claspers are generally present in the males.

In the surviving members of the group the nostrils retain their primitive

ventral position. There is a conus arteriosus with several rows of valves.

A spiracle, often furnished with a spiracular pseudobranch, is generally

present, and, as a rule, {432}there is a hyoidean hemibranch supplied with

venous blood from the ventral aorta. The gill-filaments are attached

throughout their length to the interbranchial septa. There is an optic

chiasma. An air-bladder is not developed. The intestine has a spiral valve,

and there is a cloaca. The gonoducts in both sexes are derived from the

kidney system. The ova are large, few in number, and enclosed in horny

egg-cases, and they are fertilised before extrusion. The segmentation is

meroblastic, and the embryo is furnished with long external gills.

The Elasmobranchs are for the most part active predaceous Fishes, living at

different depths in the sea, from the surface to nearly a thousand fathoms,

and ranging from mid-ocean to the shallower waters round the coasts in

almost every part of the world. Although typically marine, they sometimes

ascend rivers beyond the reach of tides, and a few are permanent

inhabitants of fresh water. They are most abundant in tropical and

subtropical areas, where they also attain their greatest size, and are

numerous in temperate regions, but there are some species which are

typically Arctic. None of them are small, and some of the Sharks are the

largest of living Fishes. All are carnivorous, but so diversified is their

food that in different species it may range from other Fishes of no mean

size to Molluscs, Crustaceans and other Invertebrates, or even to plankton.

In their breeding habits the Sharks and Dog-Fishes present many interesting

features. Unlike the generality of Fishes, the eggs are fertilised

internally as a sequel to the copulation of the sexes. For this purpose the

males are furnished with special intromittent organs, the myxopterygia or

so-called claspers, which are developed as modifications of the hinder

portions of the pelvic fins.[515] Each clasper is supported by an internal

skeleton, consisting of several cartilages derived from the radialia of the

fins, and is traversed along its inner aspect by a groove. When sexual

congress takes place the claspers are thrust through the cloaca of the

female into the oviducal orifices, and in some instances it is probable

that they are retained in this position by hook-like denticles developed at

their free extremities. The seminal fluid then flows along these conduits

into the oviducts, in the upper portions of which it meets and impregnates

the eggs. After fertilisation the egg is enclosed in a dark brown horny

egg-case, secreted by the oviducal gland.

{433}[Illustration: FIG. 245.—Egg-case of _Heterodontus_ (_Cestracion_)

_galeatus_. (From Parker and Haswell, after Waite.)]

As a rule each egg-case has but a single egg, but in _Rhinobatus_ and

_Trygonorhina_ (Batoidei), both of which are viviparous, each case contains

three to four eggs. Generally the egg-cases are somewhat quadrangular in

shape, with the four angles, two at each end, prolonged either into short

horns, or into long tapering tendrils (Fig. 246). The oval egg-cases of the

Heterodontidae are remarkable not only for their size, but also for the

presence of a broad spiral lamina winding round the exterior of the case

from one pole to the other (Fig. 245). The majority of the Sharks,

Dog-Fishes, and Rays are viviparous, that is, the young are born alive; the

rest, like the Scylliidae (_e.g._ the common British Dog-Fishes, _Scyllium

canicula_ and _S. catulus_), the Heterodontidae, and the Raiidae are

oviparous, that is, the young are hatched out after the extrusion of the

eggs. In the oviparous species the eggs are extruded either singly or in

pairs, and generally deposited on the sea-bottom. When, however, the

egg-cases are provided with tendrils, as, for example, in the two British

Dog-Fishes just mentioned, these organs act as anchoring filaments. When

extruding an egg, the female swims round and round some piece of upright

seaweed, and the curling tendrils become entwined round it in such a way

that the egg becomes securely attached thereto (Fig. 246).[516] The embryos

are long in developing, and in _Scyllium_ it may be several months after

fertilisation (200 to 275 days) before they are hatched, the young Fish

finally escaping through a rupture in the egg-case.

{434}[Illustration: FIG. 246.—Egg of the Spotted Dog-Fish (_Scyllium

canicula_), showing its mode of attachment after extrusion. (From Hertwig,

after Kopsch.)]

In the oviparous species the nutritive food-yolk stored up, first in the

egg and subsequently in the yolk-sac (Fig. 248), suffices for the

nourishment of the embryo until the period of hatching, but in viviparous

forms, whose embryonic development is completed within special uterine

dilatations of the oviducts, additional means of nutrition are provided for

the young. Such Elasmobranchs as _Spinax_, _Acanthias_, _Centrina_,

_Scymnus_, _Trygon_, _Torpedo_, and _Myliobatis_ have long filaments (villi

or trophonemata) developed from the inner surface of the uterus, which

secrete a nutritive fluid, and this fluid is either absorbed by the

blood-vessels of the embryonic yolk-sac, or it is taken up by the embryo in

some more direct manner. In some of the Trygonidae and Myliobatidae of the

Indian Ocean it seems probable that the secretion is taken into the

alimentary canal of the embryo either through the mouth or through the open

spiracles.[517] One species, _Pteroplatea micrura_, has its long and highly

vascular and glandular trophonemata gathered into two bundles, which are

thrust through the huge spiracles into the pharynx of the embryos, of which

there may be from one to three, and the nutritive secretion is apparently

digested in the alimentary canal of the embryo and absorbed by the foetal

blood-vessels (Fig. 247). A few Sharks, like most species of _Mustelus_,

develop a placenta when the food-yolk in the yolk-sac of the embryo is

nearly used up. Folds or projections from the highly vascular wall of the

yolk-sac interlock with similar vascular folds of the lining membrane of

the uterus, and a diffusion of nutrient material takes place from the

maternal blood in the uterine blood-vessels to the foetal blood in the

{435}vessels of the yolk-sac.[518] Each embryo has its own placenta, and in

_Mustelus antarcticus_ the uterine portion of the oviduct is divided by

septa into several chambers, each containing a single embryo.[519] It is

worthy of note that in the viviparous species a distinct but very thin,

delicate egg-case is formed, occasionally even with the rudiments of

tendrils, which may either be retained or thrown off in the uterus. The

Greenland Shark (_Laemargus borealis_) is unique amongst Elasmobranchs. Its

eggs are small and unprotected by egg-cases, and their fertilisation is

said to be effected in the water after deposition, as in the generality of

Fishes.

Fossil remains of Elasmobranchs in the shape of fin-spines

(ichthyodorulites) and dermal denticles, associated with various

Ostracodermi (Coelolepidae, Pteraspidae, and Cephalaspidae), are amongst

the earliest undoubted indications of Vertebrate life. They first appear in

the Upper Ludlow Bone Bed and in Silurian rocks in other parts of Europe,

and in North America; and in greater or less abundance the group is

represented in almost every subsequent geological period. It cannot be said

that the group shows signs of decadence, for Elasmobranchs still survive in

apparently undiminished numbers and variety in the marine fauna of the

present day.

[Illustration: FIG. 247.—Embryo of an Indian Sting Ray (_Pteroplatea

micrura_) as seen when the uterus is laid open. _t. t_, Two bundles of

trophonemata inserted into the spiracles, _sp_, _sp_. (From Wood-Mason and

Alcock.)]

The Elasmobranchs are certainly a very primitive race of Fishes. Their

earliest representatives of whose structure we have any precise knowledge

(e.g. _Cladoselache_ and _Pleuracanthus_) are in many respects the most

archaic of known gnathostomatous {436}Craniates, and from such types as

these, amongst others, we may reasonably look for the ancestors of all or

most of the remaining groups of Fishes. It has been well said of

_Pleuracanthus_ that "it is a form of Fish which might with little

modification become either a Selachian, Dipnoan, or Crossopterygian,"[520]

while the condition of the primary upper jaw in the Chondrostean _Polyodon_

suggests that even the more primitive Actinopterygii had an Elasmobranch

origin. The important researches of Dr. Traquair render it also highly

probable that the ancient Ostracodermi may claim kinship through their

Coelolepid ancestors with some primitive type of Elasmobranch; and within

the limits of the group there is ample evidence that differentiation has

taken place on many divergent lines, of which we have notable examples in

such specialised offshoots as the Acanthodei and the Holocephali, to say

nothing of several highly specialised families which became extinct at

successive periods in the history of the group.

[Illustration: FIG. 248.—An embryo Shark, with its yolk-sac (_y.s_); _sp_,

spiracle.]

ORDER I. PLEUROPTERYGII.

The only certain representative of this group is the Palaeozoic form

_Cladoselache_, probably the most primitive Elasmobranch at present known

(Fig. 249). Elongated and somewhat cylindrical in shape,

_Cladoselache_[521] has a terminal mouth, five or possibly seven pairs of

branchial clefts, and a pair of olfactory organs, lateral in position near

the extremity of the snout. Wide-based, triangular pectoral and pelvic

fins, a low anterior and a posterior dorsal fin, devoid of spines, and a

heterocercal caudal fin with homocercal tendencies, are present, but no

anal fin has yet been detected.

{437}[Illustration: FIG. 249.—Restoration of _Cladoselache fyleri_. Lateral

and ventral views. (From Parker and Haswell, after Dean.)]

The exoskeleton consists of minute lozenge-shaped denticles, which invest

the body and extend on to the surfaces of the fins, and there is also a

circumorbital ring of several concentric rows of small square plates. A

lateral line, in the form of a groove between two rows of denticles,

extends along each side of the body. The notochord is persistent. Calcified

neural and haemal arches (basidorsals and basiventrals) have been observed

in the caudal region, where they correspond numerically with the remains of

the myotomes, but interdorsal or intercalary arcualia seem to be absent.

The upper and lower jaws, similar in size and shape, are apparently

supported by a hyomandibular cartilage; hence the skull is hyostylic. The

endoskeletal supports of the pectoral, and especially those of the pelvic

fins, exhibit a more primitive disposition than in any other Fishes. They

extend nearly to the distal margins of the fins, where they seem to

interdigitate with the proximal ends of feebly-developed ceratotrichia

(Fig. 145). The extension of the fins in the horizontal plane, the gradual

shading off of their broad bases into the sides of the body, and the

resemblance between their radialia and those supporting median fins, are

very suggestive of the origin of the paired fins from continuous lateral

fin-folds. Claspers are absent. The dentition is well developed, and

several rows of teeth seem to be functional at the same time. {438}Each

tooth consists of a broad base, supporting a long pointed central cusp and

a variable number of similarly shaped but much shorter lateral cusps. The

teeth in the various transverse rows from without inwards are closely

wedged together by the interlocking or overlapping of their bases.

FAM. 1. CLADOSELACHIDAE.—Several species of _Cladoselache_, varying from 2

to 5 feet in length, have been found in the Cleveland Shale (Upper

Devonian) of Ohio. Isolated teeth similar to those of _Cladoselache_ occur

in the Lower Carboniferous of Europe, India, and North America, and have

been referred to various species of the genus _Cladodus_, but with one

exception nothing more is known of the structure of these Fishes, and

consequently their relationship to _Cladoselache_ is doubtful. _C.

neilsoni_,[522] from the Lower Carboniferous (Calciferous Sandstones) of

Kilbride in Scotland, has a very different type of pectoral fin, which

appears to be distinctly uniserial, but intermediate in structure between

the biserial fin of _Pleuracanthus_ and that of the modern sharks. There

are several other genera from the Devonian and Lower Carboniferous whose

claims to inclusion in this group rest on no better foundation.

ORDER II. ICHTHYOTOMI.

While more specialised than the Pleuropterygii the Fishes included in this

group represent an extremely generalised type of Elasmobranch, which, as

already indicated, may easily have been the ancestor of more than one group

of Fishes. In the typical genus _Pleuracanthus_[523] (Fig. 250)[524] the

body is elongate, but slightly depressed, with a terminal mouth, and a

tapering diphycercal tail fringed above and below by a continuous caudal

fin. A long dorsal fin, two small anal fins, and well-developed paired fins

with contracted bases, are present. The head is armed with a prominent,

serrated, dorsal spine, but it is doubtful if dermal denticles (shagreen)

are present. The vertebral column {439}is acentrous, and the persistent

notochord supports a series of basidorsal cartilages, which alternate with

small interdorsals, a series of basiventrals supporting small ribs, and in

the caudal region well-developed haemal arches. The dorsal fin is supported

by slender, tri-segmented radialia, which appear to be twice as numerous as

the neural arches in the trunk; but in the dorsal lobe of the caudal fin

the two structures agree in number. Ventrally-prolonged haemal spines are

the sole endoskeletal supports of the inferior lobe of the caudal. The

coraco-scapular cartilages of opposite sides remain distinct, and each

supports a biserial fin. The pelvic girdle is represented by a pair of

small cartilages supporting the basipterygia. The pelvic fins are

uniserial, with post-axial skeletal supports for claspers in the males.

Both the median and the paired fins are provided with marginal

ceratotrichia. The skull is probably amphistylic. Five, possibly six or

seven, branchial arches, bearing clusters of minute denticles, are present.

Circumorbital plates are wanting. All the endoskeletal structures are

partially calcified. The teeth are tricuspid, each with two long divergent

lateral cusps and a minute median cusp; the broad bases of the teeth

overlap and articulate with one another by means of facets.

[Illustration: FIG. 250.—Restoration of _Pleuracanthus ducheni_. A',

Ventral fin; B, basal cartilages of the paired fins; D, ceratotrichia; DS,

head-spine; HA, haemal arches; _HM_, hyomandibular; IC, interdorsal

cartilages; MC, Meckel's cartilage; N, notochord; NA, neural arch; P,

supposed pelvic cartilage; the triangular cartilage behind it is the

basipterygium; PQ, palato-quadrate; _R_, radialia of the paired fins; _R′_,

rib; _RB_, radialia of the dorsal fin; _SG_, shoulder-girdle. (From Parker

and Haswell, after Dean.)]

{440}FAM. 1. PLEURACANTHIDAE.—The single family included in the group

ranges from the Lower Carboniferous to the Lower Permian. Within these

limits the family is widely distributed in different formations in Great

Britain, Continental Europe, New South Wales (Lower Hawkesbury Formation),

and North America. _Pleuracanthus_, of which complete skeletons and skulls

have been found, is the best known genus.

ORDER III. ACANTHODEI.

The Fishes comprising the Acanthodei[525] may be regarded as a highly

specialised and terminal offshoot from some primitive race of early

Elasmobranchs. The Elasmobranch kinship of the Acanthodei is indicated by

their exoskeleton of shagreen tubercles; the completely heterocercal tail;

the absence of an operculum, the external gill-clefts apparently being

exposed; the position of the lateral line of the trunk between two rows of

shagreen denticles; the nature of the powerful spines in connexion with the

dorsal and anal, and the pectoral and pelvic fins; and the formation of the

hard parts of the skeleton, not by ossification involving the presence of

bone-cells, but by the calcification of cartilage, or of more superficial

membranous or fibrous tracts. On the other hand, it may be noted that the

Acanthodei appear to have undergone much specialisation on lines in some

respects parallel to those which have marked the evolution of the

Teleostomi, but by methods which are simply an exaggeration of features

normally characteristic of Elasmobranchs. Perhaps the most striking

illustration of this is to be seen in the development of a species of

secondary skull by an extension of a process of calcification as

distinguished from ossification. Hence the presence of

membrane-calcifications in relation with the upper and lower jaws, whose

development is proportional to the size of the teeth they support, and of

smaller investing plates of the cranial roof. Similar exoskeletal

calcifications, when most completely developed (e.g. _Diplacanthus_), form

a dorsally incomplete arch, apparently corresponding to a secondary

pectoral girdle for the support of the stout pectoral spines, in which

elements {441}analogous to clavicles or cleithra and infra-clavicles can be

recognised. Each pectoral spine forms the preaxial margin of the fin, and

behind it there is a series of ceratotrichia. Nothing is known of the

endoskeletal supports, but having regard to the nature and proportions of

the pectoral spines it may be inferred that the exoskeletal elements of the

fins predominate over the former to an extent which is only paralleled

elsewhere in the Teleostei.

Apparently the notochord is persistent, and there are long and slender

neural and haemal arches, but no ribs. The dermal denticles are uniform in

size, and so small as to give a granular appearance to the skin. In

structure they are thick, with a flat, enamelled, often sculptured,

external surface, quadrate or rhombic in shape, and fitting closely

together. Teeth are either absent or very minute, but sometimes (e.g.

_Acanthodopsis_ and _Ischnacanthus_) they are few in number and large,

conical in shape, occasionally with minute cusps between the larger teeth.

Claspers are absent. The Acanthodei are small Fishes, most of them being

less than .3 m. in length, and ranging from the Upper Silurian to the Lower

Permian inclusive. Two families are recognised.

FAM. 1. DIPLACANTHIDAE.—Two dorsal fins are present. Usually there is a row

of lateral spines extending along each side of the body between the

pectoral and pelvic fins. Exclusively Upper Silurian and Devonian.

The genera _Diplacanthus_, _Climatius_, _Parexus_, _Euthacanthus_, and

_Ischnacanthus_ are all found in the Lower Old Red Sandstone of Scotland.

_Climatius_ and _Diplacanthus_ are also represented in the Devonian of

Canada.

[Illustration: FIG. 251.—Restoration of _Acanthodes wardi_. Carboniferous

of England and Scotland. (From Smith Woodward.)]

FAM. 2. ACANTHODIDAE.—A single dorsal fin; lateral spines vestigial or

absent. Lower Devonian to the Lower Permian.

{442}The widely-distributed genus _Acanthodes_ (Fig. 251) is represented in

the Lower Old Red of Scotland, the Devonian of Siberia and Canada, the

Carboniferous of England and Scotland, and the Lower Permian of France,

Germany, and Bohemia. _Acanthodopsis_ (Coal Measures), and _Mesacanthus_

and _Cheiracanthus_ (Lower Old Red) are the remaining genera.

ORDER IV. PLAGIOSTOMI.

Head prolonged in front of the ventrally-situated mouth as a more or less

prominent preoral rostrum, vertebral column consisting of alternating basi-

and inter-dorsal cartilages, generally supported by more or less

well-developed chorda-centra. Pectoral and pelvic fins uniserial. Pelvic

girdle and claspers present. Except in two families the branchial arches

and clefts are invariably five in number. An operculum is not

developed.[526]

SUB-ORDER 1. SELACHII.

Body elongate or fusiform, shading imperceptibly into a powerful swimming

tail. Pectoral fins of moderate size, with contracted bases; not confluent

with the sides of the head. Branchial clefts lateral in position. Vertebral

centra generally asterospondylic or cyclospondylic.

This sub-order includes such typical Elasmobranchs as the modern Sharks and

Dog-Fishes as well as numerous fossil representatives ranging from the

Carboniferous, and probably from still earlier periods, to the present day.

FAM. 1. NOTIDANIDAE.—Body moderately elongate, the spineless dorsal fin

opposite the anal. Mouth ventral; nostrils ventral, near the extremity of

the snout, without oro-nasal grooves. Branchial arches and clefts six or

seven. Interbranchial septa devoid of marginal frills. Notochord persistent

and continuous, partially constricted by simple chorda-centra, each

consisting of two distinct rings, without either concentric or radial

lamellae, except {443}in one species (_Notidanus cinereus_), which exhibits

a feeble asterospondylism in the caudal vertebrae. Skull amphistylic. Teeth

unlike in the two jaws; those in the upper jaw usually with a large central

cusp and smaller lateral cusps; those in the lower jaw comb-like, each

consisting of numerous graduated pointed cusps inclining in the same

direction, and supported on a long basal plate.

The very few species included in this family are widely distributed in the

tropical and subtropical regions of the Atlantic and Pacific Oceans.

_Notidanus_ (_Heptanchus_) _cinereus_, which has seven branchial arches and

clefts, inhabits the Mediterranean and Atlantic. _N_. (_Hexanchus_)

_griseus_, with six branchial arches and clefts, has a similar

distribution, but besides being an occasional visitant to the British

coasts, it is not uncommon at Cuba in the West Indies. It is said to grow

to a length of 26 feet.

Fossil remains of _Notidanus_, principally teeth, occur in the Middle and

Upper Jurassic, in the Cretaceous, and in the Eocene and Pliocene of

England and the Continent.

FAM. 2. CHLAMYDOSELACHIDAE (Frilled Sharks).—Body much elongate. Median

fins as in _Notidanus_. Mouth nearly terminal. Nostrils lateral, nearly

terminal, and without oro-nasal grooves. Branchial arches and clefts six.

The outer margins of the interbranchial septa are produced into overlapping

cutaneous frills, the first of which is developed from the hyoid arch and

overlaps the hyobranchial cleft, like a rudimentary operculum. Vertebral

column as in the preceding family, but in the hinder part of the trunk the

notochord is unconstricted and uniform in diameter, centra being absent.

Skull hyostylic. Lateral line an open groove. Teeth alike in both jaws,

each consisting of a broad basal plate supporting three slender curved

cusps, separated by a pair of much smaller cusps.

The only living species known is _Chlamydoselachus anguineus_ (Fig.

252),[527] which occurs in the Pacific near Japan, in deep water off

Madeira, and also off the Azores and the coast of Norway. It reaches a

length of 4 to 5 feet. Teeth from the Pliocene deposits of Tuscany have

been referred to an extinct species, _C. lawleyi_.

{444}Scarcely anything is known of the habits of the Notidanidae and the

Chlamydoselachidae. It is evident that they are closely-related forms, and

from the unusual number of their gill-clefts and branchial arches, and the

condition of the vertebral column, it is also obvious that they are the

most archaic of modern Selachians.

[Illustration: FIG. 252.—_Chlamydoselachus anguineus_. (From Günther.)]

FAM. 3. HETERODONTIDAE (Bullhead Sharks).—Head large and high, with a blunt

snout projecting but little in front of the small and almost terminal

mouth, and with prominent supraorbital crests. Trunk thick-set and somewhat

trihedral, covered with fine shagreen. Nostrils ventral but nearly

terminal, with oro-nasal grooves. Spiracles small, beneath the eyes. Two

dorsal fins, each with a spine in front, the first opposite the interval

between the pectorals and pelvics, the second in front of the anal.

Vertebral centra asterospondylic when fully developed. Palato-quadrate

cartilages with an extensive articulation with the sides of the preorbital

regions of the cranium, the normal suspensoria of a hyostylic skull

(hyomandibular cartilages) taking little share in their support. Dentition

similar in both jaws. Teeth at the symphyses numerous, small, and conical,

furnished with three to five cusps in the young; those behind broad and

pad-like, arranged in oblique rows, the teeth forming the two middle rows

being much larger than those in the front or behind. Living species,

oviparous. Egg-cases large, with an external spiral lamina (Fig. 245).

About four species belonging to one genus, _Heterodontus_ (= _Cestracion_)

(Fig. 253), or possibly to two, represent this dwindling family. All are

inhabitants of the Pacific Ocean (Japan, Amboyna, Australia, the Galapagos,

and the Californian coast of North America). Little is known of their

habits. They feed {445}principally on Molluscs, the shells of which are

crushed by their massive grinding teeth. The different species vary in size

from 2 to 5 feet.

The Heterodontidae were the most characteristic and abundant Sharks of the

Mesozoic period. Amongst extinct genera _Hybodus_ ranges from the Middle

Trias to the Lower Cretaceous (Wealden); an allied genus, _Acrodus_, from

the Middle Trias to the Upper Cretaceous (Gault). _Palaeospinax_ occurs in

the Lias and possibly in the Upper Trias. _Synechodus_ is a Cretaceous

genus, and _Asteracanthus_, which has large hooked spines on the head, is

characteristic of the Middle and Upper Jurassic. An even greater antiquity

may be claimed for the Heterodontidae if, as is not improbable, such

Palaeozoic Sharks as _Orodus_, _Sphenacanthus_, _Tristychius_

(Carboniferous), and _Wodnika_ (Permian) belong to this family. Many

ichthyodorulites are probably the spines of various extinct Heterodontidae.

[Illustration: FIG. 253.—Port Jackson Shark (_Heterodontus philippi_). A,

lateral view; B, mouth and nostrils. _d_, Clasper. (From a specimen in the

Cambridge University Museum.)]

FAM. 4. COCHLIODONTIDAE.[528]—This Palaeozoic family includes a number of

Sharks probably related to the Heterodontidae, but of which little is known

except their dentition. The teeth are in some respects similar to those of

_Heterodontus_, except that those which appear to correspond to one or both

of the middle rows of the latter genus tend to fuse and form a few large,

convex, and often scroll-like plates. The typical Cochliodonts are

exclusively Carboniferous (Europe and North America). _Psephodus_,

_Pleuroplax_, _Deltodus_, _Poecilodus_, _Cochliodus_, _Deltoptychius_,

_Helodus_, and _Menaspis_ (Permian) are characteristic genera.

{446}Probably some ichthyodorulites described under various generic names

belong to this family.

FAM. 5. PSAMMODONTIDAE.—Teeth large, flat or slightly arched, oblong or

quadrate, and arranged in one, two, or more longitudinal rows. Only the

teeth are known, and from differences in their shape, size, and surface

markings, the genera _Psammodus_, _Archeobatis_, and _Copodus_ have been

recognised. The family is confined to the Lower Carboniferous of Great

Britain and Ireland, Russia, Belgium, and North America.

FAM. 6. PETALODONTIDAE.—Teeth transversely elongated, with a blunt or a

sharply-ridged crown, separated from a single or multiple root by a

constricted neck, and disposed in transverse and longitudinal pavement-like

rows; exoskeleton of smooth, oval, rounded or quadrate shagreen denticles.

Only the teeth, and in some genera the dermal denticles, are known, except

in _Janessa_, which has a Ray-shaped body, with large pectoral fins

prolonged towards the head. The family is mainly confined to the

Carboniferous formations of Great Britain, Europe, and North America.

_Petalodus_, _Janessa_ (also represented in the Permian), _Glossodus_,

_Polyrhizodus_, and _Callopristodus_ are characteristic genera.

FAM. 7. SCYLLIIDAE (Dog-Fishes).—Dorsal fins two in number, small, and

without spines, the first above or behind the pelvic fins, the second

usually behind the anal. Tail not bent upwards or but slightly so, without

lateral keels. Spiracles present. Nictitating membranes absent. Vertebrae

asterospondylic. Teeth small, each with a median cusp, and one to four

small cusps on each side. Oviparous. Egg-cases (Fig. 246) large, quadrate,

with long twining tendrils at the angles for attachment.

The genus _Scyllium_ includes the true Dog-Fishes (Fig. 254). The species

are coast Fishes of small or moderate size, and are widely distributed in

temperate and tropical seas, at depths not as a rule exceeding 400 fathoms.

Two species, _S. canicula_ and _S. catulus_, are common on the British

coasts, living near the bottom and feeding on Crustaceans and Molluscs. An

allied form, _Pristiurus_, is also common in European and British waters.

_Chiloscyllium_ is a widely-distributed genus ranging from the Cape of Good

Hope through the Indian Ocean to the coasts of Australia, China, and Japan.

_Stegostoma tigrinum_ of the {447}Indian Ocean attains a length of 10 to 15

feet, and is remarkable for its handsome coloration of dark bands on a

yellow ground, which has suggested the name of Tiger- or Zebra-Shark. The

pelagic genus _Ginglymostoma_ has the terminal portion of the tail bent

upwards, and grows to a length of 6 to 12 feet. It is represented by

species in the Indian Ocean and the tropical parts of the Atlantic (West

Indies and the west coast of Mexico). _Crossorhinus_ includes species of

large size, some of which are 10 feet long. They are ground-sharks,

frequenting the coasts of Australia and Japan, which lie on the bottom

watching for their prey, and in accordance with this habit their coloration

closely resembles that of their surroundings.[529] A large North Atlantic

Shark (_Pseudotriakis microdon_), of which only two specimens are known,

one taken on the Portuguese coast, and the other, 10 feet in length, off

Long Island, on the Atlantic coast of North America, has the general

characters of the Scylliidae, except that the first dorsal fin is opposite

the interval between the pectoral and pelvic fins. Some Scylliidae live at

great depths, _Scyllium_ (_Scylliorhinus_) _profundorum_ having been

obtained from a depth of 816 fathoms in the North Atlantic.[530]

[Illustration: FIG. 254.—A female Dog-Fish (_Scyllium canescens_), from the

south-western coast of South America. (From Günther.)]

Most of the fossil Scylliidae belong to existing genera. The earliest known

representatives of the family occur in the Upper Jurassic (Lithographic

Stone of Bavaria), where the extinct genus _Palaeoscyllium_, a near ally of

the existing _Scyllium_, and _Pristiurus_, are found, nearly complete.

_Scyllium_ itself ranges from the Cretaceous through the different Tertiary

formations. A species of _Chiloscyllium_ has been recorded from the Miocene

Tertiaries, and detached teeth of _Ginglymostoma_ from the Eocene of

Belgium and North America. An extinct genus (_Mesiteia_), which is found in

the Upper Chalk of Mount Lebanon and the Upper Eocene of Monte Bolca, is

remarkable for the enclosure of {448}its lateral sensory canals in a series

of incomplete calcified rings, as in the Holocephali.

FAM. 8. CARCHARIIDAE.—Sharks with two dorsal fins, the first in front of

the pelvic fins and the second opposite the anal fin, both devoid of

spines. Tail without lateral keels. Preoral rostrum elongated. Mouth

crescentic. Eyes with nictitating membranes. Spiracles small or absent.

Vertebrae asterospondylic. Teeth usually consisting of a single triangular

cusp, with smooth, trenchant, or serrated margins, rarely with basal cusps;

generally with an axial cavity when fully developed. Viviparous. The family

comprises about twenty genera, and approximately sixty species; found in

all seas, often in mid-ocean. Amongst the more important genera may be

mentioned _Carcharias_ (_Carcharhinus_), _Galeocerdo_, _Triakis_,

_Thalassorhinus_, _Galeus_, _Mustelus_ and _Scylliogaleus_.

[Illustration: FIG. 255.—The Blue Shark (_Carcharias glaucus_). (From

Müller and Henle.)]

Species of _Carcharias_ are found in nearly all tropical and subtropical

seas. The genus is a somewhat comprehensive one, and groups of its species

have been distinguished as sub-genera under the names of _Prionodon_,

_Hypoprion_, _Scoliodon_, _Aprionodon_,[531] etc. One of the most widely

distributed of the thirty to forty species is the Blue Shark, _C_.

(_Prionodon_) _glaucus_ (Fig. 255), of the Atlantic and Pacific Oceans,

which may grow to a length of 25 feet, although the young forms not

infrequently captured in British waters do not exceed 6 to 8 feet. It is a

slender, swift, pelagic Shark, of a slaty-blue colour above and white

underneath, and a voracious hunter of other Fishes. _C. nicaraguensis_, a

Shark about 7 feet long, is confined to Lake Nicaragua and its outlet the

Rio San Juan, and is one of the very rare strictly freshwater Sharks.

_Galeocerdo_ is a large Shark found in temperate and tropical waters, but

one species, _G. arcticus_, is confined to Arctic seas. The variegated _G.

tigrinus_, or West Indian Tiger-Shark, is said {449}to reach a length of 15

to 20 feet. The genus _Galeus_ includes the small Sharks commonly known as

"Topes," which are common in nearly all tropical and temperate seas. The

British species, _G. canis_, which ranges from 4 to 6 feet in length, is a

bottom-feeding Fish, preying on Molluscs, Crustacea, Star-Fish, and small

Fishes. The various species of _Mustelus_, or "Hounds," resemble the Topes

in their habits and distribution. Living principally on Molluscs and

Crustaceans, the dentition has lost the trenchant, unicuspidate type

characteristic of most other Carchariidae, and is adapted for crushing and

grinding, the teeth being flat, without cusps, and arranged in

pavement-like rows. Two species, _M. vulgaris_ and _M. laevis_, are

abundant on the coasts of Europe and the British Isles. _Scylliogaleus_,

which combines the general characters of _Mustelus_ with nostrils similar

to those of a _Scyllium_, is known only from a single specimen from the

coast of Natal.[532]

The Carchariidae are comparatively modern Sharks. No undoubted remains are

known earlier than the Eocene, in which, as in the succeeding Miocene and

Pliocene deposits, they are represented principally by their characteristic

teeth. The extinct fossil genera are few in number, and so far as their

dentition is concerned they differ but little from their living allies.

FAM. 9. SPHYRNIDAE (Hammer-head Sharks).—In their general characters the

Hammer-head Sharks agree with the Carchariidae. They are distinguished,

however, by the remarkable shape of the head, which is prolonged into two

conspicuous lateral lobes, supported internally by corresponding

cartilaginous outgrowths from the post-orbital and the lateral ethmoidal or

nasal regions of the skull, with the eyes at their distal extremities, and

the nostrils in relation with their anterior margins. One genus and five

species.

The Sphyrnidae are denizens of nearly all tropical and subtropical seas.

_Sphyrna_ (_Zygaena_) _tudes_ occurs in the Mediterranean, and _S. zygaena_

is a very rare visitant to the British coasts. A specimen over 13 feet in

length was captured at Ilfracombe in 1865, and other examples have been

taken off Banffshire, at Newlyn in Cornwall, at Yarmouth, and in Carmarthen

Bay.[533] The shape of the head differs in different species, and in young

{450}forms the peculiarities of the adult are less marked. In the Bonnet

Shark (_S. tiburo_) (Fig. 256, A), the head is crescentic or kidney-shaped,

with prominent postero-lateral angles, and between this type of head and

the more pronounced "hammer" of _S. zygaena_ (Fig. 256, B) an almost

perfect gradation is supplied by other species. The Hammer-heads are

voracious Sharks, usually living in deep water, and they may grow to a

length of 15 feet. As many as thirty-seven embryos have been taken from the

oviducts of a female nearly 11 feet in length.[534]

[Illustration: FIG. 256.—Ventral view of the head and trunk (A) of a young

Bonnet Shark (_Sphyrna tiburo_), and (B) of a young male Hammer-head (_S.

zygaena_). _c_, Clasper; _cl_, cloacal aperture; _e_, eye; _n_, nostril;

_n′_, nasal groove.]

Teeth assigned with more or less probability to _Sphyrna_ are found in the

Miocene of Europe and North America.

FAM. 10. LAMNIDAE (Porbeagle Sharks).—Large, stout-bodied Sharks with two

dorsal fins, the first just behind the pectoral fins, the second, which is

small, opposite the small anal fin; both {451}without spines. Tail with a

prominent lateral keel on each side. Nictitating membranes absent.

Spiracles minute or wanting. Branchial clefts very wide. No oro-nasal

grooves. Vertebrae asterospondylic. When fully developed the teeth are

solid.

In the genus _Lamna_, which includes the Porbeagle Sharks, the teeth are

large, each consisting of a long narrow central cusp, usually with smaller

cusps at the base. The common Porbeagle (_L. cornubica_), a fierce pelagic

Shark, which may reach a length of 10 feet, frequents the North Atlantic

and the North Pacific (Fig. 257). It has often been captured off the coasts

of Great Britain and Ireland in Mackerel or Salmon nets, or by lines laid

for food Fishes. An allied genus, _Isurus_, is represented by species on

the Atlantic coast of North America, in the Mediterranean and the

neighbouring parts of the Atlantic, and also in Asiatic seas. _Carcharodon

rondeletii_[535] is a pelagic Shark with large, triangular, finely-serrated

teeth, without basal cusps, and is found in all tropical and subtropical

seas from the Mediterranean to Australia and New Zealand. It is one of the

largest and most formidable of Sharks, and it is said to grow to a length

of 40 feet. Nothing is known of its breeding habits. _Odontaspis_, which

has minute pore-like spiracles, but no lateral caudal keels, is a Shark of

moderate size, chiefly inhabiting the Atlantic, but found also in the

Mediterranean and the Southern Pacific. Its teeth are long and awl-like,

with small basal cusps.

[Illustration: FIG. 257.—The Common Porbeagle (_Lamna cornubica_). (From

Parker and Haswell, after Bashford Dean.)]

The Thresher or Fox Shark (_Alopecias vulpes_) is remarkable for the

extraordinary length of the upper lobe of the caudal fin, {452}which is as

long as the rest of the body (Fig. 258). Its teeth are of moderate size,

triangular in shape, and without serrations. The "Thresher" has a wide

distribution, being abundant in the Atlantic and Pacific Oceans, besides

being the commonest of the larger Sharks frequenting the British coasts. It

grows to a length of 15 feet, of which the tail forms at least one-half.

Quite inoffensive to man, the Thresher feeds on the shoals of smaller

Teleosts, such as Pilchards, Herrings, and Sprats. When feeding it swims in

gradually diminishing circles round the shoal, splashing the water with its

long tail, and keeping its victims so crowded together that they become an

easy prey. A remarkable Lamnoid Shark (_Mitsukurina owstoni_),[536] which

has the snout produced into a "long, flat, flexible, leaf-like blade,"

somewhat resembling that of _Polyodon_, but narrower and more pointed, and

has protractile jaws and large spiracles, is found in deep water near

Yokohama, and may prove to be generically identical with the Cretaceous

Shark _Scapanorhynchus_.[537]

[Illustration: FIG. 258.—The Thresher Shark (_Alopecias vulpes_). (From

Jordan and Evermann.)]

Lamnoid Sharks are not certainly known to have existed until the Upper

Cretaceous formations, in which, as well as in different Tertiary deposits,

teeth indistinguishable from those of the existing genera _Lamna_,

_Odontaspis_, and _Carcharodon_ are found. The interesting genus

_Carcharodon_ has one extinct species in the Cretaceous and several others

distributed in Tertiary formations in nearly every part of the world. The

teeth of some of the Tertiary species measure 5 inches along the margin and

4 inches across the base, and it is evident that they belonged to Sharks so

gigantic as completely to dwarf the existing species. That these giant

Lamnidae have only recently {453}become extinct is proved by the fact that

similar teeth have been dredged from the bottom of the Pacific. Teeth and

detached vertebrae from various Tertiary deposits have been referred to

species of _Alopecias_. Entire Fishes, with an elongated rostrum and an

extensive anal fin, from the Cretaceous of Mount Lebanon, have been

assigned to an extinct genus, _Scapanorhynchus_.

FAM. 11. CETORHINIDAE (Basking Sharks).—Two dorsal fins, without spines,

the anterior midway between the pectoral and pelvic fins. Tail without

lateral keels. Nictitating membranes absent. Spiracles small, situated just

above the angles of the mouth. Branchial clefts wide and of great vertical

extent, extending from the dorsal to the ventral surface. Teeth small, very

numerous, conical in shape, without serrations. Claspers of the male

provided with horn-like denticles.

[Illustration: FIG. 259.—The Basking Shark (_Cetorhinus_ (_Selache_)

_maximus_). (From Goode and Bean.)]

The single species included in this family, the Basking Shark,

(_Cetorhinus_ (_Selache_) _maximus_), is one of the largest of living

Fishes, reaching a length of 40 feet (Fig. 259). It is a pelagic Shark,

inhabiting the Arctic seas, but wandering as far south on opposite sides of

the Atlantic as the Mediterranean, the coasts of Portugal and Virginia, and

in the Pacific to the Californian coast. Although generally described as a

northern form, _Cetorhinus_ is known to occur in Australian waters.[538] It

is fairly common off the coasts of Scotland, and it has been seen or

captured at various points on the western coast of Ireland, and {454}the

eastern and southern coasts of England. The Fish is gregarious in its

habits, often swimming in shoals near the surface. The name "Basking Shark"

has been suggested by its habit of lying motionless on the surface in warm

or calm weather, as if basking in the sun, with its dorsal fin protruding

from the water. Unless attacked, this Shark is quiet and inoffensive. It

derives its food-supply from small pelagic Fishes, and also from marine

Invertebrates, which are strained from the water by the fringes of long,

slender gill-rakers with which the branchial arches are provided. At one

time harpooned and caught off the Irish, Scotch, and Norwegian coasts for

the sake of the oil obtained from its liver, the Fish is now of little

economic importance. Nothing is known of its mode of reproduction.

Extinct species of _Cetorhinus_ have been founded on detached vertebrae and

isolated teeth from deposits of Pliocene age in Belgium and Italy, and

possibly from still earlier Tertiary formations. Dermal spines similar to

those found on the claspers of the males in the existing species occur in

the Antwerp Crag, and in the Red Crag of Suffolk.

FAM. 12. RHINODONTIDAE.—Two dorsal fins, without spines, the anterior a

little in front of the pelvic fins, the second opposite the anal. Tail with

lateral keels and a pit at its root. Spiracles small. Nictitating membranes

absent. Mouth and nostrils nearly terminal. Teeth very minute, numerous,

and conical in shape.

One genus, _Rhinodon_, with one or two species, is known. These Sharks are

very widely distributed, specimens having been seen or captured in the

neighbourhood of Ceylon, at the Seychelles, the Cape of Good Hope, Callao

on the Peruvian coast, in the Gulf of California, and off the coast of

Florida. _Rhinodon_ is probably the largest known Shark. It is stated to

exceed 50 feet in length, but to be quite harmless. Scarcely anything is

known of its habits, but the small size of the teeth, and the length of the

gill-rakers, which resemble those of the Basking Shark, suggest a similar

kind of food.

FAM. 13. SPINACIDAE.—Two dorsal fins, the first in advance of the pelvic

fins. Anal fin absent. Nictitating membrane absent. Spiracles rather large.

Vertebrae cyclospondylic. Teeth variously modified in different genera.

{455}The more typical representatives of this family are the Spiny

Dog-Fishes, which are distinguished by the presence of a strong spine in

front of each dorsal fin. They are more abundant in temperate regions than

in the intervening tropics. The more important genera are _Acanthias_,

_Centrina_, _Centrophorus_, _Spinax_, and _Centroscyllium_. _Acanthias

vulgaris_, the Picked or Piked Dog-Fish, is a gregarious, voracious Shark,

about 3 to 4 feet in length, and is frequently seen in huge shoals all

round the British coasts, especially during the summer months. It is very

destructive to food Fishes, and its ravages result in serious loss to

fishermen. _Acanthias_ is viviparous. _Centrina salviani_ is a much smaller

Shark, which frequents the Mediterranean and the Bay of Biscay; on rare

occasions it has been taken off the southern coast of England.

_Centrophorus_ occurs in deep water in the Mediterranean and adjacent

portions of the Atlantic, and off the coasts of Japan. _Centroscyllium_ is

found on opposite sides of the North Atlantic (Greenland and

Massachusetts), and in the opposite hemisphere at the Falkland Isles. A

deep-water form, _Paracentroscyllium_, has been obtained in the Bay of

Bengal at depths from 285 to 405 fathoms.[539]

[Illustration: FIG. 260.—The Greenland Shark (_Laemargus borealis_). (From

Goode and Bean.)]

Three remaining genera (_Scymnus_, _Laemargus_, and _Echinorhinus_) differ

from the preceding in the absence of dorsal spines.

_Scymnus lichia_ is common in the Mediterranean and the neighbouring parts

of the Atlantic. The Greenland Shark (_Laemargus borealis_) (Fig. 260) is

an inhabitant of the Arctic regions, wandering as far southwards on

opposite sides of the Atlantic as the French coast and Cape Cod. It is a

huge, clumsy shark, reaching a length of 26 feet. Numerous instances are

recorded of its capture off the coasts of Great Britain, especially in

northern waters. The Greenland Shark is said to be a determined foe to

{456}the Right Whale, which it attacks, biting pieces out of its body.

_Scymnus_ is viviparous, _Laemargus_ oviparous, and the latter is unique

among Sharks in producing eggs devoid of a horny shell, which are deposited

on the sea-bottom. _Echinorhinus_ has dermal denticles in the form of

relatively large rounded tubercles, each surmounted by a tuft of fine

spines. One species only is known, _E. spinosus_, a large Shark attaining a

length of 10 feet, and frequenting deep water off the Atlantic coasts of

Europe and Africa from the North Sea to the Cape of Good Hope. A single

specimen has been taken at Cape Cod on the eastern coast of the United

States, and another off Dunedin, New Zealand. The capture of thirty

examples in British waters since 1828 has been recorded,[540] the largest a

female 9 feet in length.

Most of the existing genera of Spinacidae are represented by teeth or

detached spines in the later Tertiary deposits, but none are certainly

known to occur earlier than the Pliocene.

[Illustration: FIG. 261.—The Angel-Shark (_Rhina squatina_). A, dorsal

view; B, view of the mouth and nasal barbels. _p.f_, Pectoral fin; _pv.f_,

pelvic fin; _sp_, spiracle.]

FAM. 14. RHINIDAE (Angel-Sharks).—Ray-like Sharks with a flattened head and

body, and nearly terminal mouth and nostrils. Pectoral fins very large,

horizontally expanded, but constricted at the base and not adherent to the

sides of the head or trunk. Two dorsal fins, both small, without spines,

and situated on the tail behind the pelvic fins. Anal fin absent. Spiracles

large {457}and crescentic. Vertebrae tectospondylic. Teeth conical and

pointed. A single species only is known.

_Rhina squatina_, the Angel-Shark or Monk-Fish (Fig. 261), is intermediate

between the ordinary Sharks and the Skates and Rays, both in external

appearance and internal structure, but is more Ray-like than Shark-like in

its habits. Within the temperate and tropical regions of both hemispheres

it is almost cosmopolitan in its distribution, frequenting the coasts of

Europe, including the British Isles, the Atlantic and Pacific coasts of

North America, and the shores of South Australia and Japan. The Angel-Shark

is viviparous, producing about twenty young at a time. Not rarely it grows

to a length of 5 feet.

The family ranges from the Upper Jurassic to the present time. Species of

_Rhina_ are represented by more or less complete skeletons in the

Lithographic Stone of Bavaria, and in the Upper Cretaceous of Westphalia

and Mount Lebanon, and by teeth and vertebrae in the English Chalk, as well

as in different European Tertiary formations.

FAM. 15. PRISTIOPHORIDAE.—Prenasal portion of the head and cranium produced

into a long flattened rostrum, furnished with a pair of long tentacles on

its under surface, and, as in Saw-Fishes, with a series of large,

tooth-like, dermal denticles, of equal or unequal size, along each of its

lateral margins. Two dorsal fins, without spines, the first in front of the

pelvics. No anal fin. Pectoral fins large, distinct from the head and

trunk, with a contracted base. Spiracles large and crescentic. Teeth small,

with a conical cusp and a broad base.

These singular Sharks closely resemble the true Saw-Fishes (Pristidae), but

they differ in the lateral position of their gill-clefts, the presence of

rostral tentacles, and their smaller size. The few species known belong to

the genus _Pristiophorus_, and are confined to the Australian and Japanese

seas.

_Pristiophorus_ is represented in the Upper Cretaceous of Mount Lebanon,

and in the Miocene deposits.

SUB-ORDER 2. BATOIDEI.

Body generally discoidal or rhombic in shape, the axial portion being

formed by the flattened head and trunk, and the lateral portions by the

enormously expanded pectoral fins, which {458}are usually confluent with

the sides of the head. Tail slender, sharply marked off from the trunk, to

which it usually appears as a mere appendage. Dorsal fins, when present, on

the tail. Anal fin absent. Branchial clefts ventral in position. Spiracles

large, usually crescentic. Vertebrae tectospondylic.

For the most part the Batoidei are sluggish ground-Fishes, slowly moving

over the sea-bottom by the gentle undulatory vibrations of the margins of

their huge pectoral fins, the tail being of little use in locomotion. They

feed principally on Crustacea, Molluscs, and the smaller Teleosts. As with

other Fishes of similar habits, the coloration of the dorsal surface

harmonises with that of the sea-bottom, while the ventral surface is either

deficient in pigment or white. The majority of them are coast Fishes,

rarely descending to a greater depth than 500 fathoms, but some are

pelagic. The Batoidei are a relatively modern race, first appearing towards

the middle of the Mesozoic period, and evidently representing an assemblage

of specialised Elasmobranchs adapted for a bottom-living existence. As

remarked by Smith Woodward, the three families, Rhinobatidae, Raiidae, and

Trygonidae, are not so clearly differentiated before the close of the

Cretaceous period as they subsequently become.[541]

The first two families, the Pristidae and the Rhinobatidae, are interesting

connecting-links between such Selachii as the Rhinidae and the

Pristiophoridae and the more specialised Batoidei like the Skates, Rays,

and Trygons. While they agree with the latter in the ventral position of

the gill-clefts, the absence of an anal fin, and the caudal position of the

dorsal fins, the body still retains an elongated and somewhat Shark-like

shape, and shades off imperceptibly into a powerful swimming tail, and in

the Pristidae at all events the pectoral fins are of moderate size and free

from any fusion with the sides of the head. It must be admitted that the

institution of the two sub-orders introduces a somewhat arbitrary

distinction between certain families of Plagiostomes which has little to

recommend it except custom and some measure of convenience. The two series

of Fishes shade almost imperceptibly into one another, and the importance

of the ventral position of the gill-clefts has probably been overestimated.

Primitively, the gill-clefts are lateral, and lie wholly in front of the

pectoral fins, a position which is retained in many {459}Selachii. In

others, however, the hinder gill-clefts tend to extend backwards above the

base of the pectoral fins, while in some the clefts assume a more ventral

position, and extend beneath the pectoral fin; hence, even within the

limits of the Selachii the position of the gill-clefts varies to the extent

that these structures may be lateral, or they may tend to become either

dorsal or ventral.[542] On the score of convenience the customary usage is

adopted here.

FAM. 1. PRISTIDAE (True Saw-Fishes).—Although somewhat depressed, the body

is still elongate and Shark-like, with a well-developed tail terminating in

a heterocercal caudal fin. Dorsal fins large, the first opposite the pelvic

fins. Head and skull prolonged into a long flattened rostrum, the lateral

margins of which are armed with a series of strong tooth-like denticles,

firmly implanted in sockets in the calcified rostral cartilage. No rostral

tentacles. Teeth in the jaws minute and obtuse. One genus and about four or

five species are known, all inhabitants of tropical and subtropical seas.

[Illustration: FIG. 262.—The Saw-Fish (_Pristis antiquorum_). (From

Cuvier.)]

Some of the true Saw-Fishes attain a considerable size, 10 to 20 feet or

even longer, and "saws" 6 feet long and a foot in width across the base are

not uncommon. By means of powerful lateral strokes of its saw the Fish is

capable of lacerating the bodies of other animals and tearing off pieces of

flesh, which it then devours. Indian species are known to ascend rivers

beyond tidal influence, and an American species, ranging northwards to the

West Indies and the Gulf of Mexico, where it is abundant, enters the lower

Mississippi. _P. antiquorum_ occurs in the Mediterranean and the Atlantic,

but does not extend so far northward as the British coasts.

The earliest known representative of the family is the {460}extinct genus

_Sclerorhynchus_ from the Upper Chalk of Mount Lebanon, in which the

smaller size and more superficial position of the rostral "teeth," and the

absence of sockets in the rostral cartilage, prove that the "teeth"

approximate more to ordinary dermal spines in this genus than in any of the

more recent Saw-Fishes. An extinct genus _Propristis_, from the Upper

Eocene of Egypt, with non-socketed teeth, and species of the existing genus

_Pristis_ from the English Middle Eocene, are also known.

FAM. 2. RHINOBATIDAE.—Owing to the increased expansion of the pectoral fins

and the forward growth of their anterior cutaneous portions along the sides

of the head, as well as backwards along the trunk, the body now assumes a

sub-rhombic shape, and approximates to the disc of the more typical

Batoidei, but the tail with its dorsal and caudal fins is still strongly

developed, and blends imperceptibly with the trunk in front. Teeth very

obtuse. No electric organs. About five genera and twenty species are known,

distributed in most tropical and subtropical seas.

[Illustration: FIG. 263.—_Rhinobatus granulatus_. (From Müller and Henle.)]

The cosmopolitan _Rhinobatus_ is represented by species from the

Mediterranean, the Red Sea, the west coast of Africa, the Indian Ocean,

Australia and China, as well as from the Atlantic and Pacific coasts of

America, and the Galapagos. _Rhynchobatus_ ranges from the Red Sea through

the Indian Ocean to China, _Zapteryx_ occurs at San Diego and Panama, and

_Platyrhinoidis_ on the Californian coast. _Trygonorhina_ is an Australian

genus.

The family dates from the Upper Jurassic. _Rhinobatus_ is {461}represented

by complete skeletons in the Lithographic Stone of Bavaria, the Upper

Cretaceous of Mount Lebanon, and the Upper Eocene of Monte Bolca.

_Trygonorhina_ occurs in the Eocene.

FAM. 3. RAIIDAE (Skates or Rays).—The endoskeletally supported portions of

the large pectoral fins extend along the lateral margins of the trunk and

head from the pelvic fins to the snout, and are confluent therewith,

forming the lateral portions of a large rhombic disc. The tail is slender,

and sharply marked off from the trunk. Usually two small dorsal fins on the

tail. Caudal fin small or absent. No serrated spine on the tail. Caudal

electric organs are often present. Larger or smaller denticles or spines

are generally present on the skin. Oviparous. Egg-cases four-horned,

without tendrils. Four genera and from thirty to forty species. Found in

all temperate seas, a few ranging into deep water.

[Illustration: FIG. 264.—_Raia murrayi_, from Kerguelen Island. A, male; B,

female. (From Günther.)]

The great majority of the species belong to the genus _Raia_ (Fig. 264),

which chiefly inhabits temperate seas, but is more abundant in the northern

than in the southern hemisphere, and approaches nearer to the Arctic and

Antarctic regions than any other Batoidei. The colour of the upper surface

of the body is closely assimilated to that of the sandy or gravelly bottom

on {462}which they live, and thus concealed, small Fishes, Crustaceans, and

other organisms are lured unsuspectingly within the reach of the

comparatively inactive and sluggish Ray. From the ventral position of the

mouth the Ray cannot at once seize its prey, but the Fish darts over its

victim and covers it with its body, and then readily devours it. The sexes

are usually distinguished by secondary sexual characters, which take the

form of differences in size and coloration, in the dentition, and also in

the presence and position of patches or rows of specially modified dermal

spines on the dorsal surface (Fig. 264). Some of the larger species reach a

great size, the disc measuring 7 to 8 feet in width. A few species range

into deep water. _R. mamillidens_, a uniformly jet-black species, has been

obtained from a depth of 597 fathoms in the Bay of Bengal,[543] and _R.

abyssicola_ from 1588 fathoms off Queen Charlotte Islands, British

Columbia.[544] The following are British species: the Thornback (_R.

clavata_); the Spotted Ray (_R. maculata_); the Painted Ray (_R.

microcellata_); the Starry Ray (_R. radiata_); the Cuckoo or Sandy Ray (_R.

circularis_); the Skate (_R. batis_); the Flapper Skate (_R.

macrorhynchus_); the White Skate (_R. alba_); the Long-nosed Skate (_R.

oxyrhynchus_); and the Shagreen Ray (_R. fullonica_).[545] Most of the

species are of some economic value as food Fishes. _Psammobatis_, with a

circular disc, frequents the southern coasts of South America, and

_Platyrhina_ the coasts of India, China, and Japan.

The family ranges from the Upper Cretaceous, in which, as well as in

different Tertiary deposits, it is represented by species of _Raia_. An

extinct genus, _Cyclobatis_, with a circular or oval disc, occurs in the

Upper Cretaceous of Mount Lebanon.

FAM. 4. TAMIOBATIDAE.—The systematic position of the only representative of

this family, _Tamiobatis vetustus_,[546] from the Devonian or Lower

Carboniferous of Kentucky, is very uncertain, but in some respects this

unique type seems to be intermediate between the modern Sharks and the

Rays.

FAM. 5. TORPEDINIDAE (Electric Rays).—A disc is formed as in the Raiidae,

but it is sub-circular in shape rather than rhombic, and in the nature of

its endoskeletal supports it is in some respects unique. Its semicircular

anterior margin is supported {463}in the centre by a branched prenasal

rostrum, and laterally by the curiously branched preorbital cartilages,

each of which radiates outwards and forwards from a common basal

articulation with the lateral ethmoid regions of the skull. Tail relatively

short and thick, with two dorsal fins, a caudal fin, and two lateral

longitudinal folds. Skin smooth, without denticles. Mouth transverse and

ventral. A characteristic quadrangular naso-frontal lobe, with a free

hinder margin, which forms the anterior lip, is enclosed by the two nasal

organs and the oro-nasal grooves leading from them to the corresponding

angles of the mouth. A pair of large electric organs between the pectoral

fins and the head. Seven genera and about fifteen species. Inhabitants of

most warm seas.

[Illustration: FIG. 265.—The Electric Ray (_Torpedo ocellata_). Dorsal (A)

and ventral (B) views. _p.f_, Pectoral fin; _pv.f_, pelvic fin; _sp_,

spiracle.]

The well-known genus _Torpedo_ (Fig. 265) is represented by species in the

Mediterranean (_T. marmorata_, _T. narce_, _T. hebetans_), the Red Sea, and

the Atlantic and Pacific Oceans. _T. hebetans_ {464}has been taken at

several places in British waters. An American _Torpedo_ (_Tetronarce_) is

represented by species on the Atlantic and Pacific coasts. _Narcine_ is a

very widely distributed genus, species having been recorded from the East

Indies, Tasmania, China, Japan, South Africa, and the Atlantic coasts of

North and South America. _Discopyge_ is an eastern Pacific genus (Peru and

Panama). _Hypnos_ frequents the Australian seas.

The family seems to be exclusively Tertiary, and its earliest fossil

representatives are from the Upper Eocene of Monte Bolca.

FAM. 6. TRYGONIDAE (Sting- or Whip-tailed Rays).—Disc sub-rhombic, broader

than long. Pectoral fins confluent with the sides of the head, their

preaxial endoskeletal radialia meeting in front of the skull along the

lateral margins of a slender prenasal rostral cartilage. Tail usually

whip-like, terminating in a small caudal fin, and generally armed with a

sharp, serrated spine, which takes the place of a dorsal fin. Skin smooth

or spinose. A rectangular naso-frontal flap in front of the mouth. About

ten genera and fifty species. Found in nearly all tropical and subtropical

seas.

Of the more important genera, _Trygon_ (_Dasyatis_) is represented by

numerous species in the tropical parts of the Atlantic and Pacific Oceans,

including the Pacific coasts of North and South America. Two species occur

in the Mediterranean, and one of them (_T. pastinaca_), ranges from the

coasts of Norway and the British Isles through the Atlantic and Indian

Oceans to Japan. _Urogymnus_ frequents the Red Sea and the Indian Ocean.

_Urolophus_ includes a few species of small size, distributed along the

Atlantic and Pacific coasts of Central and North America, and in Australian

seas. _Pteroplatea_ comprises rather large species, and is almost

cosmopolitan in its distribution, being represented by species on the

Atlantic and Pacific coasts of North and South America, in the

Mediterranean and the Red Sea, the Indian Ocean, the Malay Archipelago, and

on the coasts of China and Japan. The caudal spines, which may be 8 to 9

inches long in some of the larger species, are capable of inflicting very

severe wounds, the danger of which is greatly increased by the apparently

poisonous cutaneous mucus introduced into the wound. As the spines become

lost they are replaced by others developed from behind. Some Trygonidae

live in fresh waters. _Trygon_ (_Dasyatis_) _sabina_ frequents the streams

and estuaries of Florida {465}as well as on the adjacent coasts, and

specimens have been obtained from Lake Munroe at some distance from salt

water.[547] _Ellipesurus_ and _Paratrygon_ are freshwater genera, found in

Colombia, Venezuela, and Guiana.

Fossil remains of undoubted Trygonidae appear to be confined to the

Tertiary period.

FAM. 7. MYLIOBATIDAE (Eagle-Rays).—Disc much broader than long, and rhombic

in shape. The huge pectoral fins are not continued to the extremity of the

snout, but cease on the sides of the head, and reappear in front of the

snout as a pair of distinct folds, the so-called cephalic fins. The head

projects above the level of the disc, and consequently the eyes and

spiracles are lateral in position. Tail long, slender, and whip-like, with

a single dorsal fin near the root, and usually one or two serrated spines

behind the fin. A rectangular naso-frontal fold is present. The dentition

consists of flat, hexagonal, pavement-like crushing teeth arranged from

before backward in arched rows in both jaws, and there is either a single

median row of large teeth, with (e.g. _Myliobatis_) or without (e.g.

_Aëtobatis_) the addition of several rows of much smaller teeth on each

side, or there are numerous rows, the teeth then decreasing in size from

the middle line laterally (e.g. _Rhinoptera_). Skin smooth. Sexes similar.

Five genera and about twenty-seven species are known; all inhabitants of

tropical and subtropical seas.

_Myliobatis_ is represented in the Mediterranean by two species, and one of

them, the almost cosmopolitan _M. aquila_ (Fig. 266), has been taken at

various points on the eastern and southern coasts of England. _Aëtobatis_

is also widely distributed in tropical seas, but is unknown in European

waters. _Rhinoptera_ has one species in the Mediterranean, while others

have been recorded from Brazil, the Atlantic and Pacific coasts of North

America, and the East Indies. The two tropical genera _Dicerobatis_ and

_Ceratoptera_ have the cephalic fins prolonged anteriorly into a pair of

horn-like appendages, which are said to be used in conveying food to the

mouth. The teeth are small, flat or tubercular, and are arranged in

numerous rows. In _Ceratoptera_ they are wanting in the upper jaw. The

Eagle-Rays feed principally on Molluscs, the shells of which they crush

with their large grinding-teeth. Some of them attain {466}an enormous size,

and are among the largest of Fishes. _Ceratoptera vampyrus_ of the West

Indies, for example, grows to a width of 20 feet, and an embryo extracted

from the oviduct of a gravid female 15 feet wide, and from 3 to 4 feet in

thickness, measured 5 feet across the disc and weighed twenty pounds.[548]

This Fish is much dreaded by the divers engaged in the pearl fisheries near

Panama, whom it is said to devour after enveloping them with its vast

wings.[549]

[Illustration: FIG. 266.—The Eagle-Ray, _Myliobatis aquila_.]

The family is exclusively Tertiary, and with the exception of an extinct

genus, _Promyliobatis_, from the Eocene of Monte Bolca, all the fossil

species belong to the existing genera _Myliobatis_, _Rhinoptera_, and

_Aëtobatis_.

ORDER V. HOLOCEPHALI

The propriety of including the Holocephali in the sub-class Elasmobranchii

is scarcely open to doubt. Like the Acanthodei {467}they seem to represent

a divergent and specialised offshoot from some primitive Elasmobranch type,

and while retaining most of the essentially distinctive features of their

ancestors, they have acquired, perhaps independently, certain characters

distinctive of the Teleostomi, combined with others peculiar to themselves.

In the few surviving genera agreement with the Elasmobranchs is to be seen

in the wholly cartilaginous condition of the endoskeleton and the complete

absence of cartilage- and membrane-bones. The vertebral column is acentrous

and ribless, and the notochord is persistent; the dorsal arcualia include

supradorsals and regularly alternating basi- and inter-dorsals. The limbs

and limb-girdles are essentially Elasmobranch. Dermal denticles are

present, either locally, or, as in some of the fossil types, in the form of

a general investment. The brain and the reproductive organs agree more

closely with the corresponding structures in the Elasmobranchs than with

those of any other Fishes, and the agreement extends to the large size of

the eggs and their enclosure in horny egg-cases. In both groups the

nostrils are connected with the mouth by oro-nasal grooves; the hyoidean

hemibranch is a true gill, and there is no air-bladder. The Holocephali

also agree with the Elasmobranchs in retaining such primitive features as

an intestinal spiral valve and a conus arteriosus. On the other hand,

indications of specialisation in the Teleostome direction are to be noticed

in the tendency to the concentration of the branchial arches towards and

beneath the skull; the reduction of the interbranchial septa to the extent

that they are no longer continuous with the skin, and the gill-filaments

project beyond their outer margins; the presence of an operculum; the

suppression of the spiracles; and the absence of a cloaca, the rectum

opening externally by an anus in front of the urino-genital apertures.

Among the more notable features evolved within the limits of the group

mention may be made of the autostylic condition of the skull, probably an

adaptive modification induced by the large size of the crushing dental

plates which have taken the place of ordinary teeth; and the singular

development of anterior and frontal "claspers."

The group is one of great antiquity. Apart from the isolated spines or

"ichthyodorulites" common in Devonian and Carboniferous strata, some of

which are probably the frontal or the {468}fin-spines of ancient

Holocephali, dental plates, closely resembling those of modern Chimaeroids

and referred to the Ptychodontidae, are probably the earliest indications

of the existence of the group. The Holocephali become more abundant in the

Mesozoic period, but of the four families usually recognised, only one, the

Chimaeridae, has survived.

FAM. 1. PTYCTODONTIDAE.—This Palaeozoic family is known only by the dental

plates, of which there is a single pair in each jaw, meeting at the

symphysis. _Ptyctodus_[550] and _Rhynchodus_ occur in the Devonian of

either Russia or Germany, and in North America, and _Palaeomylus_ only in

the Devonian of North America.

FAM. 2. SQUALORAIIDAE.—General shape of the body similar to the existing

_Harriotta_. There is a long, depressed, preoral rostrum, and in the male

the head carries a long slender frontal spine. Conical denticles are

sparsely present on the head and body. No dorsal fin-spine. Dental plates

similar to those of the living Chimaeroids, but thinner, the tritoral areas

being less well defined. The only genus is _Squaloraia_ from the English

Lias, of which nearly complete skeletons are known.[551]

FAM. 3. MYRIACANTHIDAE.[552]—Body elongate, but less depressed. A dorsal

fin-spine is present, and in the males a frontal spine. The dentition

consists of a median incisor-like tooth at the symphysis of the lower jaw,

in addition to dental plates similar to those of _Squaloraia_. There is a

symmetrical series of tuberculated dermal plates on the lateral surfaces of

the head, which probably represent groups of fused denticles. One species

(_Myriacanthus granulatus_) has its rostrum terminating in a cutaneous

flap, as in _Callorhynchus_. _Myriacanthus_, from the Lower Lias of Lyme

Regis, and _Chimaeropsis_, from the Lithographic Stone of Bavaria, are the

only two genera.

FAM. 4. CHIMAERIDAE.—Body elongate and shark-like in form, but the head is

compressed and the mouth is small. Pectoral and pelvic fins large,

especially the former, which are somewhat ventrally placed. Two dorsal

fins, the anterior over the pectorals, with a stout spine in front; and a

small anal fin. Dermal denticles restricted to the claspers, and to

localised areas {469}on the dorsal surface in young forms. Dental plates

large and thick, including a single pair in the lower jaw and two pairs,

vomerine and palatine teeth, above, which combine trenchant edges with

well-marked grinding areas. Three genera are known.

[Illustration: FIG. 267.—_Chimaera monstrosa_ (male). _m_, Mouth; _n.p_,

frontal clasper; _op_, operculum.]

In _Chimaera_ (Fig. 267) the mouth and nostrils are ventral, posterior to a

bluntly conical snout. Head surmounted in the males by a club-shaped

appendage armed with a pad of recurved denticles, the frontal clasper;

there is also an anterior clasper armed with similar denticles and

retractile into a shallow glandular pouch in front of each pelvic fin, in

addition to the ordinary clasper behind the fin. The caudal fin consists of

nearly equal-sized dorsal and ventral lobes, between which the slightly

up-tilted caudal axis is prolonged as a long tapering filament: hence the

tail appears to be nearly diphycercal. _C. monstrosa_ occurs off the coasts

of Europe from Norway to Portugal, including the Mediterranean, and also in

the neighbourhood of the Azores, as far south as the Cape of Good Hope, and

eastwards off the coast of Japan. It is the largest of the living species,

reaching a length of 3 feet. _C. affinis_ was first taken off the coast of

Portugal, and subsequently on the North American side of the Atlantic, at

depths ranging from 200 to 1200 fathoms. _C_. (_Hydrolagus_) _colliei_ is

restricted to the North Pacific, and is especially plentiful off

South-eastern Alaska, and about the wharves at Esquimalt. Unlike most other

Chimaeroids this species swims at the surface, and there is no evidence

that it is a deep-sea form. In its breeding habits, and in the mode in

which its eggs are fertilised, _Chimaera_ probably resembles the oviparous

Sharks and Dog-Fishes.

{470}[Illustration: FIG. 268.—Egg-case of a species of _Chimaera_. _a_,

Transverse section across the case at _x_, showing the lateral valvular

slits; _b_, similar section across _x′_, showing the vertical ridges. (From

Günther.)]

[Illustration: FIG. 269.—_Callorhynchus antarcticus_. Male. A, lateral

view; B, front view of the mouth; C, front view of nasal process. _a.c_,

Anterior clasper; _b.a_, external branchial aperture; _f.c_, frontal

clasper; _p.c_, posterior clasper. (From a specimen in the Cambridge

Museum.)]

The eggs appear to be deposited on the sea-bottom in deep water, but they

are very rarely obtained. An egg-case dredged up off the south-west coast

of Ireland, at a depth of 315 fathoms, and about 6½ inches in length, is

shown in Fig. 268.[553] It consisted of a broad, somewhat oval, flattened

portion which contained the egg, and terminated at one end in a truncated

margin, while at the other it was produced into a long tapering styliform

process, traversed by dorsal, ventral, and lateral ridges. The cavity of

the egg-case was open in front, and also along each side, where linear,

slit-like valvular apertures freely admitted sea-water into the central

cavity. A similar egg-case from Japan, measuring 9 inches in length, had

its surface traversed by longitudinal and {471}transverse ridges, and no

doubt belonged to a Japanese _Chimaera_.[554] In neither egg-case was there

any trace of tendrils. The eggs probably lie on the sea-bottom, or, when

the cases have styliform prolongations, it is possible that they are

implanted in the ooze.

_Callorhynchus_ (Fig. 269) is distinguished by a singular prolongation of

the rostrum, which terminates in a downwardly-directed cutaneous flap,

evidently from its abundant nerve-supply an important tactile organ. A

frontal clasper is present in the male. The prolonged caudal axis is

up-tilted, and the tail is more distinctly heterocercal than in _Chimaera_.

The only species, _C. antarcticus_, is confined to the Antarctic basin and

the South Pacific. The egg-cases of _Callorhynchus_ differ considerably

from those of _Chimaera_, and so large are they that one may measure 25 cm.

in length, or nearly as long as the abdominal cavity of the Fish. Each case

is ovoid in shape, surrounded by a wide flat margin which is covered on one

side with yellow hair-like fibres, thus giving to the case a protective

resemblance to a mass of seaweed (Fig. 270). In the central part of the

case there is a pear-shaped cavity in which the egg or the embryo is

contained. From one end of this cavity a passage, guarded by a valve, leads

to the exterior, and provides for the escape of the young. While in the

egg-case the nearly ripe embryo has long external gills, and its body is

nearly sessile on a large and singularly lobed yolk-sac.

[Illustration: FIG. 270.—Egg-case of _Callorhynchus antarcticus_, laid open

to show the embryo and its lobed yolk-sac (_y.s_); _s_, dorsal spine.

(Cambridge Museum.)]

{472}[Illustration: FIG. 271.—_Harriotta raleighana_. A, lateral view; B,

ventral view of a male. (From Goode and Bean.)]

The third genus, _Harriotta_ (Fig. 271),[555] is remarkable for its

elongated, tapering, and depressed rostrum, and for the large size and

wing-like appearance of the pectoral fins. There is no frontal clasper, and

the ordinary claspers in the young male examined {473}were very small and

simple. The caudal filament, which is longer in older specimens than in the

younger, and is not developed at all in the youngest examples at present

known (Fig. 272, A), is not uptilted, although the lower lobe of the caudal

fin is much larger than the upper. Young forms have a double row of stout

spine-like denticles in front of the second dorsal fin, and also in the

interval between the latter and the upper caudal lobe. Similar denticles

are also present on the upper surface of the head between the orbits (Fig.

272). _H. raleighana_ is found in the North Atlantic. Individuals varying

in length from 4 to 25 inches have been taken at depths ranging from 707 to

1081 fathoms. A species of _Harriotta_ has also been recorded as occurring

in Japanese waters.[556]

[Illustration: FIG. 272.—Young example of _Harriotta raleighana_, 4 inches

in length. A, side view; B, dorsal view. (From Goode and Bean.)]

With the probable exception of _Chimaera colliei_ the surviving Holocephali

are denizens of deep water; hence their comparative rarity and our almost

complete ignorance of their habits. Young forms of _C. monstrosa_, 1½ to 5

inches in length, have been dredged in the Färoe Channel at depths from 505

to 555 fathoms;[557] and the youngest specimen of _Harriotta_ was obtained

from 991 fathoms. Egg-cases are rarely obtained, and then only from

considerable depths. It is therefore reasonable to {474}infer that these

Fishes breed in deep water. As might be expected, little is known of the

embryology of any of the Holocephali, but that little adds further proof of

the Elasmobranch relationship of the group. The segmentation of the egg of

_Chimaera_ and the overgrowth of the yolk by a circular blastoderm are

essentially as in Elasmobranchs. The early embryos are said to be

shark-like, and to possess both spiracles and "external gills," and the

primary upper jaw is less completely confluent with the skull than in the

adult. It is also said that the palatine dental plates are represented at

an early stage by series of small, more or less conical elements, which,

outwardly at least, resemble the rudiments of the grinding teeth of the

Cestraciont Sharks.[558]

The Chimaeridae first appear in the Lower Oolites, and attain their maximum

development in the Cretaceous and the Eocene.[559] _Ganodus_ is an Oolitic

genus. _Ischyodus_ ranges from the Lower Oolites to the Lower Cretaceous.

_Edaphodon_ is Cretaceous and Eocene, extending, however, into the Miocene,

and _Elasmodus_ ranges from the Upper Cretaceous into the Eocene. Teeth of

the existing genus _Callorhynchus_ occur in the Cretaceous of New Zealand,

and of _Chimaera_ in the Upper Tertiary of Europe and Java. The fossil

Holocephali afford little evidence of the origin of the group from more

typical or more primitive Elasmobranchs. So far as their structure is

known, they all possess the essentially distinctive features of their

modern representatives, and offer little evidence of transitional forms.

The surviving Chimaeroids seem to have acquired a more specialised

dentition, but in other respects they are either more primitive, or

possibly somewhat degenerate.

{475}CHAPTER XVIII

TELEOSTOMI: GENERAL CHARACTERS—CROSSOPTERYGII—CHONDROSTEI—HOLOSTEI

SUB-CLASS II. TELEOSTOMI.