various sections of the alimentary canal is often developed into outgrowths
which are more or less linear.[242] In the oesophagus these may be
papilliform, as in _Box_ and _Caesio_; obtuse in _Acipenser_, hard and
almost spine-like in species of _Rhombus_; or in the form of pyramidal
retroverted processes with jagged or fringed edges, as in the Spiny
Dog-Fish (_Acanthias vulgaris_). In the Basking Shark (_Selache_) similar
processes are present, which, near the stomach, become unusually long and
branched, so that the entrance to that cavity is surrounded by a series of
backwardly-directed arborescent tufts. Peculiar papillose or tag-like
processes of the mucous membrane are frequently present on the spiral valve
of Elasmobranchs, in the intestine of such Teleosts as _Balistes_, _Mugil_
and some Pleuronectidae, and also in the rectum of _Rhombus maximus_.
Of all the outgrowths from the mucous membrane of the alimentary canal the
so-called "spiral valve" of the Cyclostomata, Elasmobranchs, Holocephali,
Chondrostei, Crossopterygii, Amiidae, Lepidosteidae and Dipnoi is the most
characteristic. The first appearance of this structure was probably in the
form of a straight longitudinal fold or ridge projecting into the cavity of
the intestine, similar, perhaps, to the typhlosole of many Invertebrata.
This primitive condition is not retained in any existing Fishes, although
it may be closely approached in the larval Cyclostome (_Ammocoetes_), and
is perhaps also indicated in the straight anterior portion of the spiral
valve of _Polypterus_. Absent altogether in the Myxinoids, the valve is
represented in its simplest condition, as in certain other Cyclostomata
(e.g. _Petromyzon_), by a ridge of mucous membrane which commences
anteriorly on the dorsal side, and, after describing a partial spiral as it
passes backwards, terminates posteriorly on the ventral side, the width of
the valve not exceeding half the diameter of the intestine. This simple
type of valve is repeated in embryo Elasmobranchs, but in the adults of
these Fishes the valve becomes much more complicated, and exhibits a wide
range of structural variation. The increased complexity of the valve
{265}seems to depend on several factors, the effect of which, in different
Elasmobranchs, is best studied in a series of valves of progressively
higher differentiation.[243]
In a hypothetical simple type of valve, easily derivable from the more
primitive type of _Petromyzon_, it may be conceived that, while not
exceeding in width the semi-diameter of the intestine, the valve becomes
disposed in several complete and more or less closely approximated spiral
turns, the free edge of the valve being on the same level as its attached
margin, and leaving an open axial canal along the centre of the gut. The
nearest approach to this hypothetical type, which has been compared, not
inaptly, to _un escalier tournant sans noyau_, is perhaps to be found in
the Thresher-Shark (_Alopecias vulpes_).
The structure of the more complicated spiral valves of other Elasmobranchs
are well illustrated within the limits of the single genus _Raia_.
In one specimen of _Raia_ sp. (Fig. 159, A) the last four coils of the
valve are similar to those of the hypothetical type, but the more anterior
ones, owing to the greater width of the valve, which here exceeds the
semi-diameter of the intestine, have their free margins deflected
downwards, while that portion of the valve which forms the first half turn
is coiled inwards upon itself, so as to form a hollow cone, open dorsally,
and having its apex directed forwards. In other examples a further
modification is introduced by the increasing width of the valve, which now,
throughout its whole length, equals the semi-diameter of the intestine; and
by the formation of an axial columella by the thickened free edge of the
valve, which is traversed by a central band of unstriped muscle, as well as
by the intra-intestinal artery and vein, and takes the place of the central
canal of the preceding types. The valve is, however, still regular, and its
free margin remains on the same level as the corresponding portion of the
attached edge. In other specimens, again, additional complications are
introduced by a still further increase in the width of the valve, which now
exceeds, often considerably, the semi-diameter of the intestine, and the
consequent deflection of the free edge of the valve either forwards or
backwards (C and D). As shown in C the valve, in consequence of the
backward deflection of its free margin, presents the appearance of a nest
of {266}imperfect truncated cones with their apices directed backwards, the
successive cones adhering so closely to one another that they combine to
form a central conical chamber with a spirally disposed cavity winding
round it. In D, on the contrary, the free edge of the valve is deflected
forwards, so that, as in C, a nest of cones is formed, but the apices of
the successive cones are directed forwards instead of backwards.
Notwithstanding these variations in the structure of the valve as a whole,
the first coil or half coil nearly always resembles that described in A.
[Illustration: FIG. 159.—Examples of various types of the spiral valve in
Elasmobranchs. A, B, C, and D in specimens of _Raia_ spp.; E, in _Sphyrna
malleus_. A, B, and D represents longitudinal sections of the intestine,
the ventral portion of the valve being removed. In C successive portions of
the ventral wall of the intestine have been cut out. In E the intestine has
been opened along the mid-ventral line and its wall reflected to the right
and left; the ventral portion of each coil of the "scroll" valve has been
removed. In most of the figures the pylorus is shown in the upper part, and
the "rectal" gland in the lower. (From T. Jeffery Parker.)]
It is obvious that the structure of the valve varies considerably within
the limits of the genus, and it may be added that various intermediate
types of structure occur between A and B, {267}A and C, and A and D. The
individual variations are perhaps even more remarkable, and appear to be
quite independent of age and sex. By way of example it may be mentioned
that valves approximating to one or other of those represented by C and D
occur in different individuals of _Raia maculata_ of the same sex and
similar in size, even in young specimens not more than three inches in
length.
As regards other Elasmobranchs, the common Dog-Fish (_Scyllium
canicula_)[244] has a well-developed spiral valve disposed in twelve coils,
which structurally represents a more highly developed example of the type
D. The existence of considerable individual variation is nevertheless
indicated by the fact that in one specimen examined the valve was
intermediate between C and D, five of the eight cones projecting forwards
and three backwards. In a specimen of _Notidanus_ sp.[245] there were as
many as twenty coils, which in disposition were intermediate between B and
C, approximating, however, more nearly to B. In a specimen of the Port
Jackson Shark (_Heterodontus_)[246] the valve had eight coils, and in
structure was also intermediate between B and C, but approached more nearly
to C. Some of the Hammer-headed Sharks (e.g. _Sphyrna malleus_)[247]
possess a type of spiral valve which differs considerably from any of those
hitherto described, and is termed a "scroll" valve (Fig. 159, E). The
attached edge of the valve pursues a straight longitudinal course, or at
any rate only describes a half turn and back again in passing from the
pyloric to the cloacal extremity of the gut. In the middle of its course
the width of the valve is about equal to two-thirds of its length, but
towards either extremity it gradually diminishes until the free and
attached margins meet. The valve thus constituted is rolled upon itself
from left to right, the successive coils being comparable to a series of
cylinders placed one inside the other, and becoming gradually larger both
in length and diameter from within outwards. A similar valve is present in
some of the Carchariidae.
In the Holocephali (e.g. _Chimaera monstrosa_)[248] the valve describes
only three and a half coils, and is further remarkable in that the attached
margin, for a considerable portion of its {268}extent, does not form a
regular spiral but describes only a slightly sinuous course. Posteriorly,
the valve is more normal, and consists of about two cones with their apices
directed forwards.
In the Dipnoi the spiral valve is well developed, and in
_Neoceratodus_[249] describes nine coils, and in _Protopterus_[250] six or
seven. The structure of the valve in the latter Dipnoid resembles that of
_Scyllium canicula_, except for the smaller number of cones.
In the more generalised Teleostomi the valve is best developed in the
Sturgeon (_Acipenser_) and in _Polypterus_. In the former[251] the valve is
restricted to the posterior half of the total length of the intestine,
often extending to within an inch of the anal aperture, and describing in
its backward course about seven or eight coils. The width of the valve is
about equal to the semi-diameter of the intestine, and the thickened free
margin forms a well-marked axial columella, round which the cavity of the
gut winds, as in the type B, except that the spiral is a more open one. In
_Polypterus_ the valve begins close to the solitary pyloric caecum, and for
some distance pursues a straight longitudinal course, but eventually forms
a few spiral coils, ceasing, however, at a considerable distance from the
anus. The evidence afforded by petrified faeces or "coprolites" proves that
certain extinct Crossopterygii (e.g. _Macropoma_, _Megalichthys_), like
their living representative, _Polypterus_, possessed a spiral valve.[252]
In _Amia_ and _Lepidosteus_[253] the valve is almost vestigial, being
restricted to the terminal portion of the intestine, and is somewhat
variable as to the precise number of its coils. In _Amia_ there are nearly
four coils, extending over 3 cm., that is less than a tenth of the total
length of the intestine, but in some specimens the coils do not exceed two
and a half or three in number. _Lepidosteus_[254] has a still shorter valve
which, in specimens of 7-10 cm. in length, may not consist of more than
three and a half coils, and in much larger specimens may be reduced to less
than two coils, a variation which suggests that a reduction takes place in
the number of coils as the fish increases in age and size. The structure of
the valve in the three last-mentioned genera resembles that described in
_Acipenser_, and in none of them does {269}the width of the valve so far
exceed the semi-diameter of the intestine as, by forward or backward
deflection, to give rise to the highly characteristic cones of
Elasmobranchs and Dipnoi.
In the more specialised Teleostomi (Teleostei) the spiral valve is wholly
wanting, except perhaps as a vestigial structure in certain Clupeoids, as,
for example, _Chirocentrus_,[255] and possibly also in some
Salmonidae.[256]
From what has been said as to the structure of the spiral valve in the
different groups of Fishes, it may be concluded that the valve most nearly
retains its primitive condition in the Cyclostomata; attains its maximum
development in the Elasmobranchs, especially in the Notidanidae, and shows
no indication of degeneration in the Dipnoi. In the Holocephali and the
lower Teleostomi, on the other hand, the valve exhibits various stages of
retrogressive modification, and in the Teleosts is either absent altogether
or persists only as a vestigial structure in a very few species.
From a physiological point of view the object of the spiral valve is to
increase the absorptive inner surface of the intestine,[257] but, from what
has been said as to the structural variability of the valve, it is obvious
that its efficacy from a functional standpoint must be equally variable.
The value of the valve as an absorptive mechanism necessarily depends on
the area of absorption-surface which it provides, as well as on the degree
of resistance which it offers to the passage of food material along the
cavity of the intestine. These factors will in turn depend on the number of
coils, on the width of the valve, and on the extent to which its free
margin is deflected in forming the series of cones, but these again are
precisely the structural features which are most liable to variation. The
total absorption area in the four types of valve characteristic of the
genus _Raia_ has been calculated, and may be expressed in square
centimetres as follows:—A, 136.64; B, 143.82; C, 254.3; and D, 276.7.[258]
Hence as regards mere absorption area a spiral valve of the type D has
twice the extent of a valve of the type A, and if, in addition, account be
taken of the retardation of the food due to the increased obstruction
offered by the columella and cones in D, it is clear that the
{270}difference in physiological value between the two types must be far
more considerable than is indicated by a comparison of their relative
superficial areas alone.
The evolution of the spiral valve was probably due to the necessity of
increasing the absorptive area of an almost straight unconvoluted
intestine, a result which in other animals is often obtained by an increase
in the length and concurrent convolution of the intestine itself. Any
attempt to correlate the variations in the degree of perfection or
imperfection of the valve considered as an absorptive mechanism with any
special variations in the nature or quality of the food is, however, a very
difficult problem, and a satisfactory explanation has yet to be found. The
difficulty, moreover, is increased by the fact that the majority of Fishes
with a spiral valve are mainly carnivorous; the Elasmobranchs, in which
this structure is at the same time most highly developed and most variable,
exclusively so. On the other hand, the term "carnivorous" covers a
multiplicity of minor differences in the nature and relative digestibility
of different forms of animal food, and it is quite possible that it is with
differences of this kind that the specific or individual variations in the
development of the spiral valve are associated. The absence of the valve in
the variously nourished Teleosts, save perhaps as a vestige in one or two,
is also difficult to account for, although it is not improbable that
compensating structural modifications exist in this group. As a rule, the
intestine is much more convoluted in these Fishes, but to an extent which
varies greatly in different species, while the characteristic pyloric caeca
and the spiral valve appear to a certain extent to be developed in inverse
proportion to one another.
THE GLANDS.
The glands associated with the alimentary canal in different Fishes are (1)
the gastric glands, (2) the liver, (3) the pancreas, (4) the pyloric
appendages, and (5) the "rectal" gland.
Oral salivary glands are wanting in all Fishes, the only secretory
structures in the mouth being numerous mucus-secreting goblet cells, which
here, as elsewhere throughout the alimentary canal, are intermixed with the
ordinary epithelial cells.
THE GASTRIC GLANDS.—The Cyclostomata and Dipnoi do not possess any
specially differentiated gastric glands, and it is {271}probable that in
these Fishes the secretion of the digestive fluids is effected by the
ordinary lining epithelium of the stomach or intestine, or both. In the
remaining groups gastric glands are generally present in the form of simple
caecal structures embedded in the submucosa and opening on the surface of
the mucous membrane into the cavity of the stomach. The glands differ in
different Fishes in the character of their lining epithelium and in the
extent to which their component cells are differentiated from the
epithelium of the stomach. There does not appear, however, to be any
distinction into "central" (pepsin-forming) and "parietal" (acid-secreting)
cells, as is the case in the higher Vertebrata. Towards the pyloric end of
the stomach the true gastric glands are often replaced by mucous glands.
There are, nevertheless, not a few Teleosts in which special gastric glands
are absent, as, for example, _Syngnathus acus_, and several species of
Cyprinidae, Labridae, and Blenniidae, etc. In at least two genera
(_Gastrosteus_ and _Cobitis_), belonging to widely different families,
gastric glands are present in certain species but absent in others. As
suggested by Edinger,[259] the absence of these glands may possibly be due
to degeneration.
It may be remarked that the formation of such digestive ferments as pepsin
and trypsin, which are associated with the stomach and pancreas
respectively, in the higher Vertebrates, is not nearly so strictly
localised in Cyclostomes and Fishes. So far from peptic digestion being
limited to the stomach, it may take place in the pharynx, stomach, and
intestine of Ammocoetes, and in some Elasmobranchs (e.g. _Scyllium_), and
in such Teleosts as the Pike, Eel, and Carp, the peptic region extends from
the stomach for some distance along the intestine, while trypsin has been
obtained from the mucous membrane of the stomach, intestine and pyloric
caeca, as well as from the pancreas.[260]
Intestinal glands analogous to the glands of Lieberkühn in the higher
Vertebrates seem to be entirely wanting in Fishes, unless represented by
the sac-like or tubular crypts which are so generally present in the
Teleostomi.
THE LIVER.—Phylogenetically the oldest gland in connexion with the
Vertebrate alimentary canal, and in size by far the {272}largest, the liver
arises as a caecal outgrowth from the embryonic mesenteron, and in this
primitive stage recapitulates a condition which is retained throughout life
in Amphioxus. By the subsequent division and branching of this outgrowth
the massive compound tubular gland of the adult Fish is eventually formed.
The liver of Fishes (Figs. 153, 154) is very variable in size, shape,
colour, and degree of lobulation. Anteriorly, it is usually moulded to the
posterior face of the transverse septum between the pericardial and
abdominal portions of the coelom, and from thence extends backwards in the
abdominal cavity to a varying distance, in some Sharks as far as the
cloaca. Externally, the gland is invested by the peritoneum, which extends
on to it from the pericardial septum and forms a suspensory fold, and also
from the oesophagus and stomach. The shape of the liver usually bears some
relation to that of the body, being, for example, longest in the Eels and
broadest in the Rays. In the great majority of Fishes the liver is bilobed,
consisting of two sub-equal lateral lobes, disposed longitudinally and
confluent anteriorly for a portion of their extent. From this normal type
there are a few minor variations.[261] In _Petromyzon_, _Lepidosteus_ (Fig.
155, B), and a few Teleosts (_e.g._ the Gymnodontes, Lophobranchii, and
some Salmonidae) the liver is unilobed. In the Myxinoids and in the Dipnoi
(e.g. _Protopterus_), the organ is bilobed, but the small anterior lobe
lies immediately in front of the much larger posterior lobe, with the
gall-bladder between the two (Fig. 155, A). In some Teleosts (e.g.
_Scomber_), the liver is trilobed. A gall-bladder is invariably present in
either the larval or adult Cyclostomata, in the Chrondrostei, Holostei,
Crossopterygii and Dipnoi, and generally also in Elasmobranchs and
Teleosts. In the Elasmobranchs it is rarely entirely wanting, as in
_Sphyrna_ and _Pristis_, and in the Teleosts in some of the Gurnards
(_Trigla_). The gall-bladder and bile-duct of _Petromyzon fluviatilis_
atrophy after the metamorphosis which follows the larval _Ammocoetes_
stage, but in _Petromyzon marinus_ the duct, although usually absent, is
sometimes retained. In the Ammocoetes the epithelium lining the
gall-bladder is ciliated. In some Fishes, as, for example, in many
Elasmobranchs, the gall-bladder is more or less completely embedded in the
substance of the liver; in others, as in most Teleostomi, the organ is
quite distinct from the gland (Fig. 154).
{273}A simple arrangement of the ducts from the liver and gall-bladder is
that found in the common Dog-Fish (_Scyllium canicula_). In this
Elasmobranch a cystic duct leaves the gall-bladder, and, after receiving
several hepatic ducts from the lobes of the liver, becomes the bile-duct
and opens into the commencement of the intestine. In the Myxinoids and in
the Dipnoi (e.g. _Protopterus_), there are but two hepatic ducts, one from
each lobe of the liver; these unite and then meet the cystic duct to form
the bile-duct (Fig. 155, A). The number of hepatic ducts may, however, be
considerably increased, as, for example, in the Siluroid _Amiurus_,[262]
where 8-10 separate ducts join the cystic duct. In a few instances one of
the hepatic ducts opens directly into the intestine, independently of that
which unites with the cystic duct in forming the bile-duct. In the Dipnoi
(e.g. _Protopterus_),[263] and in some Teleostomi (e.g.
_Lepidosteus_),[264] the bile-duct receives the duct from the pancreas
before opening into the intestine.
THE PANCREAS.—In the Cyclostomes (e.g. _Petromyzon_, _Bdellostoma_,
_Myxine_) a rudimentary pancreas is apparently present, but the evidence as
to its identity is not wholly conclusive. A well-developed pancreas occurs
in Elasmobranchs, in at least one of the Dipnoi, and probably in most
Teleostomi.[265]
In Elasmobranchs the pancreas is a compact structure, uni- or bi-lobed, and
entirely distinct from the liver. In _Scyllium canicula_ (Fig. 153), the
bilobed gland lies in the angle between the distal limb of the stomach and
the adjacent portion of the intestine, and from the smaller of its two
lobes the duct issues to pass to its intestinal aperture near the
commencement of the spiral valve. In most of the Teleostomi in which its
existence has hitherto been recorded, the pancreas is a singularly diffuse
gland; and usually a considerable portion, or even the whole of it, is
embedded in the substance of the liver, its lobules accompanying the
ramifications of the hepatic artery and duct, and the portal vein. The
pancreatic duct usually opens into the intestine near the aperture of the
bile duct (e.g. _Amiurus_); sometimes the two ducts open on the apex of a
common papilla (e.g. _Acipenser_ and _Amia_), or by their union form a
common {274}duct (e.g. _Lepidosteus_). Among the Dipnoi a well-developed
pancreas is present in _Protopterus_,[266] embedded in the wall of the
stomach and intestine, internal to the peritoneal investment of these
organs, and extending even into the first fold of the spiral valve. The
gland is traversed by fine ductules which unite together and open into the
bile-duct just before the latter enters the intestine. In the remaining
Dipnoi the existence of a pancreas has yet to be ascertained.
Developmentally, the pancreas resembles the liver, and, histologically, is
very similar to that of the higher Vertebrates, consisting of terminal
glandular alveoli continuous with intermediary tubular portions, and
eventually with the finer ductules, which, by their union, form the main
efferent duct.
THE PYLORIC CAECA.—These structures are caecal outgrowths from the
intestine, and are situated close to the pyloric extremity of the stomach
and the intestinal apertures of the bile and pancreatic ducts. Wholly
wanting in the Cyclostomata and Dipnoi, and, unless represented by a pair
of caeca opening into the long, tubular, non-valvate anterior portion of
the intestine in the Greenland Shark (_Laemargus borealis_),[267] in the
Elasmobranchs also, they are very generally present in the Teleostomi,
although extremely variable both in number and arrangement in different
families. In _Amia_ there is no trace of pyloric caeca. _Polypterus_ has a
single short caecum with a thick muscular wall. In _Acipenser_, _Polyodon_,
and _Lepidosteus_, on the contrary, pyloric caeca are unusually well
developed. In _Acipenser_ the caeca are not only numerous, but are so
connected together by connective tissue and blood-vessels, and so invested
externally by the peritoneum, as to form a large, compact, gland-like mass,
communicating with the intestine by a single wide duct. In _Polyodon_ the
organ is essentially similar, but is lobed externally. In _Lepidosteus_
(Fig. 155, B, _py.c_), the caeca are also very numerous, but relatively
short, and, although united into a compact mass, open by four pit-like
orifices into the intestinal cavity. In Teleosts the caeca are subject to
extraordinary variations in number, size, and arrangement.[268] In some
families, and even in groups of higher taxonomic value, they are entirely
absent, as is the case with the {275}Siluridae, Esocidae, Cyprinodontidae,
Labridae, Plectognathi, and Lophobranchii. The "Sand-eel" (_Ammodytes_) has
but a single caecum; the Turbot (_Rhombus maximus_) two, and other
Pleuronectidae three to five; and the Perch (_Perca_), three (Fig. 160,
_py.c_).
In other Teleosts, on the contrary, these structures are much more
numerous. In _Labrus labrax_ there are about 60, in the Whiting (_Gadus
merlangus_) 120, while in the Mackerel (_Scomber scombrus_) there are no
fewer than 191. If few in number the caeca open separately into the
intestine, but when numerous, more or fewer of them may unite to form a
smaller number of efferent ducts, as in the Whiting, where four such ducts
are formed. In some instances, as in the Tunny (_Thunnus_), the union of
the caeca by connective tissue leads to the formation of a compact mass. As
regards their arrangement, the caeca may either be disposed in a whorl
round the intestine, as in the Whiting, or in a linear series, as in the
Salmon (_Salmo_) and in some of the Clupeidae.
The mucous membrane lining the anterior pyloric caeca is often developed
into a network of ridges, limiting crypt-like or tubular depressions; and
not infrequently the epithelium is ciliated.
[Illustration: FIG. 160.—The alimentary canal of a Perch (_Perca_). _an_,
Anus; _in_, intestine; _oes_, oesophagus; _py_, pylorus; _py.c_, pyloric
caeca; _st_, stomach. (After Wiedersheim.)]
The precise function of these organs, whether digestive or absorptive, is
still uncertain.[269] That they may be digestive is suggested by the
presence of certain amylolytic and proteolytic enzymes, but this obvious
conclusion is to some extent vitiated by the close proximity of these
organs to the stomach, and more especially to the intestinal orifice of the
pancreatic duct. It is by no means improbable, however, that the caeca are
both digestive and absorptive organs. An attempt has been made to show that
the pyloric caeca and the spiral valve vary inversely as regards the extent
of their development in different groups of {276}Fishes.[270] To some
extent the reciprocal variation of these structures supports this view, but
it is also evident that there are obvious objections to its unqualified
acceptance. Thus, in some Teleostomi (e.g. _Acipenser_, _Polyodon_),
exceptionally well-developed and numerous caeca and a spiral valve are both
present. _Amia_ with an almost vestigial spiral valve has no trace of
pyloric caeca, and in Teleosts the absence of a spiral valve is associated
with the complete suppression of the caeca in many large and important
groups.
THE RECTAL GLAND.—The "rectal" gland, or appendix digitiformis, is a small
organ of unknown function with complex glandular walls, and a central duct
opening dorsally into the terminal portion of the intestine.[271] The organ
is generally present in Elasmobranchs (Fig. 153, _rct.gl_), in which group
the intestinal orifice of its duct may either be close to the termination
of the spiral valve, or, as in _Chlamydoselachus_,[272] near the cloacal
outlet of the gut. An apparent representative of the gland, the "caecum
cloacae," is also present in the Dipnoi,[273] but communicates directly
with the cloaca (Fig. 155, A, _cl.c_). The "rectal" gland is perhaps
homologous with the intestinal caecum which is to be found in some Teleosts
(e.g. _Box vulgaris_), and possibly also with the "caecum" (caecum coli),
and its vermiform appendix in the higher Vertebrata.[274] The caecum
cloacae, on the contrary, is morphologically a urogenital sinus, formed as
a dilatation of the fused hinder portions of the mesonephric ducts, and
probably comparable with the sperm sacs of male Elasmobranchs, and also
with the urinary bladder of Teleostomes.[275]
{277}CHAPTER X
THE RESPIRATORY ORGANS
The principal respiratory organs consist of a series of pairs of branchial
clefts in the form of perforations in the side walls of the throat, which
place the pharynx in free communication with the exterior. The first and
most anterior of these clefts, the mandibulo-hyoid cleft or "spiracle," is
situated between the mandibular and hyoid arches; the second, the
hyo-branchial or hyoidean cleft, between the hyoid arch and the first
branchial arch; and the remaining clefts between the succeeding branchial
arches. On the anterior and posterior walls of more or fewer of the clefts
highly vascular plate-like, or variously shaped filamentous outgrowths of
their lining membrane are developed, which subserve the purpose of exposing
the blood to the influence of the oxygen-containing water, and are termed
branchial lamellae or "gills." In addition to their usual respiratory
organs, the gills, a few Fishes utilise the air-bladder either as a
functional lung or as an oxygen reservoir, and in others accessory
breathing organs of various kinds are developed.
The arrangement of the branchial clefts and the gills may be conveniently
studied first in the Elasmobranchs. Excluding the spiracles, there are
usually in this group (Fig. 161, A), five pairs of branchial clefts, but in
certain primitive members of the group the number may be larger. Thus, in
_Notidanus griseus_ (_Hexanchus_) and in _Chlamydoselachus_ there are six,
and in _Notidanus cinereus_ (_Heptanchus_), seven clefts. The pharyngeal
apertures of the clefts are relatively wide, but their external openings,
which are freely exposed on the lateral surface of the head between the eye
and the pectoral fin, are usually narrow and slit-like.
{278}[Illustration: FIG. 161.—A, Horizontal section through the head of an
Elasmobranch; B, similar section of a Teleost (diagrammatic). _b.c_,
Branchial cavity; _b.l_, branchial lamellae; _c_, coelom; _e.b.a_, external
branchial aperture; _hy.a_, hyoid arch; _hy.c_, hyo-branchial cleft; _l.s_,
interbranchial septum; _n_, nasal organ; _oes_, oesophagus; _op_,
operculum; _p.q_, palato-quadrate cartilage; _Ph_, pharynx; _sp_, spiracle;
_s.ps_, spiracular pseudobranch; 1-5, 1st to 5th branchial arches. (From
Boas, slightly altered.)]
The successive clefts are separated from one another by a series of
inter-branchial septa, each of which consists of the lining membrane of two
contiguous clefts and a median fibrous sheet; it is further strengthened on
its pharyngeal margin by a branchial arch, and more externally by the
fringe of cartilaginous rods (branchial rays) with which the outer convex
edge of each arch is provided. The anterior and posterior walls of each
septum are produced into a number of outwardly-radiating vascular plates or
folds (branchial lamellae or "gills"), which by their free edges project
into the cavity of the cleft (Fig. 161, A). Although slightly free at their
outer extremities, the lamellae do not extend so far as the external margin
of the septum to which they are attached (Fig. 164, B). Each series of
lamellae is termed a "hemibranch," and, from what has been said, it is
obvious that each inter-branchial septum and its supporting branchial arch
carry two hemibranchs, an anterior and a posterior, the two forming a
complete biserial gill or "holobranch." The hyoid arch, however, has only a
single hemibranch, viz. that pertaining to the anterior wall of the
hyo-branchial cleft, and as the fifth or last cleft has a hemibranch only
on its anterior wall, the fifth arch is {279}gill-less.[276] The spiracle
is a vestigial cleft. At an early stage of embryonic growth it differs but
little from its fellows, but subsequently degenerating it is represented in
the adult by a tubular passage between the oral cavity and the exterior,
which, however, is often complicated by the development of caecal
outgrowths.[277] The anterior wall of the spiracle often retains a rudiment
of a hemibranch in the shape of more or fewer vascular lamellae, which, as
they are supplied with arterial blood, and not with venous blood like the
ordinary gills, are said to form a mandibular or spiracular "pseudobranch."
The spiracle varies greatly in size in different families, being largest in
the Trygons and Torpedos, and very small, or even absent in the Lamnidae.
Its pseudobranch is best developed in the Notidanidae, where it has the
essential structure of a true hemibranch, and, as in other Elasmobranchs,
but to a greater extent, probably aids in the additional aeration of the
blood which is distributed to the eye and brain. The characteristic
opercular covering of the external apertures of the gill-clefts in the
Teleostomi and Dipnoi is wanting in Elasmobranchs. It is interesting to
note, however, that in _Chlamydoselachus_[278] curious frilled cutaneous
folds are developed as extensions of the outer edges of the inter-branchial
septa, as well as of the hyoid region, and, like a series of incipient
opercula, project backwards over the successive branchial clefts (Fig.
252).
While in many respects more primitive than in Elasmobranchs the branchial
system of the Cyclostomata presents certain special and peculiar features.
The branchial clefts assume the form of oval, antero-posteriorly flattened
pouches or sacs, varying, however, in number, and in their mode of
communicating with the exterior, in different genera. In the Lamprey
(_Petromyzon_) there are seven pairs of obliquely-disposed gill-sacs
opening externally by small rounded orifices, and by similar apertures, not
directly into the pharynx, but into a branchial canal (Fig. 162, _r.t_),
which underlies the oesophagus, and, while ending blindly behind the last
pair of sacs, communicating in {280}front with the oral cavity.[279] The
first of the series of gill-sacs corresponds to the hyo-branchial or
hyoidean cleft of Elasmobranchs and other Fishes. Spiracles are absent in
the adult, but in the embryo are represented by pouch-like outgrowths of
the hypoblast of the oral cavity, which subsequently undergo singular
changes.[280] Thus, the outgrowths become converted into the lateral halves
of a complete ciliated circum-oral groove, which is retained even in the
Ammocoetes stage, and recalls the ciliated peripharyngeal ring of
Ascidians. Another archaic feature is also to be noted in the continuity of
the groove with a ciliated mid-dorsal pharyngeal ridge, which has been
compared to the "dorsal lamina" of Ascidians, and to the equally
characteristic hyperbranchial groove of _Amphioxus_.[281] Ventrally also,
the lateral halves of the groove unite to form a single groove, which,
after receiving the median aperture of the thyroid rudiment,[282] is
continued backwards in the mid-ventral line of the pharyngeal wall as far
as the last branchial arch. No trace of these ciliated structures is,
however, to be met with in the adult.
[Illustration: FIG. 162.—_Petromyzon marinus_. Transverse section through
the branchial region (semi-diagrammatic). _br.m_, Branchial membrane;
_d.ao_, dorsal aorta; _d.c_, dorsal cartilage of the branchial basket;
_d.m_, dorsal muscles; _e.a_, external aperture of a gill-sac; _f.t_,
fibrous tissue enclosing neural canal; _h_, _i_, lateral longitudinal
cartilages of the branchial basket; _i.a_, internal aperture of a gill-sac;
_i.ju_, inferior jugular vein; _ju_, jugular vein (anterior cardinal);
_my_, spinal cord; _nc_, notochord; _n.ca_, neural canal; _n.p_, neural
process; _oes_, oesophagus; _p.br_, peri-branchial lymph sinus; _r.m.t_,
retractor muscle of the tongue; _r.t_, respiratory tube or branchial canal;
_s_, circum-oesophageal lymph sinus; _v.ao_, ventral aorta; _v.c_, ventral
cartilage of branchial basket; _v.m_, ventral muscles. (From T. J.
Parker.)]
The branchial lamellae are represented by a series of vascular horizontal
and parallel ridges radiating outwards along the roof, floor, and lateral
walls of each gill-sac, and invested by an {281}epithelium which is
partially ciliated. The inter-branchial septa are much thicker than in
Elasmobranchs, and include not only the walls of adjacent sacs and the
branchial muscles, but also contain cavernous peribranchial lymph-sinuses.
The cartilaginous branchial skeleton is situated wholly external to the
gill-sacs, the so-called branchial arches lying between the external
apertures of the sacs, and directly beneath the superficial skin, or, in
other words, on the outer margins of the inter-branchial septa, and not on
the inner, as is invariably the case with the branchial arches of Fishes.
[Illustration: FIG. 163—Dissection of _Myxine glutinosa_ from the left
side. _au.c_, Auditory capsule; _br.ap_, left branchial aperture; _br.b_,
rudiment of branchial basket; _br.s.1_, first gill-sac; _c.br.t_, common
branchial tube; _cn.c_, cornual cartilage; _gul_, gullet; _ht_, heart;
_lg.m_, lingual muscles; _m.v.c_, median ventral cartilage; _na.t_, nasal
tube; _nch_, notochord; _n.t_, neural tube; _oes.ct.d_,
oesophageo-cutaneous duct; _p.l.c_, posterior lateral cartilage; _sb.oc.a_,
subocular arch; _sp.c_, spinal cord; _st.p_, styloid process. (After W. K.
Parker, from Parker and Haswell's _Zoology_.)]
In the Hag-Fish (_Myxine_) (Fig. 163), there are usually six, very rarely
seven, pairs of gill-sacs, all of which open directly into the pharynx, and
not into a branchial canal as in the Lampreys. On the other hand, _Myxine_
is unique in having the outer extremities of its gill-sacs produced into a
corresponding number of tubular canals which, after a longer or shorter
course obliquely backwards and outwards, unite to form on each side a
ventrally-situated external aperture (Fig. 163). In the same genus a short
canal, or oesophageo-cutaneous duct, passes from the pharynx {282}behind
the last gill-sac of the left side, and opens externally with the common
external branchial aperture of that side.
In _Bdellostoma_ there are usually six or seven pairs of gill-sacs, but
some species have ten or even fourteen pairs.[283] They agree with those of
the Lamprey in having independent external apertures, but resemble the
corresponding organs in _Myxine_ in opening directly into the pharynx. An
oesophageo-cutaneous duct is also present.[284]
In the Holocephali there are but four branchial clefts, the fifth cleft
being closed. Spiracles are absent in the adult, although present in the
young of _Chimaera_. The branchial lamellae resemble those of
Elasmobranchs, but the inter-branchial septa are somewhat shorter, so that
the lamellae project slightly beyond their outer margins (Fig. 164, B). A
hyoidean hemibranch is present. A noteworthy feature is the development of
a cutaneous fold from the outer surface of the hyoid arch, which grows
backwards over the gill-clefts, and, uniting above and below with the
body-wall, terminates in a free posterior margin, just behind the last
gill-cleft. By the growth of this opercular fold the gills become enclosed
in a spacious branchial cavity, and the clefts communicate with the
exterior through a slit-like opening between the free margin of the fold
and the body-wall.
The reduction in the extent of the inter-branchial septa which is initiated
in the Holocephali is carried to a still further extent in the Teleostomi.
Commencing with the Chondrostei, and passing thence to the more specialised
Teleostei, the septa become gradually reduced in length, and the branchial
lamellae project freely beyond their outer margins to an increasing extent.
This modification, least marked in _Acipenser_ (Fig. 164, C) and
_Polyodon_, attains its maximum in the Teleosts (Fig. 164, D and E), where
the branchial lamellae take the form of a double series of free filaments
disposed along the convex outer margin of each branchial arch, and attached
by their bases only to the reduced and inconspicuous septa. As a general
rule each of the first four arches supports two hemibranchs,[285] forming a
{283}biserial gill or holobranch. In shape the branchial filaments are
usually somewhat triangular, and consist of an axial supporting cartilage
or bone, invested superficially by a highly vascular mucous membrane. As in
most of the preceding groups the fifth branchial arch is gill-less. All
Teleostomi possess a well-developed movable operculum, supported by a more
or less complete series of opercular bones, with or without the addition of
branchiostegal rays (Fig. 161, B). The size of the external branchial
aperture varies considerably. Usually the hinder and lower margins of the
operculum are free, and then the aperture is spacious. Not infrequently,
however, the more or less extensive fusion of the ventral and hinder edges
of the operculum with the body-wall reduces the aperture to a narrow slit,
as in the Eels and some Siluridae, or to a small upwardly directed pore, as
in the "Sea-Horse" (_Hippocampus_). In the Symbranchidae the branchial
apertures close dorsally, but fuse ventrally, leaving a single median
orifice on the under side of the throat.
[Illustration: FIG. 164.—Transverse sections of branchial arches in
different Fishes. A, Elasmobranch; B, _Chimaera_; C, _Acipenser_; D and E,
Teleosts. _b.a_, Branchial arch; _g.l_, gill-lamellae; _gr_, gill-raker;
_i.s_, inter-branchial septum. (From Boas.)]
Open spiracles are wanting in most adult Teleostomi, but are, nevertheless,
retained in the Crossopterygii (_Polypterus_), and in the Chondrostei
(_Acipenser_ and _Polyodon_). They have been observed, however, in the
embryos of some Teleosts, as in the Salmon (_Salmo_),[286] and even in the
adults of _Amia_,[287] _Lepidosteus_, {284}and a few Teleosts[288] are
represented by pouch-like recesses of the oral cavity. A few vestigial
branchial lamellae may be developed on the anterior wall of each spiracle
in _Acipenser_ and _Polyodon_, but are wanting in _Polypterus_, and, as in
Elasmobranchs, represent a mandibular or spiracular pseudobranch.
The structure usually regarded as a hyoidean hemibranch in the Teleostomi
differs greatly in its development in different members of the group. In
_Acipenser_ it is undoubtedly the hemibranch of the hyoid arch and is a
true gill, receiving venous blood from the ventral aorta and returning
arterial blood to the dorsal aorta, as in Elasmobranchs. In _Polyodon_ and
in _Polypterus_ the hemibranch is suppressed. _Lepidosteus_,[289] on the
other hand, has two series of lamellae on the inner surface of the
operculum, a dorsal and a ventral series meeting at an angle (Fig. 197).
The ventral lamellae are supplied with venous blood, the dorsal with
arterial,[290] so that while the former retain their primitive character as
a functional hyoidean hemibranch, the latter is a pseudobranch. It is
interesting to note, however, that the development of this pseudobranch and
its blood-vessels proves that it does not represent any portion of a true
hyoidean hemibranch, but is really a spiracular pseudobranch.[291] In most
other Teleostomi a degenerate hemibranch occupies a similar position. In
_Amia_[292] it is very feebly developed, and is lodged in a canal
communicating with the branchial cavity by a small aperture, and situated
directly anterior to the dorsal end of the first branchial arch. Its blood
supply is arterial, and the organ is therefore a pseudobranch. In Teleosts
the hemibranch is invariably a pseudobranch; nevertheless, its primitive
condition as a gill is indicated either by its structure or by its
embryonic history. In some genera the pseudobranch consists of short free
lamellae, as in some Pleuronectidae; or it is partly free and partly
concealed, as in some of the Horse Mackerels (_Caranx_) and in _Salmo_; or
it may be completely hidden beneath the oral epithelium, as in the Cod
(_Gadus_), where the organ is very degenerate, and is little more than a
"rete mirabile" of blood-vessels. The nature of the Teleostean pseudobranch
is not in {285}all cases quite clear. In _Salmo_ it is said that there is
no hyoidean hemibranch, and that the pseudobranch is really a persistent
spiracular pseudobranch;[293] hence it is probable that a like significance
must be attached to this singular structure in other Teleosts. The evidence
of the cranial nerves on this point is conflicting. If the pseudobranch
pertains to the spiracular cleft its nerve supply should be derived from
the nerve of that cleft—viz. the seventh or facial nerve; but if it
represents a hyoidean hemibranch, then one would expect it to be innervated
by the ninth or glossopharyngeal nerve. As a matter of fact, however, the
organ is said to be supplied by the seventh in some Teleosts, and in others
by the ninth nerve.
In the Dipnoi the branchial system is best developed in _Neoceratodus_, the
increasing importance of the lungs as respiratory organs in _Protopterus_
and _Lepidosiren_ being associated with a corresponding reduction in the
structural and functional development of the gills. There is no trace of
spiracles in the adult.
[Illustration: FIG. 165.—Transverse section through a branchial arch of
_Neoceratodus_ (semi-diagrammatic), _a.b.a_, Afferent branchial artery;
_b.a_, branchial arch; _b.f_, branchial filaments; _e.b.v_, efferent
branchial vessel; _g.r_, gill-rakers. (From Baldwin Spencer.)]
In _Neoceratodus_[294] there are five branchial clefts, including the
hyobranchial. Each of the first four branchial arches carries a pair of
hemibranchs, and, as in the Holocephali, the gill-lamellae are attached
along nearly their whole length to a well-developed interbranchial septum
(Fig. 165). A peculiarity of _Neoceratodus_, which has no counterpart in
any other Fishes, is the extension of the branchial lamellae on to the
dorsal and ventral walls of the branchial clefts, so that the hemibranchs
on opposite sides of each cleft are continuous both dorsally and ventrally
(Fig. 166). The fifth arch is gill-less. {286}In addition to the normal
gills there is also a hyoidean pseudobranch. As in other Dipnoi, an
operculum forms the outer wall of the branchial cavity, and leaves but a
narrow, slit-like external branchial aperture.
[Illustration: FIG. 166.—The second branchial cleft of _Neoceratodus_, to
show the dorsal and ventral continuity of two hemibranchs on opposite sides
of the same cleft. _b.c_, Branchial cleft; _b.f_, branchial filaments;
_g.r_, gill-rakers. (From Baldwin Spencer.)]
In _Protopterus_[295] the number of branchial arches is increased to six,
but, in consequence of the closure of the hyobranchial cleft, there are but
five open clefts. The first, second, and third arches are wholly devoid of
branchial filaments: the fourth and fifth support each a biserial gill,
while the sixth arch retains only an anterior hemibranch, which, however,
as the source of its blood supply seems to indicate, may consist of
"emigrant" gill-filaments from the posterior hemibranch of the fifth
arch.[296] Interbranchial septa are practically non-existent, the
flattened, leaf-like gill-lamellae being free except at their attached
bases, and thus repeating a characteristic Teleostean feature. A "hyoidean"
hemibranch or pseudobranch, supplied from the ventral aorta, is present,
but as the hyobranchial cleft is closed it projects into the branchial
cavity immediately in front of the cleft between the first and second
branchial arches. In _Lepidosiren_[297] the branchial arches are reduced to
five and the clefts to four, the hyobranchial and fifth clefts being
closed. There is a "hyoidean" hemibranch resembling that of _Protopterus_.
The facts furnished by the study of the numerical and structural variations
in the gill-clefts, gills, and gill-arches of different groups of Fishes
prove that atrophy of these structures takes place at opposite ends of the
series. We have examples of this anteriorly in the suppression of the
hyo-mandibular cleft and its hemibranch, and of the hyoidean hemibranch, as
the result of {287}the conversion of the mandibular and hyoid arches into
jaws, or into skeletal supports for the jaws; and posteriorly, in the
reduction which is evident when the generality of Fishes are compared with
such primitive Elasmobranchs as _Chlamydoselachus_ and _Notidanus_.
In most Fishes the concave pharyngeal margins of the branchial arches are
fringed with a double series of either cartilaginous or bony tubercles or
filaments, the "gill-rakers" (Figs. 161 and 164). The anterior row of
gill-rakers on each arch usually interdigitate with those of the posterior
row on the preceding arch, and in this way the two rows form a sieve-like
mechanism to prevent any solid particles, which may enter the pharynx with
the respiratory current of water, from passing into the gill clefts and
clogging or otherwise injuring the branchial filaments.
In a few Fishes the gill-rakers are enormously developed, and subserve a
function similar to that of the baleen plates of the Whalebone Whales in
acting as a filter for straining from the water the small pelagic organisms
on which the Fish feeds. This is notably the case in the great Basking
Shark (_Selache maxima_)[298] in which the closely-set, flattened, tapering
gill-rakers may be so long as four or five inches, and, while somewhat
resembling "whalebone" in appearance, have the histological structure of
vascular dentine. The nature of the food, which in the stomach of one
specimen examined consisted solely of an immense quantity of plankton,
including Copepods and the larvae of other Crustaceans,[299] affords clear
evidence of the great value of such a filtering mechanism to this Shark,
and, at the same time offers an explanation of the striking and significant
reduction in the size of the teeth, which, relatively to the dimensions of
the Fish, are so small as to be almost vestigial. A similar filter has been
observed in an extinct _Selache_ (_S. aurata_)[300] from the Antwerp Crag,
and also in an existing South African Shark (_Rhinodon typicus_);[301] and
in the latter, as in the Basking Shark, is associated with a marked
reduction in the importance of the dentition. The long slender gill-rakers
of the Chondrostean {288}_Polyodon_ also constitute an efficient filter,
and the same may be said of several plankton-eating Teleosts.
THE MECHANISM OF RESPIRATION.—The aeration of the blood is effected by the
rhythmical suction of water into the oral cavity, and its subsequent
expulsion through the gill-clefts, bathing the highly vascular
gill-lamellae in its course. In any single act of inspiration the mouth is
opened, and the oral cavity enlarged by the lateral expansion of its walls.
When the oral cavity is filled with water, the mouth is closed and the
expiratory process begins. By the lateral contraction of the oral walls the
water is driven outwards through the gill-clefts, and over the
gill-lamellae. During this process the branchial arches become widely
separated by the contraction of their muscles, the operculum is elevated,
and the oesophagus is closed by the contraction of its muscular wall. In
many Fishes the course of the expiratory water-current is controlled by
special valve-like folds of the oral mucous membrane, the maxillary and
mandibular "breathing-valves."[302]
The rate of "breathing" varies considerably in different Fishes, even in
allied species.[303] In the Blue Wrasse (_Labrus_), and the Rockling
(_Motella_), the number of respirations per minute is 15, in the Minnow
(_Leuciscus_), and Stickleback (_Gastrosteus_), as many as 150. A
deficiency of oxygen in the water accelerates the respiratory movements,
and the Fish appears to "pant" or breathe hurriedly. In the Lampreys, both
inspiration and expiration may take place through the external
gill-apertures by the alternate expansion and contraction of the gill-sacs,
more especially when the suctorial buccal funnel is used for the attachment
of the animal. On the other hand, the singular habits of the Myxinoids
involve a further modification of the respiratory process. In these
Cyclostomata the inspiratory current enters the external naso-pituitary
aperture and reaches the pharynx through the naso-pituitary canal, and
thence, as an expiratory stream, traverses the gill-sacs on its way
outwards. The pharynx is closed behind the last pair of gill-sacs by a
constrictor muscle, which prevents the entrance of the water into the
oesophagus, and converts the pharynx into a respiratory tube for the time
{289}being; but, when food is being swallowed, the pharyngeal constrictor
is relaxed and the internal apertures of the gill-sacs are closed by the
contraction of their own sphincter muscles.
In addition to the usual respiratory organs it is probable that in not a
few Fishes the superficial skin may share with the gills the function of
breathing. In this connexion may be mentioned the fact that in
_Periophthalmus_ the tail is used for respiration. Hickson[304] observed
that a species of this genus, frequenting the extensive sandy shores of the
Island of Celebes, often rests with its tail in the water, the head and
trunk being exposed. Under such circumstances the gills are probably of
little use, and the tail is utilised as a breathing organ, principally, as
Haddon[305] subsequently pointed out, through the agency of its extremely
vascular caudal fin.
[Illustration: FIG. 167.—Embryos of the Electric Torpedo (_Torpedo
ocellata_). A, dorsal view; B, ventral view of a slightly younger specimen.
_cl_, Cloaca; _el.o_, electric organ; _ex.b_, external gills; _p.f_,
pectoral fin; _pv.f_, pelvic fin; _sp_, spiracle; _y.s_, stalk of
yolk-sac.]
Some Fishes possess larval breathing organs; others, even when provided
with gills, either utilise the air-bladder, or develop special accessory
organs, for aquatic or, more usually, for aerial respiration.
{290}[Illustration: FIG. 168.—Head of young _Polypterus_. _ex.g_, External
gill of the left side. (From Steindachner.)]
LARVAL GILLS.—In early life many Fishes acquire larval gills, either as the
result of the precocious growth of the normal gills, or by reason of the
development of evanescent structures. In the embryos of Elasmobranchs
"external gills," in the form of long filiform processes invested by
hypoblast, are developed from the walls of all the branchial clefts,
including the spiracles, and protrude outwards for some distance through
the external apertures of the clefts (Fig. 167, B). They perhaps facilitate
respiration within the egg, as they completely disappear after hatching;
but there is also reason for believing that they aid in the absorption of
nutriment. Similar gills are present in young Holocephali. In some larval
Teleosts, as in certain genera of the Osteoglossidae and Mormyridae (e.g.
_Heterotis_ and _Gymnarchus_)[306] these structures are remarkably
developed (Fig. 239). The young of the Loach (_Misgurnus_) and of the
Salmon (_Salmo_) also have the ordinary gill-filaments prolonged externally
as filiform structures, which subsequently become reduced to their normal
size.[307] In its larval state _Polypterus_[308] has a pair of
pinnately-fringed ectodermal or cutaneous gills projecting from the lateral
surfaces of the head behind and above the external branchial apertures
(Figs. 168 and 281). Apparently as an individual peculiarity the right gill
has been retained in a specimen of _P. congicus_ so large as 22 cm. in
length, although the left one had entirely disappeared.[309] Each gill is
supplied with blood from the ventral aorta by a vessel which ascends the
{291}hyoid arch, and is apparently the representative of the artery
supplying the hyoidean hemibranch in Elasmobranchs. The efferent vessel of
each gill joins the common trunk formed by the union of the efferent
vessels of the normal gills of the same side.
The cutaneous gills of the Dipnoid _Protopterus_ may also be included in
the category of larval breathing organs. They consist of three simple
unbranched filaments on each side of the head, and, as in _Polypterus_, are
situated at the dorsal extremity of the external gill aperture (Fig. 309).
Although usually represented in the relatively young or half grown
specimens which, so far, have reached Europe, it is extremely probable that
these organs atrophy in older individuals. Similar gills are present in the
larval _Lepidosiren_ (Fig. 311), but disappear at a much earlier stage. At
no period of its development are larval gills present in
_Neoceratodus_.[310]
THE AIR-BLADDER AS A RESPIRATORY ORGAN.—In certain Fishes the air-bladder
may become subservient to the function of respiration. In _Amia_ and
_Lepidosteus_ the internally sacculated and vascular air-bladder is
obviously adapted for air-breathing, and there are not wanting
observations[311] which suggest that the organ is actually used for this
purpose after the fashion of a lung. According to Jobert,[312] this is also
the case with the sacculated air-bladder of certain Brazilian Teleosts,
viz. _Sudis gigas_, _Erythrinus taeniatus_ and _E. braziliensis_, since
these Fishes die of asphyxia when the organ is cut off from communication
with the exterior by the ligature of its ductus pneumaticus. It is in the
Dipnoi, however, that the air-bladder becomes most completely a true lung.
In _Neoceratodus_[313] the lung is probably of the greatest use to the Fish
when the rivers are low during the hot season and the water is charged with
foul gases from decomposing vegetable matter, and possibly also when the
water is filled with sediment in the rainy season. In _Protopterus_, and
more especially in _Lepidosiren_, the partial atrophy of the gills renders
it highly probable that the lungs are the principal breathing organs at all
times. Nevertheless, it must be emphasised that in all these Fishes
respiration by means of the air-bladder {292}necessarily involves a transit
of air to and from that organ through the ductus pneumaticus, and at
present nothing is known as to the method by which such inspiratory and
expiratory currents can be produced.[314]
There is also some experimental evidence for the belief that the
air-bladder of some Teleosts may be subsidiary to respiration by acting as
a reservoir for the superabundance of oxygen which is taken into the blood
through the gills, and subsequently reabsorbed into the blood when the Fish
is in water containing relatively little oxygen.[315] It is clear, however,
that the conditions under which the air-bladder can be used in this way are
by no means fully understood, for, under experiment, such Fishes died of
asphyxia even though after death the air-bladder still contained upwards of
fifty per cent of oxygen.
ACCESSORY ORGANS OF RESPIRATION.—In certain Fishes of peculiar habits, or
living under special external conditions, accessory respiratory organs are
developed.
Although in this particular instance no special organs are formed, mention
may first be made of the singular method of intestinal respiration in vogue
in some Teleosts. In one of the Loaches (_Misgurnus fossilis_),[316] air is
swallowed and passed along the alimentary canal until it is finally voided
at the anus. The mucous membrane of the intestine is extremely vascular,
and hence the blood comes into sufficiently intimate relations with the
swallowed air to admit of it exchanging carbon dioxide for oxygen.
Intestinal respiration also occurs in species of the South American
freshwater genera of Siluridae and Loricariidae, _Callichthys_, _Doras_,
_Loricaria_, and _Plecostomus_;[317] and in some cases the area of
respiratory surface is considerably increased by the development of folds
and processes of the intestinal mucous membrane.
In a few tropical Teleosts curious labyrinthiform organs are developed in
connexion with certain of the branchial arches, and serve as accessory
breathing organs. In the Indian "Climbing Perch" (_Anabas scandens_),[318]
of the family Anabantidae, the organ (Fig. 169) consists of three or more
concentrically-arranged bony {293}laminae, with wavy, crenulated margins,
attached by a common bony base to the upper extremity of the fourth
branchial arch, and enclosed in a special dorsal enlargement of the
branchial cavity. The vascular membrane which invests the laminae is
abundantly supplied with venous blood by a branch of the fourth afferent
branchial artery, the equivalent efferent vessel joining the dorsal aorta.
Essentially similar organs are found in several genera of Osphromenidae
(e.g. _Polyacanthus_, _Osphromenus_, and _Trichogaster_). A simpler form of
respiratory organ of somewhat the same type occurs in the Indian family
Ophiocephalidae.[319] In these Fishes there is, on each side, an accessory
branchial cavity, situated above that which contains the gills, but freely
communicating with it (Fig. 170). The cavity is lined by a thickened and
puckered vascular membrane, but otherwise contains no special respiratory
structures.
[Illustration: FIG. 169.—Labyrinthiform organ of _Anabas scandens_, exposed
by the removal of the greater part of the operculum. _b.a′_, First
branchial arch; _l.o_, labyrinthiform organ; _op_, operculum; _sb.c_,
supra-branchial cavity.]
[Illustration: FIG. 170.—Supra-branchial cavities of _Ophiocephalus_.
Ventral view, as seen after the removal of the ventral halves of the
branchial arches, _b.a^{1-4}_, The first four branchial arches; _o.c_, roof
of oral cavity; _oes_, oesophagus; _p.t_, pharyngeal teeth; _sb.c_; left
supra-branchial cavity; _v.f_, folds of the lining membrane of the cavity.]
In the Siluroid genera _Clarias_ and _Heterobranchus_ the accessory organ
takes the form of branched, arborescent and highly vascular structures,
developed as outgrowths from the dorsal extremities of one or two branchial
arches, and enclosed within a posterior and dorsal expansion of the proper
branchial cavity (Fig. 171).
{294}Another example of these interesting structures occurs in _Chanos
salmoneus_ and a few other Clupeidae[320] in the shape of a coiled
gill-like organ ("gill-helix"), which is supported by the dorsal segment of
the fourth branchial arch, and enclosed in a similarly curved caecal
extension of the branchial cavity. Each gill derives its blood from the
fourth afferent branchial artery, the corresponding efferent vessel joining
the fourth efferent branchial artery. A similar spirally-coiled
"gill-helix" is found also in _Heterotis ehrenbergii_,[321] amongst the
Osteoglossidae, and in several species of Characinidae.[322]
[Illustration: FIG. 171.—Accessory respiratory organ of _Clarias_, as seen
after the removal of the left operculum. _a_, Anterior arborescent organ;
_b.a^{1-4}_, the first four branchial arches and their holobranchs;
_d.b.c_, dorsal extension of the left branchial cavity; _f_, modified
gill-filaments; _op_, base of the operculum; _p_, posterior arborescent
organ.]
In other Teleosts the accessory breathing organ assumes the condition of
paired lung-like outgrowths of the branchial cavity. Thus, in one of the
Symbranchidae, the Indian "Cuchia Eel" (_Amphipnous cuchia_),[323] there is
a pair of small bladder-like sacs, with membranous and vascular walls, each
of which opens into the branchial cavity above the first gill-cleft, and is
supplied with blood by the afferent branchial artery of the gill-less first
branchial arch. An extreme modification in the same direction {295}is
presented by the Indian Siluroid _Saccobranchus_.[324] In this Fish a long
caecal diverticulum of the branchial cavity extends backwards on each side
from the dorsal region of the first branchial cleft to the tail, and in its
course is situated internally to the lateral trunk musculature, and close
to the vertebral column (Fig. 172). The walls of the caeca are vascular,
but no special respiratory structures are developed within their cavities,
which, during life, only contain air. In _S. singio_ the right caecum is
supplied with blood by an extension backwards of the dorsal portion of the
first afferent branchial artery of that side; the left, on the contrary,
being supplied by the corresponding portion of the fourth afferent artery
of the same side. In _S. fossilis_[325] both air-sacs are supplied by the
fourth afferent branchial artery. The efferent vessels join the fourth
efferent branchial artery, right or left as the case may be.
[Illustration: FIG. 172.—Air-sacs of _Saccobranchus singio_. _a.b_, The
air-bladder enclosed in its bony capsule; _a.c_, right air-sac; _a.s_, left
air-sac; _c.a_, bulbus aortae; _l.a.v_, afferent vessel of the left
air-sac; _r.a.v_, afferent vessel of the right air-sac; _r.e.v_, efferent
vessel of the right sac. (After Hyrtl, altered by Hubrecht.)]
With perhaps one or two exceptions, the accessory respiratory organs of
Fishes seem to exist for the purpose of enabling their possessors to
breathe in air. This is certainly the case with the labyrinthiform organs
of _Anabas_ and its allies, and also in such Fishes as _Amphipnous_,
_Saccobranchus_, and the Ophiocephalidae, and probably in others. Nearly
all these Fishes are tropical in geographical distribution, more or less
amphibious in their habits, and usually possess a remarkable capacity for
sustaining life out of water, under conditions which are promptly fatal to
ordinary Fishes. Thus, _Anabas scandens_ may be kept alive for days in
earthen pots without water, and when free is able to travel short distances
on land, especially in the early morning when the dew is on the ground,
while _Amphipnous_ frequents {296}marshes, lurking in holes in the grass
and about the sides of ponds. In fact, even when in the water, access to
air, which is probably swallowed and passed over their accessory breathing
organs, is indispensable to their existence. Experiments conclusively prove
that if the Fish is artificially prevented from obtaining air in this way
asphyxiation speedily ensues.[326]
In addition to breathing air through the agency of special organs evolved
for the purpose, there are many freshwater Fishes which, like those just
mentioned, periodically rise to the surface and swallow air in order to
saturate the water which bathes the gills with oxygen.[327]
{297}CHAPTER XI
THE AIR-BLADDER
[Illustration: FIG. 173.—Transverse section of the body of a Teleost, to
show the position of the air-bladder (diagrammatic). _a.b_, The
air-bladder; _c_, coelom; _d.p_, ductus pneumaticus; _k_, the kidneys;
_oes_, oesophagus; _p.p_ and _v.p_, parietal and visceral layers of the
peritoneum; _r_, rib; _v.c_, vertebral column.]
In the Crossopterygii, Chondrostei, and Holostei, in the Dipnoi, and in the
great majority of Teleosts, there is situated on the dorsal side of the
coelom, between the alimentary canal below and the kidneys and vertebral
column above, a more or less elongated sac with membranous walls, an
internal epithelial lining and gaseous contents—the air-bladder (Figs. 154
and 173). Usually developed in the embryo as a caecal outgrowth from the
dorsal surface of the oesophagus, the air-bladder grows anteriorly and
posteriorly, and may either retain throughout life its primitive connexion
with the alimentary canal by means of a longer or shorter tubular canal,
the ductus pneumaticus, or become completely separated therefrom in the
adult by the atrophy of the duct. Its walls sometimes, but rarely, contain
muscle-fibres, as in _Lepidosteus_, _Amia_, and the Dipnoi, and are always
more or less vascular, while laterally and ventrally the organ is invested
externally by the peritoneum (Fig. 173). In addition to the muscle-fibres
distributed in its walls, the bladder is often provided with powerful
extrinsic muscles, more especially in those Fishes in which it is used as
an organ for {298}sound-production. In the different groups of Fishes in
which it is present the air-bladder frequently undergoes remarkable
structural modifications and becomes adapted for various distinct
functions.
In the Cyclostomata there is no trace of an air-bladder, and, unless
represented in certain Sharks (e.g. _Mustelus_, _Galeus_, and
_Acanthias_),[328] by a small caecum embedded in the dorsal wall of the
oesophagus and communicating with its cavity, it is also absent in all
Elasmobranchs. In the Crossopterygii (e.g. _Polypterus_),[329] the
air-bladder is double, but while the right sac is long and somewhat
tubular, the left is much smaller and oval in shape (Fig. 174). Near their
anterior extremities the two sacs fuse into a single unpaired chamber,
beyond which they again project in the form of two short caeca. The median
chamber opens into the oesophagus on the ventral side by an orifice (_gl_)
bounded by prominent lips and furnished with a muscular sphincter. The
organ is devoid of internal sacculations. In the Chondrostei (e.g.
_Acipenser_) the air-bladder is oval in shape, with a smooth,
non-sacculated, inner surface, and a lining of ciliated epithelium, and it
communicates with the oesophagus by means of a relatively wide, dorsally
placed, funnel-like orifice.
[Illustration: FIG. 174.—Air-bladder of _Polypterus_. _gl_, glottis. (From
Wiedersheim.)]
In the Lepidosteidae the single air-bladder extends the whole length of the
abdominal cavity, and, as in _Polypterus_, communicates with the exterior
through a larynx-like vestibule provided with a glottis,[330] which,
however, opens dorsally into the oesophagus (Fig. 175). A strong fibrous
band runs along the median line of the inner surface of its dorsal wall,
from which extends ventrally on each side a series of transverse
fibro-muscular ridges, forming the boundaries of a double row of regularly
arranged alveoli (Fig. 176). The bottom of each alveolus {299}is still
further sacculated by finer branches of the principal fibrous bands.[331]
In the Amiidae the bladder is very large, and, except that a short median
cleft divides it in front into two short caeca, it is unpaired.
Internally, its walls are much sacculated, but the alveoli are smaller and
arranged less regularly than in _Lepidosteus_. The aperture of
communication with the oesophagus is dorsally situated.
[Illustration: FIG. 175.—Portion of the air-bladder, with the ventral wall
removed, and the glottis, of _Lepidosteus_. _a.b_, Air-bladder; _gl_,
glottis; _s_, bulging of the hinder wall of the vestibule into the cavity
of the air-bladder; _v_, cleft leading from the air-bladder into the
vestibule. (From Wiedersheim.)]
It may be mentioned that in all the preceding Teleostomi the ductus
pneumaticus is remarkably short, the connexion between the air-bladder and
the oesophagus being almost direct by means of a larger or smaller orifice,
which, except in _Acipenser_, is more anteriorly placed than in most other
Teleostomi; and further that, unlike many Teleosts, there are no special
"retia mirabilia," "red bodies," or "red glands."
[Illustration: FIG. 176.—Portion of the air-bladder of _Lepidosteus_,
opened along the mid-ventral line to show the alveoli. _av_, Alveolus;
_f.b_, medio-dorsal fibro-muscular band. (From Wiedersheim.)]
In the Dipnoi the structural resemblance of the air-bladder to a true lung,
which to some extent is indicated in _Polypterus_, _Amia_, and
_Lepidosteus_, becomes still more marked.
In _Neoceratodus_[332] the organ is not unlike that of _Lepidosteus_, and
takes the form of a spacious unpaired sac, extending from one end of the
abdominal cavity to the other. On its inner surface two fibrous bands, one
of which is dorsal and the other ventral, traverse the whole length of the
bladder, and project slightly into its cavity. {300}Between these median
ridges extend a number of transverse septa, forming the boundaries of a
series of pairs of bilaterally symmetrical oval alveoli, the walls of which
are still further sacculated by a network of finer ridges (Fig. 177). The
short ductus pneumaticus seems to be an anterior continuation of the right
half of the bladder, and opens into the oesophagus by a small glottis,
situated on the ventral side, a little to the right of the median line.
[Illustration: FIG. 177.—Interior of a portion of the air-bladder of
_Neoceratodus_. _av_, Alveolus; _f.r_, the two fibrous ridges. (From
Günther.)]
The more complicated and much more lung-like air-bladder of _Protopterus_
(Fig. 178)[333] is essentially double, consisting of an anterior unpaired
portion, and of two sac-like prolongations which extend backwards the whole
length of the coelom, gradually tapering towards the cloaca. Anteriorly,
the unpaired portion of the organ is continued into a vestibule or
pneumatic duct, which, after passing ventrally on the right side of the
oesophagus, opens into the latter by a ventrally-situated, slit-like
glottis, immediately behind the last pair of gill-clefts. The margins of
the glottis are provided with radially-arranged dilator muscles, and in
connexion with its anterior border there is an epiglottis-like
fibro-cartilaginous plate.[334] The central cavity of each lung (Figs. 178
and 179) communicates with a series of larger or smaller alveoli in the
lung-wall, and each of the latter opens in succession into smaller tubular
cavities, and then into still smaller terminal caecal sacculi. Hence, much
more than in _Neoceratodus_, the lungs approximate in structure to those of
the higher terrestrial {301}Vertebrata. Non-striated muscle cells, pigment
cells, and blood capillaries are abundantly present in the connective
tissue external to the lining epithelium of the lung-cavities.
[Illustration: FIG. 178.—A, the air-bladder of _Protopterus_, viewed from
the ventral side. Portions of the ventral walls of the pharynx and bladder
have been removed. _gl_; Glottis; _lg_, undivided portion of the lung;
_l.l_, left lung; _oes_, oesophagus; _p.a_^{1}, _p.a_^{2}, the left and
right pulmonary arteries; _ph_, pharynx; _p.v_, pulmonary vein; _r.l_,
right lung; _vb_, vestibule. (From Newton Parker.) B, portion of one lung
of _Protopterus_, opened from the dorsal side to show the alveoli. _al_;
Alveolus. (From Baldwin Spencer.)]
The air-bladder of _Lepidosiren_ closely resembles that of _Protopterus_,
and, as in the latter Dipnoid, the glottis seems to be furnished with an
epiglottis.[335]
In all the Dipnoi the air-bladder is highly vascular, but nevertheless
presents no trace of "red bodies" or "red glands."
The most striking features in the remarkably polymorphic air-bladder of
Teleosts relate to (_a_) its presence or absence; (_b_) differences in
shape and relative size; (_c_) the development of caecal outgrowths; (_d_)
the subdivision of its cavity by the formation of internal septa; (_e_) the
retention or suppression of the ductus pneumaticus, and the occasional
development of secondary ducts communicating directly with the exterior;
(_f_) the presence of "red glands" or "red bodies"; (_g_) its connexion
with the auditory organ; (_h_) its adaptation as an organ for
sound-production.
{302}(_a_) The air-bladder is by no means universally present in Teleosts.
It is absent in several entire families,[336] such as, for example, the
Flat Fishes or Pleuronectidae, the Scopelidae, and the "Lump-suckers"
(Cyclopteridae). In a few families, as in the Mackerels (Scombridae), the
"Blennies" (Blenniidae) and the Polynemidae, the organ is present in most
genera, but absent in a few, or even present or absent in different species
of the same genus. Thus, of the three British species of Mackerel, viz. the
Spanish Mackerel (_Scomber colias_), _S. pneumatophorus_, and the common
Mackerel (_S. scombrus_), an air-bladder is present in the first two, but
absent in the third.[337]
(_b_) As might be anticipated, the shape of the air-bladder is extremely
different in various Teleosts, and usually conforms to the shape of the
body, while differences in relative size are of frequent occurrence, even
in closely related species. Sometimes the organ is more or less tubular,
fusiform, ovoid, or heart-shaped; occasionally it is shaped like a
"dumb-bell," consisting of two lateral sacs connected by a median tubular
portion, as in the Siluroids _Clarias_ and _Callichthys_; or it may be
horse-shoe-shaped, as in the Silurid _Ailia_.[338] Not unfrequently a
transverse constriction divides the air-bladder into two intercommunicating
sacs, as in most of the Carp family (Cyprinidae), or three such sacs may be
formed by two constrictions (e.g. _Ophidium_). In the "Electric Eels"
(Gymnotidae) there are two sacs, connected by a slender canal, from which
the ductus pneumaticus takes its origin.[339]
[Illustration: FIG. 179.—Showing the structure of one of the larger alveoli
of the air-bladder of _Protopterus_. 1, Central cavity of the lung; 2,
alveolus; 3, tubular cavities communicating with 4, the small terminal
sacculi. (From Baldwin Spencer.)]
The air-bladder is either more or less free in the abdominal cavity, or
firmly attached to the vertebral centra and their rib-bearing processes by
fibrous extensions passing between the two structures. Not rarely the organ
extends from the abdominal {303}cavity into the tail, sometimes penetrating
for a short distance into the expanded haemal canal of the anterior caudal
vertebrae, or extending unsymmetrically along either the right or left side
of the tail. More frequently, perhaps, where the air-bladder is prolonged
into the tail, it assumes the form of two bilaterally arranged and
symmetrical caeca, which extend backwards for a variable distance internal
to the caudal muscles and in contact with the adjacent skeletal elements,
as in Notopteridae, and in some Sparidae, Carangidae, and Scombridae. The
extension of the air-bladder into the tail is often associated with a
short, laterally-compressed trunk, which, if the bladder is to attain its
normal degree of development, necessitates its prolongation into the caudal
region.
(_c_) A characteristic feature in the air-bladder of many Teleosts
belonging to widely different families is the development of a more or less
complex system of simple, or variously branched, caecal outgrowths, which,
like the internal septa, are specially characteristic of those Fishes in
which the bladder is used as a vocal organ without, however, being peculiar
to them.
In some of the Gadidae, as in the Cod (_Gadus morrhua_), the air-bladder
divides anteriorly into a pair of caecal prolongations which extend
forwards to the head, and are often curiously coiled. Somewhat similar
caeca are also present in species of Berycidae, Sparidae, Siluridae,
Clupeidae, and Notopteridae. Caecal prolongations may also be developed
from the hinder end of the bladder, and, as already mentioned, extend into
the tail; or even from both ends in the same species (e.g.
_Notopterus_).[340] In the Silurid, _Rita crucigera_,[341] a long tubular
caecum is developed from each side of the heart-shaped bladder, and thence
is prolonged backwards to the anus. In certain species of _Doras_ of the
same family (e.g. _D. maculatus_),[342] an elegant series of variously
sized branched caeca fringe each of the lateral margins of the bladder. It
is, however, in the Physoclist family of the Sciaenidae[343] that the
branching of the air-bladder attains its greatest development in extent and
variety.
{304}[Illustration: FIG. 180.—Air-bladder of _Otolithus_. (From Cuvier and
Valenciennes.)]
In _Otolithus_ (Fig. 180) two short tubular canals are given off from the
antero-lateral angles of the bladder, each subsequently dividing into two
elongated, tapering sacs, of which one is directed forwards and the other
backwards. In _Corvina lobata_ (Fig. 181) the lateral margins of the
bladder are everywhere fringed with a series of tufts of caeca, each tuft
being connected by a short common canal with the cavity of the organ. In
the "Drum" (_Pogonias chromis_) (Fig. 182) each side of the anterior third
of the air-bladder has a series of digitately branched caecal appendages,
the most posterior of which on each side are connected by a tubular canal,
also bearing branched caeca, with the corresponding postero-lateral
extremity of the bladder.
_Collichthys_[344] has a still more remarkable arrangement. In this
Sciaenoid (Fig. 183) twenty-five tubular branches are given off from each
side of the bladder, all of which soon subdivide into a dorsal and a
ventral division. These still further divide, and their branches either end
blindly or are prolonged into a series of arches to the mid-dorsal or
mid-ventral line as the case may be, where they become continuous with the
corresponding branches of the opposite side. The series of dorsal branches,
enveloped in their peritoneal investment, extend between the body of the
air-bladder and the roof of the body-cavity, while the corresponding
ventral branches, similarly invested, surround that part of the coelom
which contains the stomach, intestine, and liver.
[Illustration: FIG. 181.—Air-bladder of _Corvina lobata_. (From Cuvier and
Valenciennes.)]
(_d_) In addition to the subdivision of the cavity of the air-bladder by
the externally obvious, {305}transverse, or longitudinal constrictions
already described, or by the growth of simple or branched prolongations,
the organ is often chambered or sacculated by the development of internal
septa or partitions.
[Illustration: FIG. 182.—Air-bladder of _Pogonias chromis_. (From Cuvier
and Valenciennes.)]
In many of the Gurnards (_Trigla_)[345] the cavity of the bladder is
divided into two intercommunicating compartments by a transversely-disposed
and centrally-perforated diaphragm. The large air-bladder of some species
of _Erythrinus_[346] is subdivided internally into numerous alveoli or
sacculi. In _Notopterus_ a longitudinal septum divides the cavity of the
abdominal portion of the bladder into two lateral chambers, which, however,
freely intercommunicate anteriorly. In the great majority of the
Siluridae[347] the cavity of the organ is divided by a characteristic
T-shaped arrangement of a primary transverse and a longitudinal septum into
three communicating chambers, of which one is anterior and transversely
disposed, and two are posterior and longitudinally arranged (Fig. 222). The
posterior compartments in many genera are still further divided by the
growth of secondary transverse septa, extending outwards from the median
longitudinal septum, without, however, reaching the external lateral walls
of the chambers. In a few genera, as in certain species of
_Pangasius_,[348] additional fibrous bands and ridges passing between the
primary and secondary {306}septa give to the cavities of the lateral
compartments the appearance of being occupied by a coarse spongy network.
[Illustration: FIG. 183.—Transverse section through the abdominal region of
_Collichthys lucida_. _a.b_, Air-bladder; _d.b_ and _v.b_, the dorsal and
ventral branches of the air-bladder; _l_, liver; _m_, mesentery; _s_,
stomach; _v.c_, vertebral column. The dotted and broken lines surrounding
the bladder and its branches represent the peritoneal investment of these
parts. (From Günther.)]
(_e_) In its relations to the oesophagus and to the air-bladder the ductus
pneumaticus exhibits striking modifications in different Teleosts. With
very rare exceptions, an open ductus is wanting in the Heteromi,
Catosteomi, Acanthopterygii, Opisthomi, Pediculati, Jugulares, and the
Plectognathi, for which reason the term "Physoclisti" has often been used
as a collective name for these Fishes. On the other hand, a permanently
open ductus is generally present in the Malacopterygii, Ostariophysi,
Apodes, and the Haplomi, which, in consequence, have been designated
"Physostomi." It must be emphasised, however, that all Teleosts are
Physostomous in the embryonic condition, and whether they eventually become
Physoclistous or remain Physostomous depends entirely on the abortion or
retention of the primitive communication between the air-bladder and the
alimentary canal. When present in Teleosts, the ductus pneumaticus, with a
few exceptions (e.g. _Notopterus_), where it is both short and relatively
wide, is almost invariably much longer and narrower than in the other
orders of Teleostomi and in the Dipnoi, sometimes passing directly from the
air-bladder to the oesophagus, but not infrequently describing a sigmoid
curve, as in some Cyprinidae, or an even more tortuous course. The opening
into the alimentary canal is, with perhaps a single exception, dorsal, but
may vary from the commencement of the oesophagus to the hinder end of the
stomach. In _Erythrinus_ the oesophageal aperture is lateral. In two
instances the air-bladder has acquired secondary openings to the exterior,
and of these one occurs among the Physostomi and the other {307}in the
Physoclisti. In the Herring (_Clupea harengus_),[349] in addition to the
proper ductus, which is connected with the distal end of the caecal
stomach, a tubular canal leaves the hinder extremity of the bladder and
opens externally on the left side of the genital aperture; consequently, in
this Fish the air-bladder has a secondary and direct connexion with the
exterior in addition to the primary and indirect communication by means of
its proper duct. The Horse-Mackerel (_Caranx trachurus_)[350] is even more
peculiar. This Teleost has no true pneumatic duct, but instead a special
duct which passes from the bladder to open into the right branchial cavity
by a very minute aperture. In neither case is anything known of the mode of
origin or morphological nature of the secondarily acquired duct.
(_f_) The air-bladder differs greatly in its degree of vascularity in
various Teleosts, as well as in the extent to which its capillary
blood-vessels accumulate at special points on the inner surface to form the
so-called "red bodies" or "red glands." In some Teleosts the distribution
of capillaries is uniform or nearly so; in others, as in the Carp
(_Cyprinus carpio_) the vessels are arranged in fan-like, radiating tufts
over almost the whole extent of the inner surface; in others again, as in
the Pike (_Esox lucius_) the tufts are larger and more definitely
localised. A more extreme modification occurs in some of the Physostomi, in
which a remarkable concentration of capillaries takes place at one or more
points on the inner surface of the bladder, which project into the cavity
of the organ in the form of variously shaped blood-red masses. These "red
bodies" are essentially retia mirabilia, consisting of masses of
interlacing, tightly-packed capillaries with their afferent arteries and
efferent veins. The flattened lining epithelium of the bladder is continued
over these bodies without undergoing any special modification. In the
common Eel (_Anguilla vulgaris_) there are several of these bodies, of
which the largest are near the entrance of the pneumatic duct.
In the Physoclisti the "red bodies" seem to be replaced by true
glands,[351] which nevertheless in appearance closely resemble the former.
Some of the Gadidae, such as the Cod (_Gadus {308}morrhua_), the Haddock
(_G. aeglefinus_), and the Hake (_Merluccius vulgaris_), have a single
large "red gland" projecting into the interior of the bladder from its
dorsal or ventral wall (Fig. 184, A). The John Dory (_Zeus faber_) has five
such glands, worm-like and curved in shape, with their concavities facing a
central point between them (Fig. 184, B). In these Fishes a "rete mirabile"
of blood-vessels forms the vascular basis of the glands. The ordinary
pavement epithelium of the bladder becomes replaced by faintly granular,
columnar, and evidently glandular cells as it passes over the retia
mirabilia, and at the same time becomes invaginated into the mass of
capillaries in the form of a number of simple caecal glands (Fig. 185). So
far as is at present known, the "red glands" are only found in those
Teleosts in which the air-bladder has no ductus pneumaticus, whereas in
those Fishes which retain the ductus throughout life there are either no
special retia mirabilia, or, as in the Eel, only the so-called "red
bodies."[352]
[Illustration: FIG. 184.—Red glands, A, of the Cod (_Gadus morrhua_), and
B, of the John Dory (_Zeus faber_), seen from the interior of the
air-bladder. _bv_, Blood-vessels; _r.g_, red glands. (From Swale Vincent
and Stanley Barnes.)]
{309}(_g_) and (_h_) The structural modifications involved in the connexion
of the air-bladder with the auditory organ, and its adaptation for
sound-production, as well as its use in respiration, are considered
elsewhere.[353]
THE GASES OF THE AIR-BLADDER.—The gaseous contents of the air-bladder
consist of oxygen and nitrogen, but the relative proportions of the two
gases differ in different Fishes, and even in the same Fish, under
different conditions. Normally the proportion of oxygen is considerably
less in freshwater than in marine Fishes, and amongst the latter the
proportion of oxygen is often enormously greater, amounting in some cases
to 87 per cent., in deep-sea species as compared with their shallow water
congeners. A trace of carbon dioxide is also usually present. The gases are
derived from the blood as the latter circulates through the capillaries in
the walls of the bladder, and it is highly probable that the "red glands"
take an important part in the process; at all events, experimental research
has shown that the "secretion" or diffusion of gas into the air-bladder, as
well as the absorption of gas from the bladder into the blood, take place
most rapidly in those Fishes in which "red glands" or "red bodies" are
present.[354]
[Illustration: FIG. 185.—Vertical section through a "red gland"
(diagrammatic). _c_, Capillary blood-vessels; _g_, tubular glands. (From
Vincent and Barnes.)]
THE FUNCTIONS OF THE AIR-BLADDER.—Probably no single organ in any group of
Vertebrata is associated with the performance of a greater variety of
functions than the air-bladder of Fishes. Originally evolved, it may be, as
a glandular caecum in certain {310}Sharks, the air-bladder in the Dipnoi,
and some of the more generalised Teleostomi (e.g. _Amia_ and
_Lepidosteus_), and perhaps also in a few of the more specialised members
of the latter group (_e.g._ certain Teleosts), is to a greater or less
extent an accessory respiratory organ. In not a few Teleosts it is an organ
for sound-production, and in others again it is sometimes regarded as
having an important relation to the sense of hearing. But omitting such
subordinate functions which, as it were, have been grafted on to the
air-bladder, there can be no doubt that in the great majority of Fishes its
primary use is to act as a hydrostatic organ or "float." From this point of
view experimental investigations[355] seem to justify the following
conclusions:—
The function of the air-bladder is to render the Fish, bulk for bulk, of
the same weight as the water in which it lives. In this condition of
equilibrium, or plane of least effort, the Fish floats _in_ the water, and
therefore it is able to swim with a minimum of muscular effort. It is
obvious, however, that as a Fish rises or sinks it becomes exposed to an
increase or a diminution of hydrostatic pressure, which will necessarily
bring about the expansion or contraction of the volume of gas in the
air-bladder, and, therefore, by decreasing or increasing the specific
gravity of the animal, will tend to remove the Fish from its plane of least
effort. To counteract this, and to restore the Fish to a plane of
equilibrium at the new level, gas is either absorbed from the air-bladder,
or more gas is secreted into the bladder, as the case may be. According to
Moreau, by this process of automatic adjustment a Fish will always find,
sooner or later, a plane of least effort, whatever may be its depth in the
water; and further, this process takes place much more readily in those
Fishes which possess "red glands" or "red bodies", and with extreme
slowness in those in which these organs are absent. Nevertheless, it seems
doubtful if this process of adjustment can be of much use to a Fish in
ordinary vertical movements, inasmuch as gaseous secretion and absorption
are comparatively slow processes, the length of which in different Fishes,
and under different conditions, varies from a few hours to several days. On
the whole it seems more probable that adjustment to the varying pressures
of different depths by such means is far more likely to be useful during
such slow and gradual changes of level as are encountered {311}in the
course of diurnal, seasonal, or other periodic migrations than during the
rapid changes of level which may take place in ordinary vertical
locomotion.[356] In the generality of Fishes, and more especially in the
Physoclisti, it may be concluded that the possession of an air-bladder
restricts freedom of movement in the vertical direction, and confines
ordinary locomotion within more or less well-defined vertical limits above
or below the plane of least effort for the time being. As illustrating this
point, and as a proof of the danger incurred by a too rapid rise in the
water, the following remarks with reference to the "Kilch," a small
Salmonoid (_Coregonus_) inhabiting the Lake of Constance, and a favourite
article of food, may be quoted:[357]—The Fish "are caught in nets, and
brought to the surface of the water; they come up invariably with the belly
much distended, the air in the swimming-bladder, being relieved from the
pressure of the column of water, has expanded greatly and occasioned this
unnatural distension, which renders the Fish quite incapable of swimming.
Under these conditions the Fish is naturally unable to live for any length
of time. But the fishermen of the lake have a very simple remedy; they
prick into the air-bladder with a fine needle; the air escapes with some
force, the distension subsides, and the fishes are enabled to live under
totally changed conditions as to pressure, even in quite shallow water and
at the surface, swimming quite as freely as their companions, the natives
of the surface water. Hence the Kilch is confined to a certain depth,
because it is not capable of accommodating the tension of its
swimming-bladder to the change of pressure in the column of superincumbent
water."
It is not improbable that the Physostomi, or at any rate most of them, are
somewhat more advantageously placed in this respect. From the general
absence of "red glands" in this group, it may be inferred that whatever
capacity for gaseous secretion or absorption they possess must be exercised
with exceptional slowness, and, therefore, as a means of
pressure-adjustment may be neglected. On the other hand, they seem to
possess the compensating advantage of being able to substitute for
absorption the mechanical liberation of gas through the ductus pneumaticus.
It would seem, therefore, that the Physostomi have a distinct advantage
over the Physoclisti in that during ascent in the water they {312}can more
readily adapt themselves to the diminished pressure of a higher level by
ejecting the needful amount of gas than by relying upon the process of
gaseous absorption.[358] This conclusion is in harmony with the results of
experiment and with much that is known of the habits of these Fishes and
their greater freedom of locomotion in the vertical direction.
These briefly summarised conclusions as to the hydrostatic function of the
air-bladder must, however, be accepted only in a general sense. There are
many structural anomalies in the air-bladder of Fishes which are very
difficult to explain, or to correlate with any variations in the habits or
in the locomotor activities of its possessor.
In this connexion it may be mentioned that the presence or absence of an
air-bladder in different Fishes seems to some extent to be governed by two
causes. First, whenever the requirements of a Fish necessitate exceptional
freedom of locomotion in all directions the restrictions imposed by the
presence of an air-bladder are removed by its partial or complete
suppression; a result produced, secondly, by the assumption of a bottom
feeding or ground habit on the part of the Fish. Fishes like the Flat
Fishes or Pleuronectidae, when not in motion by the exercise of their fins,
habitually rest on the sea-bottom, and, as an air-bladder is useless under
such conditions, it has, in consequence, undergone complete atrophy. Not a
few Siluridae, and some Cyprinidae, inhabit the comparatively shallow
waters of rapidly flowing mountain torrents, and are often provided with
suckers for attachment to stones or rocks. To such Fishes as these a
hydrostatic organ is obviously useless, and it has hence become greatly
reduced in size, and in other respects approaches the condition of a
vestigial organ.
{313}CHAPTER XII
THE VASCULAR SYSTEM, THE LYMPHATICS, AND THE BLOOD-GLANDS
The Cyclostomata and Fishes possess a closed vascular system, consisting of
a heart, arteries, capillaries, and veins, the whole forming a continuous
series of blood-containing channels provided with definite limiting walls,
through which the blood is propelled in a constant direction by the
rhythmical contractions of the heart. In the course of the circulation the
blood flows from the heart through a single large trunk, the ventral aorta,
to the capillaries of the gills. From the gills the arterialised blood is
collected into a large dorsally-situated vessel, the dorsal aorta, and
thence is distributed through a system of arteries to the capillaries of
the various organs of the body. Finally, the blood is collected from the
capillaries and returned to the heart by the veins.
Although in most instances the organs of the body are supplied with
arterialised blood conveyed to them by arteries, there are nevertheless
cases in which an organ may receive venous blood by a vein in addition to
arterial blood supplied by an artery. For example, the capillaries of the
liver not only receive blood from the hepatic artery, but also venous blood
by a large vein (hepatic portal vein), formed by the union of a number of
smaller veins by which venous blood is collected from the capillaries of
the stomach, intestine, spleen, and pancreas. In this and similar
instances, where a vein formed by the union of the capillaries of an organ,
or of a series of organs, instead of uniting with other veins and
proceeding towards the heart, becomes continuous with a second set of
capillaries in some other organ, a "portal" system is said to be formed,
and in the particular example of the liver it is termed the "hepatic
portal" system. A similar, or "renal {314}portal," system also exists in
connexion with the kidneys in the majority of Fishes.
There is little doubt that, primarily, the vascular system of Vertebrate
animals consisted of a dorsal artery (dorsal aorta), running along the
median dorsal line of the alimentary canal, and a ventral or subintestinal
vein similarly related to the ventral surface of the digestive tube. The
two vessels were connected by a series of pairs of lateral branches, which
had their origins from the dorsal vessel, and, by their subdivision, formed
a capillary network in the walls of the alimentary canal. From these
networks paired veins issued and opened into the subintestinal vein. The
simplicity of this primitive arrangement was somewhat disturbed in the
region of the pharynx by the development of gill-clefts, in the walls of
which the blood circulated for respiratory purposes from the ventral to the
dorsal vessel; and also by the development of a hepatic portal circulation
in connexion with the liver. In the latter instance the subintestinal vein
entered the liver and subdivided into capillaries in the substance of that
organ, the corresponding efferent vessel, or hepatic vein, becoming
continuous with the anterior or pharyngeal section of the subintestinal
vein, or, as it is usually termed, the ventral aorta. In this low grade of
vascular system, which is perhaps most completely retained in Amphioxus,
the circulation of the blood was probably effected by the wave-like
contractions of more or fewer of the larger vessels; but subsequently a
definite chambered heart was developed at the origin of the ventral aorta.
Of Fishes in general it may be said that the primitive dorsal and ventral
vessels are present in the embryo, and for a time retain their original
relations and physiological importance. To a very unequal extent they may
also be retained in the adult, where, however, they co-exist with numerous
other vessels, which the increasing differentiation of the body has called
into existence. Thus, at a later period of embryonic life, the
subintestinal vein becomes somewhat fragmentary. Its caudal section (caudal
vein) ceases to be continuous with the precaudal portion, and the blood
collected from the muscles and other structures of the tail is conveyed to
the heart by a pair of posterior cardinal veins, which are either directly
continuous with the caudal vein, or indirectly through the intervention of
a renal portal system in the kidneys. {315}The precaudal portion of the
subintestinal vein is represented by a vein which runs forwards in the
intestinal wall, and is one of the minor affluents of the hepatic portal
vein, while its prehepatic section is represented in succession by the
hepatic vein, the heart, and the ventral aorta. Of the additional veins
which supplement these remnants of a primitively continuous subintestinal
vein, the largest and most constant are (_a_) the posterior cardinal veins
which, commencing in the kidneys and receiving the blood from those organs,
pass forwards to the heart; (_b_) a pair of anterior cardinal veins, formed
by the union of smaller veins from the head, including the brain, and
passing backwards towards the heart. At the level of the latter organ each
anterior cardinal vein joins the posterior cardinal of the same side of the
body to form a short but wide transverse vessel, the Cuvierian duct or
precaval vein, which opens into the hindermost of the cavities of the
heart, viz. the sinus venosus; (_c_) a pair of inferior jugular veins by
which the nutrient blood of the branchial apparatus is returned to the
right and left Cuvierian ducts. In addition to these principal veins there
may also be a pair of lateral veins collecting the blood from the lateral
walls of the trunk, and also opening into the Cuvierian ducts; and
subclavian and femoral veins from the pectoral and pelvic fins.
On the other hand, the primitive dorsal vessel (dorsal aorta), retains not
only its original position and relations, but also its primary function as
the main channel for the distribution of arterialised blood to all parts of
the body. The system of lateral and probably segmentally arranged vessels,
by which the dorsal and subintestinal vessels were connected in the
primitive Vertebrata, have undergone considerable modification in all
existing Fishes, but nevertheless retain much of their original disposition
and relations in the pharyngeal region of the alimentary canal, where they
are represented by the afferent and efferent vessels of the gills.
A more detailed account of the condition of the vascular system in the
Cyclostomata and Fishes will now be given.
THE VENOUS SYSTEM.—The Cyclostomata,[359] as might be expected, exhibit a
more primitive condition of the venous system {316}in certain features than
is the case in any other group. The precaudal portion of the subintestinal
vein retains much of its original importance and runs in the rudimentary
intestinal spiral valve as far as the liver, where it becomes the hepatic
portal vein. From the liver the blood is collected into a _single_ hepatic
vein, and by it is conveyed to the sinus venosus. The caudal section of the
subintestinal vein, now known as the caudal vein, bifurcates near the anus,
and its two branches become directly continuous with the right and left
posterior cardinals, without forming a renal portal system. In their
forward course to the heart the posterior cardinals are situated directly
beneath the notochord, and after receiving the blood from the kidneys and
gonads, and from the numerous pairs of segmental veins of the body-wall,
join the corresponding anterior cardinal veins, and form on each side a
short transverse Cuvierian duct which opens into the sinus venosus. There
is also a pair of inferior jugular veins which, however, unite opposite the
fifth pair of gill-sacs to form a single trunk; this vessel is continued
backwards, externally to the medio-ventral cartilage of the branchial
basket, and finally opens directly into the sinus venosus.
In Elasmobranchs (e.g. _Mustelus antarcticus_)[360] the caudal vein (Fig.
186) lies in the haemal canal of the caudal portion of the vertebral
column. On reaching the kidneys the vein divides into two renal portal
veins, which, however, are not directly continuous with the posterior
cardinal veins as in the Cyclostomata, but, on the contrary, after
receiving the posterior segmental and oviducal veins, become continuous
with the capillaries of the kidneys.
From the latter organs the blood is collected by a series of renal veins,
and by them conveyed to the posterior cardinals, and thence to the
Cuvierian ducts. In the adult, therefore, there is a well-developed renal
portal system, but it is worthy of note, nevertheless, that this system is
developed comparatively late in embryonic life, and that at an earlier
stage the caudal vein is directly continuous with the two posterior
cardinals, precisely as is the case in the Cyclostomata throughout life.
The posterior cardinal veins are situated in the dorsal wall of the coelom
(Fig. 187), beneath the vertebral column. For the hinder portion of
{317}their extent they are embedded in the kidneys (Fig. 186); and in this
region the two veins are in close relation in the median line, and here and
there freely communicate with each other. More anteriorly, they enlarge so
much that they present the appearance of cavernous sinuses. In addition to
the anterior segmental and oviducal veins, the posterior cardinals receive
the spermatic or the ovarian vein from the male or female gonad.
The precaudal section of the primitive subintestinal vein, now termed the
internal intestinal vein (Figs. 186 and 187), traverses the spiral valve as
it passes forwards to the liver, but from a physiological point of view is
now merely one of the factors of the great hepatic portal vein, the
principal tributaries of which are the veins from the stomach and
intestine, including the rectal gland, and the pancreas and spleen. On
entering the liver the hepatic portal vein divides into two principal
branches for the right and left halves of the gland. From the liver the
blood is conveyed by two hepatic veins to the sinus venosus.
[Illustration: FIG. 186.—Venous system of _Mustelus antarcticus_. _a_,
Auricle; _a.c_, anterior cardinal; _b.a_, conus arteriosus; _br.v_,
brachial vein; _c.d_, Cuvierian duct or precaval vein; _c.v_, caudal vein;
_cl.v_, cloacal vein; _f.v_, femoral vein; _h.s_, hyoidean sinus; _h.v_,
hepatic vein; _i.i.v_, internal intestinal vein; _i.j_, inferior jugular;
_k_, kidney; _l_, liver; _l.v_, lateral vein; _md.v_, mandibular vein;
_n.h.v_, nutrient hyoidean veins; _o.s_, orbital sinus; _p.c_, posterior
cardinal; _p.v_, hepatic portal vein; _rp.v_, renal portal vein; _sc.v_,
subscapular vein; _sp.v_, spermatic vein; _s.v_, sinus venosus; _v_,
ventricle; _v.a_, ventral aorta. (After T. J. Parker.)]
The lateral veins (Fig. 186) are situated in the lateral walls {318}of the
abdomen, immediately external to the peritoneum (Fig. 187). Each vein
begins near the pelvic fin, where it is connected with its fellow across
the dorsal face of the ischio-pubic cartilage, and thence runs forward
towards the pectoral fin. At its origin the lateral vein receives a femoral
vein from the pelvic fin and a cloacal vein, and also, near its anterior
end, a brachial vein from the pectoral fin, finally joining the Cuvierian
duct of its side.[361]
[Illustration: FIG. 187.—Diagrammatic transverse section of an
Elasmobranch, showing the position of the principal longitudinal
blood-vessels. _c_, Coelom; _d.a_, dorsal aorta; _d.c.v_, dorsal cutaneous
vein; _d.i.v_, dorsal intestinal vein; _i_, intestine; _i.i.v_, internal
intestinal vein; _l.c.v_, lateral cutaneous vein; _l.v_, lateral vein;
_m.v.a_, myelonic vein and artery; _p.c.v_, posterior cardinal vein;
_sp.c_, spinal cord; _sp.v_, spiral valve; _v_, vertebral centrum; _v.c.v_,
ventral cutaneous vein; _v.i.v_, ventral intestinal vein. (From T. J.
Parker.)]
The anterior cardinal vein is situated directly above the gill-arches of
its side of the head, and extends forwards from its junction behind with
the Cuvierian duct to the outer side of the auditory capsule, where it
communicates by a valvular orifice with a large sinus surrounding the
eye-muscles (orbital sinus), and ventrally, by means of a similar aperture,
with another large sinus, the hyoidean sinus, which lies on the outer face
of the corresponding hyoid arch, and is continuous ventrally with its
fellow of the opposite side. Into the orbital sinus open the anterior
facial vein from the anterior and external regions of the head, and the
anterior cerebral vein from the lateral half of the brain, and, into the
hyoidean sinus, the nutrient veins from the hyoidean hemibranch.
The inferior jugular veins are situated beneath the branchial apparatus.
Each vein begins anteriorly by communicating with {319}the hyoidean sinus
of its side, and, after receiving the nutrient veins from the holobranchs
of the first four branchial arches, opens into the corresponding Cuvierian
duct.
The venous blood from the heart itself is collected into two coronary
veins, which open into the sinus venosus.
In addition to the more important veins already described, there is also a
series of median and lateral cutaneous veins communicating at different
points with certain of the more deeply seated veins (Fig. 187).
Characteristic features in the venous system of _Mustelus_, as also of
Elasmobranchs in general, are the development of transverse connexions
between certain of the principal paired veins, and the tendency of many of
the main veins to enlarge into more or less irregularly-shaped sinuses.
In its broad outlines the venous system of the Teleostomi agrees with that
of Elasmobranchs, but is nevertheless characterised by several more or less
important modifications, while at the same time exhibiting many differences
in minor details.
A renal portal system is usually present, but is singularly variable in the
source of its tributary veins, even in closely allied forms.[362] In the
Sturgeon (_Acipenser_) and in some Teleosts, as in the Siluroid, _Amiurus
catus_, it resembles that of Elasmobranchs. In other Teleosts, on the
contrary, the renal portal system presents various grades of degeneration,
or, possibly, of imperfect evolution, as will be seen from the following
illustrations of its condition in different genera.
In _Amiurus_ the caudal vein, after giving off right and left renal portal
veins to the renal capillaries, emerges from the ventral surface of the
kidneys, and is then continued forwards between the gonads, the veins from
which it receives, as the radicle of the hepatic portal vein.
In the Eel (_Anguilla vulgaris_) the caudal vein (Fig. 188) traverses the
fused hinder portions of the kidneys, receiving several segmental veins
from the body-wall and also giving off from each side numerous renal portal
branches. More anteriorly, where the two kidneys become distinct, the
caudal vein also divides into two renal portal veins and, as each vein
extends {320}forwards along the outer border of the kidney of its side, it
receives a number of segmental veins, and, at the same time, gives off
branches to the renal capillaries. In addition, each renal portal vein is
connected with the hepatic portal vein by a series of singular arch-like
vessels into which the ovarian or spermatic veins open.
It is obvious, therefore, that in both _Amiurus_ and _Anguilla_ the
primitive direct continuity of the caudal and posterior cardinal veins has
been interrupted by the formation of a well-developed renal portal system,
and further, that the residue of the caudal venous blood finds its way to
the liver through the hepatic portal vein; hence it follows that, as in so
many of the lower air-breathing Vertebrates, the whole of the venous blood
from the tail is distributed either to the kidneys or liver in the course
of its return journey to the heart.
The Tench (_Tinca vulgaris_) exhibits the interesting anomaly of possessing
two caudal veins, a dorsal and a ventral (Fig. 189). The dorsal vein is
directly continuous with the right posterior cardinal, while the ventral
one divides into three branches, two forming right and left renal portal
veins and receiving numerous segmental veins, and the third becoming one of
the affluents of the hepatic portal vein. In this Teleost it is clear that
a portion of the caudal blood passes directly to the heart through the
right posterior cardinal without traversing either the renal portal or
hepatic portal system.
[Illustration: FIG. 188.—Renal portal circulation in the Eel (_Anguilla
vulgaris_). _c.v_, Caudal vein; _i.v_, intestinal vein; _l.p.c_, _r.p.c_,
left and right posterior cardinal veins; _p.v_, hepatic portal vein; _R_,
kidney; _r.p.v_, _r.p.v′_, renal portal veins; _sg.v_, segmental veins;
_x_, arch-like anastomoses between the renal portal and hepatic portal
veins; _y_, vein from the urinary bladder. (From Jourdain.)]
In the Cod (_Gadus morrhua_) the caudal vein divides into two branches.
The larger right vein retains its direct continuity {321}with the
corresponding posterior cardinal; the left, on the contrary, has ceased to
be continuous with the greatly reduced left posterior cardinal and forms a
renal portal vein, the distribution of which is, however, restricted to the
hinder portion of the left kidney (Fig. 190). As in _Amiurus_, a branch of
the caudal vein forms one of the tributaries of the hepatic portal vein. In
the Cod it would therefore seem that only a relatively small proportion of
the caudal blood flows through the imperfectly developed renal portal
system, the bulk of it traversing the right posterior cardinal and passing
directly to the heart, leaving, nevertheless, a modicum for transmission to
the liver. Finally, it may be mentioned that in some Teleosts the caudal
vein retains its embryonic continuity with one, usually the right,
posterior cardinal, without giving off a renal portal affluent, as in the
Perch (_Perca fluviatilis_); or, after division, with both posterior
cardinals, as in the Lump-sucker (_Cyclopterus lumpus_). In such instances
as these no portion of the caudal blood traverses the kidneys, and if a
renal portal system exists at all, the only true renal portal veins are the
adjacent segmental veins, which transmit venous blood directly to the
kidneys, instead of first uniting with renal portal branches of the caudal
vein as in the Tench and the Eel.
[Illustration: FIG. 189.—Renal portal system in the Tench (_Tinca
vulgaris_). _d.c.v_, _v.c.v_, Dorsal and ventral caudal veins; _k_, kidney;
_l.p.c_, _r.p.c_, left and right posterior cardinal veins; _p.v_, hepatic
portal vein; _r.p.v_, renal portal vein; _sg.v_, _sg.v′_, segmental veins.
(From Jourdain.)]
Whatever may be the condition of the renal portal system, all the renal
blood is eventually collected by renal veins and conveyed to the posterior
cardinals, which are often connected by one or {322}by several transverse
anastomoses (Fig. 190). In the region of the heart each posterior cardinal
joins the corresponding anterior cardinal to form a short but wide
Cuvierian duct, which finally opens into the sinus venosus.
A subintestinal vein is present in the embryo (e.g. _Lepidosteus_,
_Acipenser_, and some Teleosts),[363] but in the adult Teleostome its
precaudal section is usually absorbed, or at all events ceases to be
recognisable except, perhaps, as one of the minor tributaries of the
hepatic portal vein.[364]
The hepatic portal vein is formed as in Elasmobranchs, but in different
Teleostomi it may also receive the veins from the pyloric caeca, from a
portion of the air-bladder, the gonads, and, as previously mentioned, a
tributary from the caudal vein. There are usually two hepatic veins opening
into the sinus venosus, and generally of equal size (Fig. 190).
[Illustration: FIG. 190.—Venous system of a Teleost (diagrammatic). _A_,
Auricle; _ab.v_, vein from the air-bladder; _a.c_, anterior cardinal;
_c.d_, Cuvierian duct; _c.p.c_, transverse anastomoses between the two
posterior cardinals; _c.v_, caudal vein; _h.v_, hepatic vein; _i.j_,
inferior jugular; _k_, kidney; _l_, liver; _p.c_, left posterior cardinal;
_p.v_, hepatic portal vein; _r.p.c_, right posterior cardinal; _r.p.v_,
renal portal vein; _sc.v_, subclavian vein; _sg.v_, segmental vein; _sp.v_,
spermatic vein; _s.v_, sinus venosus.]
Most of the veins from the air-bladder join the hepatic portal {323}vein,
as already mentioned (Fig. 190), but more or fewer of them, especially
those from the dorsal wall of the organ, open into the posterior cardinals.
They may, as in _Polypterus_, even join the hepatic veins.[365]
The veins from the gonads are very variable in their destination, sometimes
joining the posterior cardinals, as in the Salmon (_Salmo salar_); or the
hepatic portal vein, as in _Amiurus_; or, as in the Perch (_Perca
fluviatilis_), forming by their union a single trunk, which communicates
directly with the left Cuvierian duct.
Representatives of the great lateral veins of Elasmobranchs appear to be
absent in the Teleostomi, the veins from the pectoral and pelvic limbs
joining the Cuvierian duct and the posterior cardinal veins respectively.
The two large anterior cardinal veins, which collect the blood from the
head and brain, occupy their usual position directly above the branchial
apparatus, and are sometimes connected by transverse anastomoses as they
pass backwards to join the Cuvierian ducts. The inferior jugular vein is
either single (e.g. _Gadus_); or paired, as in _Perca_ (Fig. 190).
In the Dipnoi the venous system is distinguished by an interesting
combination of characters, some of which are either primitive or peculiar
to the group, while others exhibit a distinct transition to the embryonic
or the adult condition of the lower air-breathing Vertebrates.
In _Neoceratodus_[366] (Fig. 191) the renal portal system is unusually
complex, the veins distributing venous blood to the kidneys being derived
from several sources, as follows: (1) from each of the two branches into
which the caudal vein divides on its exit from the haemal canal (_af.r.v_);
(2) from a common trunk (_pt.v_) which, on each side, is formed by the
union of segmental veins from certain of the post-cloacal myotomes and is
united with its fellow by a transverse anastomosis; (3) from more
anteriorly situated intercostal or segmental veins (_i.c.v_) which enter
each kidney directly; and (4) from a vein on each side corresponding to the
renal portal vein of Amphibia. The latter vein (_rp.v_) is formed by one of
the two branches of the iliac or femoral vein, and joins {324}the
corresponding vein from the caudal myotomes; from the common trunk numerous
branches enter the kidney.
[Illustration: FIG. 191.—Venous system of _Neoceratodus_. _a.ab_, Anterior
abdominal; _af.r.v_, afferent renal veins; _b.v_, brachial; _c.d_,
Cuvierian duct; _c.v_, caudal; _h.p.v_, hepatic portal; _h.v_, hepatic;
_i.c.v_, intercostal veins; _i.j_, inferior jugular; _il.v_, iliac;
_i.v.c_, inferior vena cava or postcaval; _k_, kidney; _l_, liver; _l.a_,
left auricle; _l.p.c_, left posterior cardinal; _l.v_, lateral cutaneous
vein; _pt.v_, vein from postcloacal myotomes; _p.v_, pulmonary vein;
_pv.v_, pelvic; _r.a_, right auricle; _rp.v_, renal portal; _s.c.v_,
subscapular; _s.j_, superior jugular or anterior cardinal; _t_, testis;
_v_, ventricle; _v.v_, right vertebral vein. (After Baldwin Spencer.)]
In the derivation of renal portal veins from each of the two veins into
which the caudal vein divides, _Neoceratodus_ approaches the Elasmobranchs.
On the other hand, the utilisation of ordinary segmental veins from the
caudal and pre-caudal myotomes, some of which directly enter the kidney, is
a feature which has already been remarked in some Teleosts; while the
formation of a renal portal affluent by a branch of the femoral vein is an
even more striking Amphibian characteristic.
The efferent renal veins[367] join the root of the left posterior cardinal
and the adjacent portion of the caudal vein.
Of the two great venous trunks into which the caudal vein divides, the
right is much the larger and behaves somewhat differently to the left. The
former (_i.v.c_) passes forwards in relation with the right kidney,
receiving in its course the spermatic or ovarian veins from the gonad of
its side, and then traverses the liver, finally opening into the median
portion of the sinus venosus, between the orifices of the two hepatic
veins. The left branch of the caudal vein (_l.p.c_) also passes forwards in
relation with the left {325}kidney and receives veins from the
corresponding gonad; but, instead of traversing the liver, it passes above
that organ, and finally opens into the left Cuvierian duct. The course of
the left vein, and the relations of the vessel to the caudal vein and the
left Cuvierian duct, point to the conclusion that it represents the left
posterior cardinal of other Fishes. From its continuity with the caudal
vein it is also obvious that the hinder or renal portion of the right trunk
is a remnant of the right posterior cardinal; but the more anterior section
so closely resembles the postcaval vein, or inferior vena cava of the
higher Vertebrates, in its relations to the liver, the hepatic veins, and
the sinus venosus, that its identity as such seems beyond doubt, and this
interpretation is supported by well-known observations[368] on the mode of
origin of the inferior vena cava in Amphibia, and especially the union of
the independently formed inferior vena cava with the posterior or
inter-renal portion of the embryonic right posterior cardinal vein,
combined with the atrophy of the anterior portion of the latter vein.[369]
The singular connexions and relations of these two great veins afford an
additional illustration of the significant transitional condition of the
venous system in the Dipnoi. On the other hand, the direct continuity of
the caudal vein with vessels which, wholly or in part, represent the two
posterior cardinals, is a feature alike characteristic of the adult
Cyclostome and the embryonic Elasmobranch, Teleost, and Amphibian.
As in the Cyclostomes and Elasmobranchs, the precaudal section of the
embryonic subintestinal vein is represented in the adult by an
intra-intestinal vein which traverses the spiral valve near its free edge
and is a tributary of the hepatic portal vein.
The two veins from the undivided air-bladder unite to form a single vessel,
which, instead of joining the hepatic portal or posterior cardinal veins as
in other Fishes, opens into the left auricle, like the pulmonary veins of
the Amphibia.
A further resemblance to the Amphibia is to be found in the presence of an
anterior abdominal vein. After leaving the pelvic {326}limb each femoral
vein divides into two branches; one of these forms a renal portal vein as
previously described; the other, which may rightly be termed a pelvic vein
(_pv.v_), unites with its fellow to form a median anterior abdominal vein
(_a.ab_). Pursuing its course forwards in the ventral abdominal wall, the
vein eventually reaches the heart and opens into the sinus venosus. The
direct connexion of the anterior abdominal vein with the heart is yet
another example of the retention in the adult _Neoceratodus_ of a
transitory embryonic feature in the developing Amphibian.[370]
[Illustration: FIG. 192.—Venous system of _Protopterus_. _a_, Auricle;
_a.c_, anterior cardinal; _an.v_, anastomotic veins; _c_, intestine; _f.v_,
femoral or iliac vein; _g.b_, gall-bladder; _h.p.v_, hepatic portal vein;
_i.j.v_, inferior jugular; _ov.v_, ovarian veins; _p_, pericardium;
_p.c.v_, left posterior cardinal; _p.v′_, parietal or segmental veins; _s_,
stomach; _sb.v_, subclavian. Other reference letters as in Fig. 191. (From
Newton Parker.)]
As in other Fishes, the blood from the head is conveyed to the Cuvierian
ducts by an anterior cardinal and an inferior jugular on each side. There
are no lateral veins, the blood from the pelvic fins flowing into the renal
portal system or into the anterior abdominal vein, and that from the
pectoral fin through subscapular and brachial veins into the Cuvierian
ducts. Lateral cutaneous veins are, however, present; and, as in
Elasmobranchs (e.g. _Mustelus antarcticus_), anastomose anteriorly with the
subscapular vein and behind with the caudal vein.
{327}Less is known of the venous system of _Protopterus_,[371] but it is
certain, nevertheless, that it presents a more advanced grade of evolution
than in _Neoceratodus_, and, except for the doubt as to the existence of an
anterior abdominal vein, it is essentially similar to that of a Urodele
Amphibian in which the right posterior cardinal vein has aborted.
The caudal vein (Fig. 192) divides into right and left renal portal
branches, neither of which, however, is directly continuous with the
inferior vena cava or the left posterior cardinal; on the contrary, each
renal portal vein is joined by the corresponding iliac or femoral vein, and
also by numerous segmental veins, and then distributes the whole of its
venous blood to the kidney. The radicles of the inferior vena cava and the
left posterior cardinal are formed by the renal veins from the two kidneys,
and in their forward course to the heart both veins receive in addition
genital and segmental veins. In its course through the liver the inferior
vena cava receives several hepatic veins, and finally opens into the sinus
venosus, while the left posterior cardinal vein joins the corresponding
Cuvierian duct, which also receives anterior cardinal, inferior jugular,
and subclavian veins. There is an intra-intestinal vein as in
_Neoceratodus_, but an anterior abdominal vein has yet to be discovered.
The two pulmonary veins from the double air-bladder form a single trunk
before communicating with the left auricle.
With the exception of certain doubtful details which need further
investigation, the venous system of _Lepidosiren_[372] seems to resemble
that of _Protopterus_.
THE HEART.—The heart is more anteriorly placed than in other Vertebrates,
being situated directly behind and beneath the last pair of branchial
clefts and internal to the ventral portion of the pectoral girdle. The
organ is enclosed in a pericardial cavity, which, in the adult, is
separated from the abdominal portion of the coelom by a transverse
pericardio-peritoneal septum, and in the Lamprey (_Petromyzon_) is
partially enclosed within a cartilaginous, cup-like modification of the
hinder part of the branchial basket. In the Ammocoetes-stage of the Lamprey
the pericardium is in communication behind with the general coelom, but the
connexion is lost in the adult. In Elasmobranchs the {328}two cavities are
connected by a single pericardio-peritoneal canal, or by two such canals;
and in _Chimaera_, and in the Sturgeon (_Acipenser_) and _Polyodon_, by a
single canal.
[Illustration: FIG. 193.—Diagram of the structure of the heart in different
Fishes. A, In an Elasmobranch; B, in _Amia_; and C, in a Teleost. _a_,
Auricle; _b.a_, bulbus aortae; _c.a_, conus arteriosus; _s.v_, sinus
venosus; _v_, semi-lunar valves; _v′_, auriculo-ventricular valve; _v.a_,
ventral aorta; _vt_, ventricle. (From Boas.)]
The heart consists of at least three chambers, a sinus venosus which
receives the venous blood from the body, an auricle and a ventricle, to
which is added a conus arteriosus in the Elasmobranchs, certain Teleostomi
(Crossopterygii, Chondrostei, and Holostei), and in the Dipnoi. Through
these cardiac chambers the blood is forced in the order mentioned. In the
Dipnoi the auricle is subdivided by a more or less complete interauricular
septum into a right and left auricle,[373] the former receiving the venous
blood from the sinus venosus, and the latter the aerated blood from the
lung-like air-bladder.
The sinus venosus and the auricle have very thin walls; the ventricular
walls, on the contrary, are very thick and in great measure are composed of
a sponge-like network of muscular bundles which generally encroaches
considerably on the ventricular cavity. Membranous valves, the
sinu-auricular, and the auriculo-ventricular valves, are developed at the
junctions of the sinus venosus with the auricle, and the auricle with the
ventricle respectively. The conus arteriosus is muscular and contractile,
and is interposed between the ventricle and the root of the ventral aorta.
Internally, the conus is provided with several transverse rows of
pocket-shaped or semilunar valves. In Teleosts the conus is non-muscular
and vestigial, and has but a single row of valves, corresponding to the
most anterior of the multiple rows of valves in the Elasmobranchs. In these
{329}Fishes the vestigial conus is succeeded by a non-contractile,
bulb-like dilatation, or bulbus aortae, of the root of the ventral aorta.
In only a single Teleost, viz. _Albula_, one of the Albulidae, is the
vestigial conus muscular, and at the same time provided with two rows of
valves.[374] In the Cyclostomata there is a bulbus with a single row of two
valves, but no true conus.
In the Dipnoi (e.g. _Protopterus_) the heart, like the rest of the vascular
system, exhibits certain interesting resemblances to the Amphibian heart.
In addition to a more or less complete interauricular septum separating
right and left auricles, there is a median longitudinal ridge, partly
muscular and partly fibrous, which incompletely subdivides the cavity of
the ventricle. The spirally-twisted conus arteriosus is furnished with
several transverse rows of valves, certain of which coalesce longitudinally
to form a complete septum dividing the cavity of the conus into two
distinct lateral channels: with this septum there coalesces another septum,
which cuts off the origins of the anterior two pairs from the remaining
afferent branchial arteries. The formation of these septa has the
physiological effect of subdividing the series of cardiac cavities into two
parallel channels, of which one has its origin behind in the sinus venosus
and transmits venous blood to the posterior afferent branchial vessels;
while the other, commencing with the left auricle, conveys arterial blood
to the first two pairs of afferent branchial arteries.[375] In
_Neoceratodus_, however, the longitudinal septum in the conus is
incomplete, and hence the blood which is sent to the anterior afferent
vessels is mixed.[376]
THE ARTERIAL SYSTEM.—The ventral aorta is a median artery situated beneath
the floor of the pharynx, and having its origin, behind, either directly
from the ventricle or from the conus arteriosus.
In the Cyclostomata[377] (e.g. _Petromyzon_) the ventral aorta (Fig. 194)
is continued forwards from the heart as a single vessel to the fourth pair
of gill-sacs, where it divides into right and left branches which extend as
far as the anterior walls of the first pair of gill-sacs. Eight pairs of
afferent branchial arteries arise from the ventral aorta and its two
branches, of {330}which the first and last supply the anterior walls of the
first pair of sacs and the posterior walls of the last pair respectively.
Each of the remaining afferent vessels extends into an interbranchial
septum, and supplies the gill-lamellae of the posterior wall of one sac and
those of the anterior wall of the next sac behind. The corresponding
efferent branchial vessels have a similar distribution, and unite dorsally
to form a median dorsal aorta. Beneath the base of the skull the latter
vessel divides into two branches which, after receiving the first pair of
efferent branchial vessels, pursue a divergent course forwards, but
subsequently converge and unite to form a "circulus cephalicus," as in
Teleosts. From the cephalic circle are given off on each side (1) an
"internal carotid" artery for the brain and eye; (2) an "external carotid"
for the lateral and ventral walls of the head; and (3) a large ventral
branch which supplies the lingual apparatus; while from the abdominal
portion of the dorsal aorta are derived, first, a coeliaco-mesenteric
artery for the liver and alimentary canal, and subsequently branches for
the myotomes, kidneys, and the gonad. The terminal portion of the aorta
then enters the tail and forms the caudal artery.
[Illustration: FIG. 194.—Branchial arterial system of the Lamprey
(_Petromyzon fluviatilis_). The ventral aorta and the afferent branchial
arteries are seen on the left side, and the efferent branchial and dorsal
aorta on the right (diagrammatic). _af.b.a_, _ef.b.a_, Afferent and
efferent branchial arteries; _a.o_, auditory organ; _b.c_, branchial canal;
_c.c_, cephalic circle; _cm.a_, coeliaco-mesenteric artery; _d.a_, dorsal
aorta; _e_, eye; _ex.c_, "external carotid"; _h_, heart; _i.b.s_,
interbranchial septum; _in.c_, internal carotid; _oph.a_, ophthalmic
artery; _r.v.a_, _l.v.a_, right and left ventral aorta; _v.a_, median
ventral aorta; _v.c_, "ventral carotid"; 1-7, gill-sacs. (Modified from
Vogt and Yung.)]
In Elasmobranchs[378] (e.g. _Mustelus antarcticus_) the undivided ventral
aorta gives off five pairs of afferent branchial arteries which, on each
side, ascend in succession the outer convex sides of the hyoid and first
four branchial arches (Fig. 195).
{331}[Illustration: FIG. 195.—The branchial arterial system of _Mustelus
antarcticus_. Left lateral view. The ventral aorta and afferent branchial
vessels are in solid black, the efferent arteries and their branches have
double contours. The branchial clefts have fringed borders to indicate
their hemibranchs, and the arches are in simple outline. _a.c.a_, Anterior
carotid; _a.d.a_, anterior dorsal aorta; _af.b.a_, afferent branchial
artery; _br.a_, brachial artery; _c.m.a_, coeliaco-mesenteric; _d.a_,
dorsal aorta; _E_, eye; _ep.a_, epibranchial artery; _H_, heart; _h.b.a_,
hypobranchial artery; _hy.a_, afferent pseudobranchial or hyoidean artery;
_md.a_, mandibular artery; _op.a_, ophthalmic artery; _p.c.a_, posterior
carotid; _sb.a_, subclavian; _sp_, spiracle; _v.a_, ventral aorta; 1-5, the
hyobranchial and four succeeding branchial clefts. The hypobranchial artery
is seen immediately beneath the ventral aorta. (After T. Jeffery Parker,
diagrammatic.)]