The deviations from this ground-plan of Hexactinellid structure are few and

simple. They are due to folding of the chamber-layer, or to variations in

the shape of the chambers, and to increasing fusion of the spicules to form

rigid skeletons. A simple condition of the chamber-layer, like that of the

young sponge of Fig. 89, {202}occurs also in some adult Hexactinellids,

_e.g._ in _Walteria_ of the Pacific Ocean (Fig. 90). Thus is represented in

this order the second type of canal system described among Calcarea. More

frequently, however, instead of forming a smooth sheet, the chamber-layer

grows out into a number of tubular diverticula, the cavities of which are

excurrent canals; these determine a corresponding number of incurrent

canals which lie between them. In this way there arises a canal system

resembling the third type of Calcarea. By still further pouching so as to

give secondary diverticula, opening into the first, a complicated canal

system is formed, as, for example, in _Euplectella suberea_.

To return to the skeleton, the most complete fusion is attained by the

deposit of a continuous sheath of silica round the apposed parallel rays of

neighbouring spicules. This may be termed the dictyonine type of union, for

it occurs in all those forms originally included under the term Dictyonina,

in which the cement is deposited _pari passu_ with the formation of the

spicules. In other cases connecting bridges of silica unite the spicules,

or there may be a connecting reticulum of siliceous threads, or, again,

rays crossing obliquely may be soldered together at the point of contact.

These more irregular methods occur in species where the spicules are free

at their first formation. Spicules originally free may later be united in a

true Dictyonine fashion. The terms LYSSACINA and DICTYONINA are useful to

denote respectively: the former all those Hexactinellida in which the

spicules are free at their first formation, and the latter those in which

the deposit of the cementing layer goes hand in hand with the formation of

the spicules. But the terms do not indicate separateness of origin of the

groups denoted by them, for there is evidence that Dictyonine types have

been derived repeatedly from Lyssacine types, and that in fact every

Dictyonine was once a Lyssacine.

[Illustration: FIG. 94.—Amphidisc, at _a_ are traces of the four missing

rays.]

The real or natural cleft in the class lies between those genera possessing

amphidiscs (Figs. 94, 97) among their microscleres, and all the remainder

of the Hexactinellida which bear hexasters (Fig. {203}96). The former set

of genera constitute the sub-class Amphidiscophora, the latter the

Hexasterophora.

[Illustration: FIG. 95.—Portion of body-wall of _Hyalonema_, in section,

showing the irregular chambers.]

SUB-CLASS 1. AMPHIDISCOPHORA.—_Amphidiscs are present, hexasters absent. A

tuft of rooting spicules or basalia is always present. The ciliated

chambers deviate more or less from the typical thimble shape, and the

membrana reticularis is continuous from chamber to chamber_ (Figs. 94, 95,

97).

[Illustration: FIG. 96.—Hexasters. A, Graphiohexaster; B, floricome; C,

onychaster.]

SUB-CLASS 2. HEXASTEROPHORA.—_Hexasters are present, amphidiscs absent. The

chambers have the typical regular form, and are sharply marked off from one

another_ (Figs. 90, 96).

All the Amphidiscophora have Lyssacine skeletons; in the Hexasterophora

both types of skeleton occur. The subdivision of the Hexasterophora is

determined by the presence or absence of uncinate spicules. An "uncinatum"

is a diactine spicule, pointed at both ends and bearing barbs all directed

towards one end. This method of classification gives us a wholly Dictyonine

order, UNCINATARIA, and an order consisting partly of Dictyonine, partly of

Lyssacine genera, which may be distinguished as the ANUNCINATARIA. {204}Ova

have rarely been found, and sexually produced larvae never; but Ijima has

found archaeocyte clusters in abundance, and his evidence is in favour of

the view that they give rise asexually to larvae, described by him in this

class for the first time (see p. 231).

Both sub-classes are represented in British waters: the Amphidiscophora by

_Hyalonema thomsoni_ and _Pheronema carpenteri_; the Hexasterophora by

_Euplectella suberea_ and _Asconema setubalense_, and of course possibly by

others.

_Hyalonema thomsoni_, one of the glass-rope sponges, was dredged by the

_Porcupine_ off the Shetland Islands in water of about 550 fathoms. The

spindle-shaped body of the sponge is shown in Fig. 97. Its long rooting

tuft is continued right up its axis, to end in a conical projection, which

is surrounded by four apertures leading into corresponding compartments of

the paragaster.

[Illustration: FIG. 97.—_Hyalonema thomsoni._ A, Whole specimen with

rooting tuft and _Epizoanthus_ crust; B, pinulus, a spicule characteristic

of but not peculiar to the Amphidiscophora, occurring in the dermal and

gastral membranes; C, amphidisc with axial cross; D, distal end of rooting

spicule with grapnel. (After F. E. Schulze.)]

The crust of Anthozoa of the genus _Epizoanthus_ (p. 406) on the rooting

tuft is a constant feature in this as in other species of _Hyalonema_. It

contributed to make the sponge a puzzle, which long defied interpretation.

The earliest diagnosis the genus received was the "Glass Plant." Then the

root tuft was thought to be part of the _Epizoanthus_, which was termed a

"most aberrant Alcyonarian with its base inserted in a sponge"; next we

hear of the sponge as parasitic {205}on the Sea Anemone. Finally, the root

tuft was shown to be proper to the sponge, which was, however, figured

upside down, till some Japanese collectors described the natural position,

or that in which they were accustomed to find it.

_Pheronema carpenteri_ was found by the _Lightning_ off the north of

Scotland in 530 fathoms. The goblet shaped, thick walled body and broad,

ill-defined root tuft are shown in Fig. 98, but no figure can do justice to

the lustre of its luxuriant prostalia and delicate dermal network with

stellate knots at regular intervals. The basalia are two-pronged and

anchor-like.

[Illustration: FIG. 98.—_Pheronema carpenteri._ × ½. (From Wyville

Thomson.)]

Both the Hexasterophoran genera were dredged off the north of Scotland, and

both conform to the Lyssacine type without uncinates. _Euplectella suberea_

is a straight, erect tube, anchored by a tuft {206}of basalia. The upper

end of the tube is closed by a sieve plate, the perforations in which are

oscula, while the beams contain flagellated chambers, so that the sieve is

simply a modified portion of the wall. It is a peculiarity of this as of

one or two other allied genera that the lateral walls are perforated by

oscula. They are termed parietal gaps, and are regularly arranged along

spiral lines encircling the body.

[Illustration: FIG. 99.—Sieve plate of _Euplectella imperialis_. (After

Ijima.)]

Ijima, who has dredged Euplectellids from the waters near Tokyo, finds that

in young specimens oscula are confined to the sieve plate; parietal gaps

are secondary formations. The groundwork of the skeleton is a lattice

similar to that shown in Fig. 100. The chamber-layer is much folded.

Various foreign species of _Euplectella_ afford interesting examples of

association with a Decapod Crustacean, _Spongicola venusta_, of which a

pair lives in the paragaster of each specimen. The Crustacean is light

pink, the female distinguished by a green ovary, which can be seen through

the transparent tissues. It is not altogether clear what the prisoner

gains, nor what fee, if any, the host exacts.

Ijima relates that the skeleton of _Euplectella_ is in great demand in

Japan for marriage ceremonies. He also informs us that the Japanese name

means "Together unto old age and unto the same grave," while by a slight

alteration it becomes "Lobsters in the same cell," and remarks that the

Japanese find this an amusing pun.

[Illustration: FIG. 100.—Skeletal lattice of _Euplectella imperialis_.

(After Ijima.)]

The same _Spongicola_ lives in pairs in _Hyalonema sieboldi_. Another case

of apparently constant association is that of the Hydroid stocks which

inhabit _Walteria_. F. E. Schulze describes _Stephanoscyphus mirabilis_

(see p. 318) in a specimen of _Walteria flemmingi_; the presence of the

polyp causes the sponge to grow out into little dome-shaped elevations,

each of which shelters one polyp; while in _W. leuckarti_ Ijima finds a

similar association in every specimen examined.

{207}FOSSIL HEXACTINELLIDA.

This group has the distinction of including among its Lyssacine members the

oldest known sponge, _Protospongia fenestrata_, of Cambrian age (Salter).

As preserved it consists of a single layer of quadriradiate, or possibly

quinqueradiate spicules, which, arranged as a square meshed lattice,

supported the superficial layer of the sponge (Fig. 101). Whether or not

the fossil represents the whole of the sponge-skeleton does not

appear.[236]

[Illustration: FIG. 101.—Part of the specimen of _Protospongia fenestrata_

in the Sedgwick Museum, Cambridge. Nat. size. (After Sollas.)]

[Illustration: FIG. 102.—A portion of the outer surface of a

Receptaculitid, _Acanthoconia barrandei_, in which the expanded outer rays

of the spicules are partially destroyed, revealing the four tangential rays

beneath, × 3. (After Hinde.)]

The extraordinary RECEPTACULITIDAE are probably early Lyssacine forms: they

are cup- or saucer-shaped fossils, abundant in Silurian and above all in

Devonian strata, and have been "assigned in turn to pine cones,

Foraminifera, Sponges, Corals, Cystideans," and Tunicata. Hinde[237] brings

forward important arguments for retaining them among Hexactinellida. The

only elements in the skeleton of the simpler genera, _e.g._ _Ischadites_,

are structures comparable to Hexactinellid spicules. The surface of the

fossil presents a series of lozenges forming a regular mosaic. Each lozenge

is the expanded end of one of the rays of a spicule; it conceals four rays

in one plane, tangential to the wall of the cup-shaped fossil, while the

sixth ray projects vertically to the wall into the cavity of the cup. In

the genus _Receptaculites_ itself there is an inner layer of plates

abutting against the inner {208}ends of the sixth rays, and at present

problematic. An axial canal is present in each of the rays—the six canals

meeting at the centre of the spicule. Special chinks between the spicules

appear to have provided a passage for the water current.

The beautiful _Ventriculites_, so common in the Chalk and present in the

Cambridge Greensand, are historically interesting, for the fact that they

are fossil Hexactinellida of which the general and skeletal characters were

very minutely described by Toulmin Smith long before recent representatives

of the group were known. In common with a number of fossil Dictyonine

species they are distinguished by the perforation of the nodes, a character

due to the fact that the siliceous investment which unites the spicules

together stops short before reaching the centre of each spicule, and

bridges across the rays so as to form a skeleton octahedron. This character

is rare in recent Hexactinellids, but, as first pointed out by Carter, it

is presented by one or two forms, of which _Aulocystis grayi_ Bwk is best

known. The majority of the fossil Hexactinellida belong to the Dictyonine

section, a fact attributable to the greater coherence of their skeleton.

The "Dictyonina" are to be reckoned among the rock-builders of Jurassic and

Cretaceous times.

[Illustration: FIG. 103.—A node of the skeleton of _Ventriculites_ from the

Cambridge Greensand. (After Sollas.)]

The OCTACTINELLIDA and HETERACTINELLIDA are two classes created by

Hinde[238] to contain certain little-known Devonian and Carboniferous

sponges, possessing in the one case 8-rayed spicules, of which 6 rays lie

in one plane and 2 are perpendicular to this plane; in the other case,

spicules with a number of rays varying from 6 to 30. Bearing in mind the

manner in which octactine spicules are known to arise in recent

Hexactinellida (p. 200), it is clearly possible to derive these 8-rayed

spicules from hexactines by some similar method; while the typical

{209}spicule of the Heteractinellida is a euaster. Hence we may refer the

Octactinellid fossils to the class Hexactinellida, and the Heteractinellid

forms either to the Monaxonida or Tetractinellida.

CLASS III. DEMOSPONGIAE

_Silicispongiae in which triaxonid spicules are absent._

This class has attained the highest level of organisation known among

Porifera; the most efficient current-producing apparatus is met with here,

so, too, are protective coverings, stout coherent skeletons, and the

highest degree of histological differentiation found in the phylum.

Correspondingly it is the most successful group, the majority of existing

sponges coming within its boundaries. A few genera and species are

exceedingly specialised, for example, _Disyringa dissimilis_ (p. 215).

These, however, contribute only a very small contingent to the Demosponge

population, those species which are really prolific and abundant being, as

we should expect, the less exaggerated types.

CANAL SYSTEM.—With a few exceptions the representatives of the Demospongiae

may be said to have taken up the evolution of the canal system at the stage

where it was left in _Leucandra aspera_—a stage which the ancestral

Demosponges must have reached quite independently of the Calcarea. These

commoner members are thus already gifted with the advantages pertaining to

a spherical form of ciliated chamber, and so, too, is the Rhagon (Fig.

105), an immature stage noteworthy as the simplest form of Demosponge, and

thus the starting-point for the higher types of canal system. The

exceptions above alluded to are not without interest: they are the

Dendroceratina, of doubtful affinities, (p. 220), which possess small

tubular Syconate chambers. They may be regarded either as of independent

origin from other Demospongiae, thus making the group polyphyletic, or more

simply as representing the ancestral condition, and in this case we must

look on the possession of spherical chambers by the Rhagon as a secondary

feature. Occupying as it does the important position above indicated, the

Rhagon merits a brief description. It is a small discoid or hemispherical

body attached by a flat base. It contains a central paragaster, with a

single osculum at the free end. Into the paragaster open directly a

{210}few spherical flagellated chambers, which lie in the lateral walls of

the body. The basal wall of the paragaster, the parts of its lateral walls

between the openings of neighbouring chambers, and the entire outer surface

of the body are covered with pinacocytes. It is convenient to call the

basal part of the sponge from which chambers are absent the hypophare, the

upper chamber-bearing part the spongophare. In some of the deeper dermal

cells spicules may be already present. In the Rhagon, then, the canal

system is of the second type, but all the adult Demosponges have advanced

to the third type, and the further evolution in this system is in the

direction of improving the mode of communication of the chambers with the

canal system. The changes involved go hand in hand with increasing bulk of

the dermal layer. A glance at the accompanying figures will show at once

the connexion between the phenomena. The increase in the dermal layer (1)

greatly reduces the extent of the lumen of the excurrent canals; and (2)

results in the intervention of a narrow tube or aphodus between the mouth

of each chamber and the excurrent canal. The chamber system is then

converted from an "eurypylous" to an "aphodal" type. When the incurrent

canal also opens into the chamber by way of narrow tubes, one proper to

each chamber and termed "prosodus," the canal system is of the "diplodal"

type.

[Illustration: FIG. 104.—Diagram of (A) eurypylous and (B) aphodal canal

systems. _a_, Apopyle; _a'_, aphodus; _E_, excurrent canal; _I_, incurrent

canal; _p_, prosopyle; _p'_, short prosodus. (After Sollas.)]

CORTEX.—All the stages in the formation of a cortex are to be seen among

the adult members of the group. Certain species (e.g. _Plakina monolopha_,

F.E.S.) are destitute even of an ectosome, {211}others have a simple dermal

membrane (_Halichondria panicea_, _Tetilla pedifera_) and various others

are provided with a cortex, either of simple structure or showing

elaboration in one or more particulars. Thus a protective armature of

special spicules may be present in the cortex, _e.g._ in _Geodia_, or to a

less extent in _Tethya_, or there may be an abundance of contractile

elements, and these may be arranged in very definite ways, forming

valve-like apparatus that will respond to stimuli.

Everywhere among sponges the goal of the skeleton appears to have been

coherence. We have seen how in Calcarea and in Hexactinellida this has been

attained by the secretion around the separate elements of a continuous

mineral sheath, calcareous in the one case and siliceous in the other. Here

we had an excellent instance of the attainment of one end by similar means

in two different groups, after their separation from the common stock, and

therefore independently. In Demospongiae, on the other hand, the same

end—coherence—has been secured by two new methods, each distinct from the

former: first the spicules may be united in strands by an organic deposit,

spongin; secondly, the spicules may assume irregular shapes and interlock

closely with one another, forming dense and stout skeletons. The latter

method is that characteristic of the Lithistid Tetractinellida.

CLASSIFICATION.—It is not of great moment which scheme of classification we

maintain, seeing that all hitherto proposed are confessedly more or less

artificial, and sufficient data for framing a natural one are not yet

forthcoming. For convenience, we accept three subdivisions and define them

thus:—

I. TETRACTINELLIDA.—Demospongiae possessing tetraxon or triaene spicules

or Lithistid desmas.

II. MONAXONIDA.—Demospongiae possessing monaxon but not tetraxon

spicules.

III. CERATOSA.—Demospongiae in which the main skeleton is formed of

fibres of spongin. The fibres may have a core of sand-grains or of

foreign spicules, but not of spicules proper to the sponge.

But at the same time we admit that some of the Ceratosa are probably

descended from some of the families of Monaxonida, so that we should

perhaps be justified in separating these families of Monaxonida from the

rest, and associating them with the allied families of Ceratosa—a method of

classification due to {212}Vosmaer. Again, some Monaxonida approximate to

Tetractinellida, and we might, with Vosmaer, unite them under the title

Spiculispongiae. This proceeding, though it has the advantage of being at

least an attempt to secure a natural classification, involves too much

assumption when carried out in detail to be wholly satisfactory.

SUB-CLASS I. TETRACTINELLIDA.[239]

Tetractinellida appear to flourish best in moderate depths from 50 to 200

fathoms, but they are found to be fairly abundant also in shallower water

right up to the coast line, and in deep water up to and beyond the 1000

fathom line. Occasionally they lie free on the bottom, but are far more

commonly attached; fixation may be direct or by means of rooting spicules;

the occurrence of a stalk is rare. There is great variety in the root tuft,

which may be a long loose wisp of grapnel-headed spicules, as in many

species of _Tetilla_, or a massive tangle, as in _Cinachyra barbata_; in

these cases the sponge is merely anchored, so that it rests at the level of

the surface of the ooze; in other cases, _e.g._ _Thenea wyvillei_, the root

tuft consists of a number of pillars of spicules which raise the sponge

above the level of the ooze, into which they descend and there become

continuous with a large dense and confused mass of spicules. The

parachute-like base of _Tetilla casula_ invites comparison with the

"Crinorhiza" forms of some Monaxonids (p. 216).

Two Orders are distinguished thus:—

I. CHORISTIDA.—Tetractinellida with quadriradiate spicules, which are

never articulated together into a rigid network.

II. LITHISTIDA.—Tetractinellida with branching scleres (desmas), which

may or may not be modified tetrad spicules, articulated together to form

a rigid network. Triaene spicules may or may not be present in addition.

ORDER I. CHORISTIDA.

_Plakina monolopha_, from the Adriatic and Mediterranean, furnishes a

connecting link between the Rhagon stage and other Tetractinellida. The

choanosome is simply folded; there is no distinct ectosome; the chambers

are eurypylous. The skeleton {213}consists of microcalthrops and their

derivatives. The hypophare is well developed. _Plakina_ thus shows a

certain amount of resemblance to _Oscarella_ (p. 196), with which it shares

the very remarkable possession of flagellated pinacocytes.

One of the species of _Tetilla, T. pedifera_, continues the series. The

folds of its choanosome are more complicated than in _P. monolopha_, and

their outer ends are bridged together by a thin layer of ectosome (cf.

species of _Sycon_ among Calcarea); the chambers are still eurypylous.

The skeleton reaches a high level: it includes oxeas and triaenes radiately

disposed and microscleres (sigmata) scattered throughout the dermal layer.

The British _Poecillastra compressa_ from the north of Scotland and Orkney

and Shetland is at about the same stage of development, being without

cortex and having eurypylous chambers, but it is not so good an example, as

the folds of its choanosome are confused.

[Illustration: FIG. 105.—Diagrammatic vertical sections of A, Rhagon; B,

_Plakina_; C, _Tetilla pedifera_.]

From _T. pedifera_ we pass to the other species of _Tetilla_ and all the

higher genera of Choristida; these possess a cortex not of homologous

origin in the various cases, but probably to be classified under one of two

heads, typified by _Stelletta_ and _Craniella_ respectively (Fig. 106).

[Illustration: FIG. 106.—A, _Craniella_ type; B, Stellettid type. _ch_,

Chone; _co_, collenchyma; _d.o_, dermal ostia; _fb_, fibrous tissue; _i.c_,

intercortical cavity; _sd_, subdermal cavity; _sp_, sphincter. (After

Sollas.)]

{214}In the Stellettids the cortex arises by the centrifugal growth of a

dermal membrane such as that of _Tetilla pedifera_; in _Craniella_ directly

from the dermal tissue of the distal ends of the choanosomal folds.

In both cases the end result, after completion of cell differentiation, is

a cortex either fibrous throughout or collenchymatous in its outer portion

and fibrous in the deeper layers. In the Stellettid type the centrifugal

growth of the dermal membrane involves the addition of secondary distal

portions to the ends of the inhalant passages. These are the intercortical

cavities or canals. Their most specialised form is the "chone." A chone is

a passage through the cortex opening to the exterior by one or more ostia,

and communicating with the deeper parts of the inhalant system by a single

aperture provided with a sphincter (Fig. 106, B).

In the _Craniella_ type the intercortical cavities are parts of the primary

inhalant system. They communicate with its deeper parts by sphinctrate

apertures. Without any knowledge of the development one would certainly

have supposed that the subdermal cavity, pore-sieve and sphinctrate

passages of _Craniella_ represented a number of chones, of which the outer

portions had become fused (Fig. 106, A).

[Illustration: FIG. 107.—_Disyringa dissimilis._ Diagrammatic longitudinal

section of the Sponge. × ½. _a_, _b_, _c_, Transverse sections at the

levels indicated to show subdivision of the lumina of the excurrent and

incurrent tubes; _e.t_, excurrent tube; _i.t_, incurrent tube; _o_,

osculum. (After Sollas.)]

In both _Craniella_ and _Stelletta_ the chamber system is aphodal, and

these genera may fairly be taken as representatives of the average level

reached by Tetractinellida. The skeleton is of the radiate type: the type

which prevails in the Choristida, but which has an erratic distribution,

appearing in some genera of {215}each family but not in others. The genus

_Pachymatisma_, of which we have the species _P. johnstonia_ and _P.

normani_ in these islands, exemplifies this; it belongs to the highly

differentiated family Geodiidae, possesses an elaborate cortex with chones,

but its main skeleton is non-radiate.

_Disyringa dissimilis_ is remarkable for the perfection of its symmetry,

and for the absence of that multiplication of parts which is so common

among sponges. It possesses a single inhalant tube and a single osculum

(Fig. 107). Until quite recently it stood alone in the restriction of its

inhalant apertures to a single area. Kirkpatrick, however, has now

described a sponge—_Spongocardium gilchristi_[240]—from Cape Colony, in

which the dermal ostia are concentrated in one sieve-like patch at the

opposite pole to the single osculum. _Disyringa_ is still without

companions in the possession of an inhalant tube. The concentration of

ostia into sieve areas occurs again in _Cinachyra_, each sponge possessing

in this case several inhalant areas with or without scattered ostia also.

ORDER II. LITHISTIDA.

The characteristic spicule of Lithistida—the desma—may be a modified

calthrop (tetracrepid desma), or it may be produced by the growth of silica

over a uniaxial spicule (rhabdocrepid desma) (Fig. 110, _q_), or it may be

of the polyaxon type. It is probable that the group is polyphyletic,[241]

and that some of its members should remain associated with Tetractinellida,

while others should be removed to Monaxonida. Forms with tetracrepid

desmas, and those forms with rhabdocrepid desmas which possess triaenes,

have Tetractinellid affinities, while forms possessing rhabdocrepid desmas

but lacking triaenes, and again those in which the desmas are polyaxon, are

probably descendants of Monaxonida.

Owing to the consistency of the skeleton Lithistida are frequently found as

fossils. The commonest known example is _Siphonia_.[242] As in the case of

so many other fossil sponges the skeleton is often replaced by carbonate of

lime, a fact which {216}misled some of the earlier investigators but was

established by the researches of Sollas and Zittel.

SUB-CLASS II. MONAXONIDA.[243]

The Monaxonida inhabit for the most part shallow water, but they also

extend through deep water into the abysses, thirteen species having been

dredged from depths of over 2000 fathoms by the "Challenger" Expedition

alone. In some cases, e.g. _Cladorhiza_, _Chondrocladia_, all the species

of a genus may live in deep water, while in others the genus, or in others,

again, the species, may have a wide bathymetrical range. Thus _Axinella_

spp. occur in shallow water and in various depths down to 2385 fathoms,

_Axinella erecta_ ranges from 90 to 1600 fathoms, _Stylocordyla stipitata_

from 7 to 1600, and so on. The symmetry of the deep-water forms contrasts

strikingly with the more irregular shape of their shallow-water

allies.[244] The shallow-water species are almost always directly attached,

some few are stalked; those from deep water have either a long stalk or

some special device to save them from sinking in the soft ooze or mud. Thus

the deep-sea genus _Trichostemma_ has the form of a low inverted cone,

round the base of which a long marginal fringe of spicules projects,

continuing the direction of the somal spicules, and so forming a supporting

rim. The same form has been independently evolved in _Halicnemia patera_,

and an approach to it in _Xenospongia patelliformis_. A similar and more

striking case of homoplasy is afforded by the Crinorhiza form, which has

been attained in certain species of the deep-sea genera _Chondrocladia_,

_Axoniderma_, and _Cladorhiza_; here the sub-globular body is supported by

a vertical axis or root, and by a whorl of stout processes radiating

outwards and downwards from it, and formed of spicular bundles together

with some soft tissue.

There is recognisable in the order Monaxonida a cleft between one set of

genera, typically corticate, and suggesting by their structure a

relationship, whether of descent or parentage, with the Tetractinellida,

and a second set typically non-corticate: these latter are the

Halichondrina, the former are the Spintharophora.

{217}ORDER I. HALICHONDRINA.

We have already seen typical examples of the Halichondrina in _Halichondria

panicea_ and _Ephydatia fluviatilis_. Within the Halichondrina the

development of spongin reaches its maximum among spiculiferous sponges, and

accordingly the Ceratosa take their multiple origin here (p. 220). Among

Halichondrina spongin co-operates with spicules to form a skeleton in

various ways, but always so as to leave some spicules bare or free in the

flesh. It may bind the spicules end to end in delicate networks (as in

_Reniera_ or _Gellius_), or into strands, sometimes reaching a considerable

thickness (as in _Chalina_ and others). In a few cases there appears to be

a kind of division of labour between the spicules and spongin, the latter

forming the bulk of the fibre, _i.e._ fulfilling the functions of support,

while the spicules merely beset its surface as defensive organs, rendering

the sponge unfit for food. Fibres formed on this pattern are called

plumose, and are typical of Axinellidae. The distinctive fibre of the

Ectyoninae is as it were a combination of the Axinellid and Chalinine

types: a horny fibre both cored with spicules and beset with them. Spicules

besetting the surface of a fibre are termed "echinating." Whenever its

origin has been investigated, spongin has proved to be the product of

secretion of cells; in the great majority of cases it is poured out at the

surface of the cell, and Evans showed,[245] at any rate in one species of

_Spongilla_, that the spongin fibres are continuous with a delicate cuticle

at the surface of the sponge. In _Reniera_ spp. occurs a curious case of

formation of spongin as an intracellular secretion. A number of spherical

cells each secrete within themselves a short length of fibre; they then

place themselves in rows, so orientated that their contained rods lie end

to end in one line. The rods then fuse and make up continuous threads; the

cells diminish in breadth, ultimately leaving the fibre free.[246]

ORDER II. SPINTHAROPHORA.

These corticate forms are further characterised by the arrangement of their

megascleres, which is usually, like that of most {218}Tetractinellida,

radial, or approximating to radial. The microscleres are, when present,

some form of aster. The cortex resembles that of Tetractinellida, and v.

Lendenfeld has described chones in _Tethya lyncurium_.[247]

The existence of the above points of resemblance between Spintharophora and

Tetractinellida suggests a relationship between the two groups as its

cause. In judging this possibility the following reflections occur to us. A

cortex exists in various independent branches of Tetractinellida. It has in

all probability had a different phylogenetic history in each—why not then

in these Monaxonida also? Within single genera of Tetractinellida some

species are corticate, others not, witness _Tetilla_. The value of a cortex

for purposes of classification may easily be overestimated. If we are to

uphold the relationship between these two groups, we must base our argument

on the conjunction of similar characters in each.

The genus _Proteleia_[248] is interesting for its slender grapnel-like

spicules, which project beyond the radially disposed cortical spicules, and

simulate true anatriaenes of minute proportions. That they are not

anatriaenes is shown by the absence of an axial thread in their cladi. It

is not surprising that a form of spicule of such obvious utility as the

anatriaene should arise more than once.

Of exceptional interest, on account of their boring habit, are the

Clionidae. How the process of boring is effected is not known; the presence

of an acid in the tissues was suspected, but has been searched for in vain.

The pieces of hard substance removed by the activity of the sponge take

their exit through the osculum and have a fixed shape[249] (Fig. 108).

As borers into oyster shells, Clionidae may be reckoned as pests of

practical importance, and in some coasts they even devastate the rocks,

penetrating to a depth of some feet, and causing them to crumble away.[250]

Sponges, however, as agents in altering the face of the earth do not figure

as destroyers merely. On the contrary, it has {219}been calculated[251]

that sponge skeletons may give rise with considerable rapidity to beds of

flint nodules; in fact, it appears that a period so short as fifty years is

sufficient for the formation of a bed of flints out of the skeletons of

sponges alone.

_Suberites domuncula_ is well known for its constant symbiosis with the

Hermit crab. The young sponge settles on a Whelk or other shell inhabited

by a _Pagurus_, and gradually envelops it, becoming very massive, and

completely concealing the shell, without however closing its mouth. The

aperture of this always remains open to the exterior, however great the

growth of the sponge, a tubular passage being left in front of it, which

continues the lumen of the shell and maintains its spiral direction. When

the crab has grown too big for the shell, it merely advances a little down

this passage. The shell is never absorbed, as was once supposed.[252] The

crab, besides being provided with a continually growing house, and being

thus spared the great dangers attending a shift of lodgings, benefits

continually by the concealment and protection afforded by the massive

sponge; the latter in return is conveyed to new places by the crab.

[Illustration: FIG. 108.—A, calcareous corpuscle detached by _Cliona_; B,

view of the galleries excavated by the Sponge. (After Topsent.)]

_Ficulina ficus_ is sometimes, like _S. domuncula_, found in symbiosis with

_Pagurus_, but the constancy of the association is wanting in this case.

The sponge has several metamps, one of which, from its fig-like shape,

gives it its name.

{220}SUB-CLASS III. CERATOSA.

The Ceratosa are an assemblage of ultimate twigs shorn from the branches of

the Monaxonid tree. They are therefore related forms, but many of them are

more closely connected with their Monaxonid relatives than with their

associates in their own sub-class.

The genera _Aulena_ and _Phoriospongia_, placed by v. Lendenfeld among

Ceratosa, by Minchin among Monaxonida, show each in its own way how close

is the link between these two sub-classes.

_Aulena_ possesses in its deeper parts a skeleton of areniferous spongin

fibres, in fact a typical Ceratose skeleton; but this is continuous with a

skeleton in the more superficial parts, which is composed of spongin fibres

echinated by spicules proper to the sponge, and precisely comparable to the

ectyonine fibres of some Monaxonida.

_Phoriospongia_, as far as its main skeleton is concerned, is a typical

Ceratose sponge, with fibres of the areniferous type, but it possesses

sigmata free in the flesh.

The sub-class is confined to shallow water, no horny sponge having been

dredged from depths greater than 410 fathoms.[253] The greatest number

occur at depths between 10 and 26 fathoms.

In the majority of the Ceratosa the skeletal fibres are homogeneous, formed

of concentric lamellae of spongin, deposited by a sheath of spongoblasts

around a filiform axis. In others, however, the axis attains a considerable

diameter, so as to form a kind of pith to the fibre, which is then

distinguished as heterogeneous. In one or two cases some of the

spongoblasts of a heterogeneous fibre are included in the fibre between the

spongin lamellae. _Ianthella_ is the best-known example in which this

occurs.

Ceratosa are divided into Dictyoceratina and Dendroceratina, distinguished,

as their names express, by the nature of the skeleton—net-like, with many

anastomoses, in the one; tree-like, without anastomoses between its

branches, in the other.

The Dictyoceratina comprise by far the larger number of Ceratosa. They fall

into two main families, the Spongidae and Spongelidae, both represented in

British waters. The Spongidae {221}are characterised by a granular ground

substance and aphodal chamber system; the Spongelidae by a clear ground

substance and sac-like eurypylous chambers.

The bath sponge, _Euspongia officinalis_, belongs to the Spongidae. The

finest varieties come from the Adriatic, the coarser ones from the

Dalmatian and North African coasts of the Mediterranean, from the Grecian

Archipelago, from the West Indies, and from Australian seas. The softer

species of the genus _Hippospongia_ also form a source of somewhat inferior

bath sponges.

Among Dendroceratina, _Darwinella_ is unique and tempts to speculation, in

that it possesses isolated spongin elements, resembling in their forms

triaxon spicules.

KEY TO BRITISH GENERA OF SPONGES.

{Skeleton calcareous 2

1. {Skeleton siliceous 6

{Skeleton horny, or without free spicules 53

{Skeleton absent 55

2. {Gastral layer continuous 3

{Gastral layer discontinuous, confined to chambers 4

3. {Equiangular triradiate systems present _Clathrina_

{Triradiate systems all alate _Leucosolenia_

4. {Chambers tubular, radially arranged 5

{Chambers spherical, irregularly scattered _Leucandra_

5. {Tufts of oxeate spicules at the ends of the chambers _Sycon_

{Oxeate spicules lying longitudinally in the cortex _Ute_

{All the spicules hexradiate or spicules easily derived from

6. { hexradiate type 7

{Some of the spicules calthrops or triaenes 10

{Megascleres uniaxial 15

7. {Amphidiscs present 8

{Amphidiscs absent 9

{Rooting spicules a well-defined wisp; four apertures lead into

8. { the gastric cavity _Hyalonema thomsoni_

{Rooting tuft diffuse; sponge oval; osculum single

{ _Pheronema carpenteri_

{Sponge tubular, dermal and gastral pinuli absent

9. { _Euplectella suberea_

{Sponge a widely open cup; dermal and gastral pinuli present

{ _Asconema setubalense_

{Tetractine spicule, a calthrop or triaene with short rhabdome;

10. { microsclere aspined microxea _Dercitus bucklandi_

{Triaenes with fully developed rhabdome 11

{222}[Illustration: FIG. 109.—Microscleres of Demospongiae. _a_, _b_,

Sigmaspires viewed in different directions; _c_, _d_, bipocilli viewed in

different directions; _e_, toxaspire; _f_, _f'_, spiraster; _g_,

sanidaster; _h_, amphiaster; _i_, sigma; _j_, diaucistra; _k_, isochela;

_l_, _m_, anisochelae viewed in different directions; _n_, cladotyle; _o_,

toxa; _p_, forceps; _q_, oxyaster; _r_, spheraster; _s_, oxyaster with 6

actines; _t_, another with 4 actines; _u_, another with rays reduced to two

(centrotylote microxea); _v_, tylote microrhabdus; _w_, trichodragmata;

_x_, oxeate microrhabdus or microxea.]

11. {Microscleres sigmata _Craniella cranium_

{Sigmata absent, asters present 12

{Microscleres include spirasters _Poecillastra compressa_

12. {Microscleres include sterrasters 14

{Microscleres include euasters: spirasters and sterrasters absent 13

{Two kinds of euaster present _Stelletta_

13. {Microscleres include a euaster and a sanidaster or amphiaster

{ _Stryphnus ponderosus_

14. {Microscleres include microrhabdi _Pachymatisma johnstonia_

{Microscleres include many-rayed euasters _Cydonium milleri_

15. {Some of the microscleres asters 16

{Microscleres absent, or not asters 17

{Skeleton radiate; asters of more than one kind _Tethya_

16. {Sponge encrusting; asters of one kind only _Hymedesmia_

{Skeleton fibrous _Axinella spp._

{Megascleres all diactinal; chelae present _Desmacidon_

17. {Megascleres all diactinal; chelae absent 18

{Some or all of the megascleres monactinal 19

18. {Habitat fresh water{223} 56

{Habitat marine 22

{Megascleres include cladotyles _Acarnus_

{Megascleres include dumb-bell or sausage-shaped spicules forming

{ the main reticulum _Plocamia_

{Microscleres include bipocilli 20

19. {Microscleres include diancistra _Hamacantha_

{Megascleres include forceps _Forcepia_

{Skeleton formed of isolated monactines vertically placed

{ _Hymeraphia_

{None of the above peculiarities present 21

20. {Skeleton fibre not echinated _Iophon_

{Skeleton fibre echinated _Pocillon_

21. {Skeleton with echinating spicules 28

{Skeleton without echinating spicules 30

22. {Spongin abundant 23

{Spongin scanty 25

23. {Fibre not echinated 24

{Fibre echinated _Diplodemia_

24. {Fibre with a single axial series of spicules _Chalina_

{Fibres with numerous spicules arranged polyserially _Pachychalina_

25. {Microscleres absent 26

{Microscleres sigmata and/or toxa 27

26. {Skeleton confused _Halichondria_

{Skeleton reticulate _Reniera_

{Rind and fistulous appendages present; microscleres sigmata

27. { _Oceanapia_

{No rind; skeleton reticulate; microscleres sigmata and/or toxa

{ _Gellius_

{Skeleton confused or formed of bundles of spicules with

{ echinating spined styles 29

28. {Skeleton fibrous or reticulate, or formed of short columns 45

{Skeleton formed of a dense central axis, and columns radiating

{ from it to the surface 52

{Spicules of the ectosome styles _Pytheas_

29. {Spicules of the ectosome oxeas or absent _Clathrissa_

{Main skeleton confused. Special ectosomal skeleton absent

{ _Spanioplon_

{Megascleres of the choanosome not differing from those

30. { of the ectosome 31

{Megascleres of the choanosome differing from those

{ of the ectosome 32

31. {Chelae absent. 33

{Chelae present 44

32. {Trichodragmata present _Tedania_

{Trichodragmata absent 42

{224}

[Illustration: FIG. 110.—Megascleres. _a-l_ and _q-s_, Modifications of

monaxon type. _a_, Strongyle; _b_, tylote; _c_, oxea; _d_, tylotoxea; _e_,

tylostyle; _f_, style; _g_, spined tylostyle; _h_, sagittal triod (a

triaxon form derived from monaxon); _j_, oxytylote; _k_, anatriaene; _l_,

protriaene; _m_, sterraster (polyaxon); _n_, radial section through the

outer part of _m_, showing two actines soldered together by intervening

silica; _o_, desma of an Anomocladine Lithistid (polyaxon); _q_, crepidial

strongyle, basis of rhabdocrepid Lithistid desma; _r_, young form of

rhabdocrepid desma, showing crepidial strongyle coated with successive

layers of silica; _s_, rhabdocrepid desma.]

{Skeleton reticulate or fibrous{225} 34

33. {Skeleton radiate or diffuse 37

{Skeleton with radiating fibres forming a reticulum with

{ others crossing them at right angles _Quasillina_

{No microscleres 35

34. {Microscleres sigmata and/or toxa with or without trichodragmata

{ _Desmacella_

35. {Sponge fan- or funnel-shaped 36

{Sponge not fan- or funnel-shaped _Hymeniacidon_

36. {Megascleres slender and twisted _Phakellia_

{Megascleres somewhat stout, not twisted _Tragosia_

37. {Sigmata present, skeleton diffuse _Biemma_

{Sigmata absent 38

38. {Skeleton more or less radiate 39

{Skeleton diffuse; sponge boring _Cliona_

39. {Sponge discoid with marginal fringe _Halicnemia_

{Sponge massive or stipitate without marginal fringe 40

40. {Sponge body prolonged into mammiform projections _Polymastia_

{Sponge body without mammiform projections 41

41. {No microscleres. Megascleres tylostyles with or without styles

{ _Suberites_

{Microscleres centrotylote. Megascleres styles or tylostyles

{ _Ficulina_

42. {Choanosomal megascleres smooth 43

{Choanosomal megascleres spined _Dendoryx_

{Microscleres chelae and sigmata of about the same size

43. { _Lissodendoryx_

{Chelae, if present, smaller than the sigmata _Yvesia_

44. {Isochelae{225} _Esperiopsis_

{Anisochelae _Esperella_

45. {Fibres or columns plumose 46

{Fibres or columns ectyonine 47

46. {Microscleres toxa _Ophlitaspongia_

{Microscleres absent _Axinella_

47. {Skeleton reticulate 48

{Skeleton not reticulate 49

{Microscleres present. Spicules of the fibre core spined _Myxilla_

48. {Microscleres absent. Spicules of the fibre core smooth

{ _Lissomyxilla_

49. {Main skeleton formed of plume-like columns 50

{Main skeleton formed of horny fibres (ectyonine). Special dermal

{ skeleton wanting _Clathria_

{Dermal skeleton contains styles only _Microciona_

50. {Dermal skeleton contains diactine spicules with or without styli

{ 51

{Main skeleton columns with a core of smooth oxeas

51. { _Plumohalichondria_

{Main skeleton columns with a core of spined styles _Stylostichon_

{Central axis contains much spongin. Echinating spined styli

{ present _Raspailia_

52. {Central axis with little or no spongin. Spined styles absent.

{ Pillars radiating from the axis support dermal skeleton

{ _Ciocalypta_

53. {Ground substance between chambers clear; chambers pear-shaped

{ or oval; eurypylous _Spongelia_

{Ground substance granular. Chambers spherical with aphodi 54

54. {Fibres not pithed; sponge fan-shaped _Leiosella_

{Fibres pithed; sponge massive _Aplysina_

55. {Chambers long, tubular, branched _Halisarca_

{Chambers not much longer than broad; not branched _Oscarella_

56. {Amphidiscs present _Ephydatia_

{Amphidiscs absent _Spongilla_

{226}

CHAPTER IX

PORIFERA (_CONTINUED_): REPRODUCTION, SEXUAL AND

ASEXUAL—PHYSIOLOGY—DISTRIBUTION—FLINTS

The reproductive processes of Sponges are of such great importance in

leading us to a true conception of the nature of a sponge that we propose

to treat them here in a special section. Both sexual and asexual methods

are common; the multiplication of oscula we do not regard as an act of

reproduction (p. 174).

[Illustration: FIG. 111.—A, amphiblastula larva of _Sycon raphanus_; B,

later stage, showing invagination of the flagellated cells. _c.s_,

Segmentation cavity; _ec_, ectoderm; _en_, endoderm. (After F. E. Schulze,

from Balfour.)]

A cursory glance at a collection of sponge LARVAE from different groups

would suggest the conclusion that they are divisible into two wholly

distinct types. One of these is the _amphiblastula_, and the other the

_parenchymula_. This was the conclusion accepted by zoologists not long

ago. We are indebted to Delage, Maas, and Minchin for dispelling it, and

showing that {227}these types are but the extreme terms of a continuous

series of forms which have all the same essential constitution and undergo

the same metamorphosis.

The amphiblastula of _Sycon raphanus_ (Fig. 111) consists of an anterior

half, formed of slender flagellated cells, and a posterior half, of which

the cells are large, non-flagellate, and rounded. These two kinds of cell

are arranged around a small internal cavity which is largely filled up with

amoebocytes. The flagellated cells are invaginated into the dome of rounded

cells during metamorphosis, in fact, become the choanocytes or gastral

cells; the rounded cells, on the other hand, become the dermal cells—an

astonishing fact to any one acquainted only with Metazoan larvae.

A typical parenchymula is that of _Clathrina blanca_ (Fig. 112). When

hatched it consists of a wall surrounding a large central cavity and built

up of flagellated cells interrupted at the hinder pole by two cells

(_p.g.c_)—the mother-cells of archaeocytes. Before the metamorphosis,

certain of the flagellated cells leave the wall and sink into the central

cavity, and undergoing certain changes establish an inner mass of future

dermal cells. By subsequent metamorphosis the remaining flagellated cells

become internal, not this time by invagination, but by the included dermal

cells breaking through the wall of the larva, and forming themselves into a

layer at the outside.

[Illustration: FIG. 112.—Median longitudinal section of parenchymula larva

of _Clathrina blanca_. _p.g.c_, Posterior granular cells—archaeocyte

mother-cells. (After Minchin.)]

In the larva of _C. blanca_, after a period of free-swimming existence, the

same three elements are thus recognisable as in that of _Sycon_ at the time

of hatching; in the newly hatched larva of _C. blanca_, however, one set of

elements, the dermal cells, are not distinguishable. The difference, then,

between the two newly hatched larvae is due to the earlier cell

differentiation of the _Sycon_ larva.[254]

Now consider the larva of _Leucosolenia_. It is hatched as a

{228}completely flagellated larva; its archaeocytes are internal (as in

_Sycon_); future dermal cells, recognisable as such, are absent. They

arise, as in _C. blanca_, by transformation of flagellated cells; but (1)

this process is confined to the posterior pole, and (2) the internal cavity

is small and filled up with archaeocytes. Consequently the cells which have

lost their flagella and become converted into dermal cells cannot sink in

as in _C. blanca_: they accumulate at the hinder pole, and thus arises a

larva half flagellated, half not; in fact, an amphiblastula. Or, briefly,

in _Leucosolenia_ the larva at hatching is a parenchymula, and when ready

to fix is an amphiblastula; and, again, the difference between the newly

hatched larva and that of _Sycon_ is due to the earlier occurrence of cell

differentiation in the latter. What completer transitional series could be

desired?

Turning to the Micromastictora, the developmental history already sketched

is fairly typical (p. 172). The differences between Mega- and

Micro-mastictoran larvae are referable mainly to the fact that the dermal

cells in the latter become at once differentiated among themselves to form

the main types of dermal cell of the adult.[255] The metamorphosis is

comparable to that of _C. blanca_. Among Tetractinellida and Hexactinellida

sexually produced larvae have not been certainly identified.

Asexual reproduction takes place according to one of three types, which may

be alluded to as (1) "budding," (2) "gemmulation," (3) formation of

"asexual larvae."

By BUDDING (Fig. 113) is meant the formation of reproductive bodies, each

of which contains differentiated elements of the various classes found in

the parent. A simple example of this is described by Miklucho Maclay in

Ascons, where the bud is merely the end of one of the Ascon tubes which

becomes pinched off and so set free.

In _Leucosolenia botryoides_[256] Vasseur describes a similar process; in

this, however, a strikingly distinctive feature is present (Fig. 114),

namely, the buds have an inverse orientation with respect to that of the

parent, so that the budding sponge presents a contrast to a sponge in which

multiplication of oscula has occurred. In fact, the free distal end of the

bud becomes the base of the young sponge, and the osculum is formed at the

opposite extremity, where the bud is constricted from the parent.

{229}[Illustration: FIG. 113.—_Lophocalyx philippensis._ The specimen bears

several buds attached to it by long tufts of spicules. (After F. E.

Schulze.)]

[Illustration: FIG. 114.—_Leucosolenia botryoides._ A, a piece of the

Sponge laden with buds, _a-f_; _i_, the spicules of the buds directed away

from their free ends; _k_, the spicules of the parent directed towards the

osculum, _j_. B, a bud which has been set free and has become fixed by the

extremity which was free or distal in A. (After Vasseur.)]

Such a reversal of the position of the bud is noteworthy in view of its

rarity, and the case is worth reinvestigating, for in other animal groups a

bud or a regenerated part retains so constantly the same orientation as the

parent that Loeb,[257] after experimenting on the {230}regeneration of

Coelenterata and other forms, concluded that a kind of "polarity" existed

in the tissues of certain animals.

In _Oscarella lobularis_[258] the buds are transparent floating bladders,

derived from little prominences on the surface of the sponge. Scattered in

the walls of the bladders are flagellated chambers, which open into the

central cavity. The vesicular nature of the buds is doubtless an

adaptation, lessening their specific gravity and so enabling them to float

to a distance from the parent.

GEMMULATION.—_Spongilla_ has already afforded us a typical example of this

process. Gemmules very similar to those of _Spongilla_ are known in a few

marine sponges, especially in _Suberites_ and in _Ficulina_. They form a

layer attached to the surface of support of the sponge—a layer which may be

single or double, or even three or four tiers deep. A micropyle is

sometimes present in the spongin coat, sometimes absent; possibly its

absence may be correlated with the piling of one layer of gemmules on

another, as this, by covering up the micropyle, would of course render it

useless. Presumably when a micropyle is present the living contents escape

through it and leave the sponge by way of the canal system (Fig. 115).

[Illustration: FIG. 115.—Gemmules of _Ficulina_. A, vertical section of

gemmules _in situ_; B, vertical section of upper portion of one gemmule.

_m_, Micropyle.]

The only case besides _Spongilla_ in which the details of development from

gemmules have been traced is that of _Tethya_.[259] Mere microscopic

examination of a _Tethya_ in active reproduction would suggest that the

process was simple budding, but Maas has shown that the offspring arise

from groups of archaeocytes in the cortex, that is to say, they are typical

gemmules. As they develop they migrate outwards along the radial

spicule-bundles {231}and are finally freed, like the buds of the

Hexactinellid _Lophocalyx_ (Fig. 113).

The comparison of the process of development on the one hand by gemmules,

and on the other by larval development, is of some interest.[260] In both

cases two cell layers—a dermal and a gastral—are established before the

young sponge has reached a functional state. Differences of detail in the

formation of the chambers occur in the gemmule; these find parallels in the

differences in the same process exhibited by the larvae of various groups

of sponges. On the other hand, the order of tissue differentiation is not

the same in the gemmule as in the larva.

[Illustration: FIG. 116.—Development of the triradiate and quadriradiate

spicules of _Clathrina_. (1) Three scleroblasts; (2) each has divided: the

spicule is seen in their midst; (3) addition of the fourth ray by a

porocyte. _p_, Dermal aperture of pore; _r_, fourth ray. (After Minchin.)]

Of the reproduction of Tetractinellida extremely little is known.

Spermatozoa occur in the tissues in profusion and are doubtless functional,

but larvae have been seldom observed.

[Illustration: FIG. 117.—Three stages in the development of the triradiate

spicules of _Sycon setosum_. × 1200. (After Maas.)]

In Hexactinellida the place of sexually produced larvae is taken by bodies

of similar origin to gemmules but with the appearance of parenchymulae.

Ijima has indeed seen a few egg-cells in Hexactinellids.[261] He finds,

however, that archaeocyte congeries occur in abundance, and there is good

reason to believe with him that these are responsible for the numerous

parenchymula-like ASEXUALLY PRODUCED LARVAE he has observed. The discovery

of "asexual larvae" was first made by Wilson in the Monaxonid _Esperella_;

in this case the asexual larva is, as far as can be detected, identical

with that developed from the fertilised egg. A similar phenomenon, the

production {232}of apparently identical larvae by both sexual and asexual

methods, has been observed in the Coelenterate _Gonionema murbachii_.[262]

Artificially, sponges may be reproduced with great advantage to commerce by

means of cuttings. Cuttings of the bath sponge are fit to gather after a

seven years' growth.

The development of the various forms of SPICULES is a subject about which

little is yet known. Most spicules of which the development has been traced

originate in a single dermal cell. The triradiate and quadriradiate

spicules of Homocoela (Clathrinidae), as Minchin[263] has most beautifully

shown, form an exception. Three cells co-operate to form the triradiate;

these three divide to give six before the growth of the spicule is

complete. A quadriradiate is formed from a triradiate spicule by addition

of the fourth ray, which, again, has a separate origin in an independent

cell, in fact a porocyte. The triradiate spicules of the Sycettidae, on the

other hand, originate in a single cell,[264] but the quadriradiate spicules

are formed from these by the addition of a fourth ray in a manner similar

to that which has just been described for Clathrinidae.

[Illustration: FIG. 118.—Development of monaxon spicules. A, from

_Spongilla lacustris_, showing the single scleroblast. (After Evans.) B, a

very large monaxon, from _Leucosolenia_, on which many scleroblasts are at

work. (After Maas.)]

Monaxon spicules if not of large size undergo their entire development

within a single scleroblast (Fig. 118, A). In some cases if their

dimensions exceed certain limits, several cells take part in their

completion; some of these are derived from the {233}division of the

original scleroblast, others are drawn from the surrounding tissue. In

_Tethya_, for example, and in _Leucosolenia_[265] the scleroblasts round

the large monaxon spicules are so numerous as to have an almost epithelioid

arrangement.

The large oxeas of _Tetilla_, _Stelletta_, and _Geodia_, however, are

formed each within a single scleroblast.[266]

[Illustration: FIG. 119.—Development of spheraster. A, of _Tethya_, from

union of two quadriradiate spicules. (After Maas.) B (_a-e_), of

_Chondrilla_, from a spherical globule. (After Keller.)]

Triaenes have been shown[267] to originate as monaxons with one swollen

termination, from which later the cladi grow out. Information as to the

scleroblasts in this case is needed.

The value of a knowledge of the ontogeny of microscleres might be great.

Maas believes that he has shown that the spherasters of _Tethya_ are formed

by the union of minute tetractine calthrops (Fig. 119, A). If this view

should be confirmed, it would afford a very strong argument for the

Tetractinellid affinities of _Tethya_.

[Illustration: FIG. 120.—Stages in the development of the microscleres of

_Placospongia_. (After Keller.)]

Keller,[268] on the other hand, finds that the spherasters of the

Tetractinellid _Chondrilla_ {234}originate as spheres (Fig. 119, B); and

spheres have been observed in the gemmule of a _Tethya_; no spherasters

were as yet present in the gemmule, and spheres were absent in the

adult.[269]

In the genus _Placospongia_ certain spicules are present which outwardly

closely resemble the sterrasters so characteristic of certain

Tetractinellidae. Their development, however, as will be seen from Fig.

120, shows that they are not polyaxon but spiny monaxon spicules.

_Placospongia_ is consequently transferred to the Monaxonida

Spintharaphora.

Sterrasters originate within an oval cell as a number of hairlike

fibres[270] (trichites), which are united at their inner ends. The outer

ends become thickened and further modified. The position occupied by the

nucleus of the scleroblast is marked in the adult spicule by a hilum.

[Illustration: FIG. 121.—Three stages in the development of an anisochela.

_al_, Ala; _al'_, lower ala; _f_, falx; _f'_, lower falx; _r_, rostrum;

_r'_, lower rostrum. (After Vosmaer and Pekelharing.)]

The anisochela has been shown repeatedly to originate from a C-shaped

spicule.[271]

What little is known of the development of Hexactinellid spicules we owe to

Ijima.[272] Numerous cells are concerned in certain later developmental

stages of the hexaster; a hexaster passes through a hexactin stage, and—a

fact "possibly of importance for the phylogeny of spicules in

Hexactinellida"—in two species the first formed spicules are a kind of

hexactin, known as a "stauractin," and possessing only four rays all in one

plane (cf. _Protospongia_, p. 207).

PHYSIOLOGY

PRODUCTION OF THE CURRENT.—It is not at first sight obvious that the

lashing of flagella in chambers arranged as above {235}described, between

an inhalant and an exhalant system of canals, will necessarily produce a

current passing inwards at the ostia and outwards at the osculum. And the

difficulty seems to be increased when it is found[273] that the flagella in

any one chamber do not vibrate in concert, but that each keeps its own

time. This, however, is of less consequence than might seem to be the case.

Two conditions are essential to produce the observed results: (1) in order

that the water should escape at the mouth of the chamber there must be a

pressure within the chamber higher than that in the exhalant passages; (2)

in order that water may enter the chamber there must be within it a

pressure less than that in the inhalant passages. But the pressure in the

inhalant and exhalant passages is presumably the same, at any rate before

the current is started, therefore there must be a difference of pressure

within the chamber itself, and the less pressure must be round the

periphery. Such a distribution of pressures would be set up if each

flagellum caused a flow of water directed away from its own cell and

towards the centre of the chamber; and this would be true whether the

flagellum beats synchronously with its fellows or not.

The comparative study of the canal systems of sponges[274] acquires a

greater interest in proportion as the hope of correlating modifications

with increase of efficiency seems to be realised. In a few main issues this

hope may be said to have been realised. The points, so to speak, of a good

canal system are (1) high oscular velocity, which ensures rapid removal of

waste products to a wholesome distance; (2) a slow current without eddies

in the flagellated chambers, to allow of the choanocytes picking up food

particles (see below), and moreover to prevent injury to the delicate

collars of those cells; (3) a small area of choanocytes, and consequent

small expenditure of energy in current production.

It is then at once clear at what a disadvantage the Ascons are placed as

compared with other sponges having canal systems of the second or third

types. Their chamber and oscular currents can differ but slightly, the

difference being obtained merely by narrowing the lumen of the distal

extremity of the body to form the oscular rim. Further, the choanocytes are

{236}acting on a volume of water which they can only imperfectly control,

and it is no doubt due to the necessity of limiting the volume of water

which the choanocytes have to set in motion that the members of the Ascon

family are so restricted in size. The oscular rim is only a special case of

a device adopted by sponges at the very outset of their career, and

retained and perfected when they have reached their greatest heights; the

volume of water passing per second over every cross-section of the path of

the current is of course the same, therefore by narrowing the

cross-sectional area of the path at any point, the velocity of the current

is proportionally increased at that point. The lining of the oscular rim is

of pinacocytes; they determine a smooth surface, offering little frictional

resistance to the current, while choanocytes in the same position would

have been a hindrance, not only by setting up friction, but by causing

irregularities in the motion.

Canal systems of the second type show a double advance upon that of the

Ascons, namely, subdivision of the gastral cavity and much greater length

of the smooth walled exhalant passage. The choanocytes have now a task more

equal to their strength, and, further, there is now a very great inequality

between the total sectional areas of the flagellated chambers and that of

the oscular tube.

Canal systems of the third type with tubular chambers are an improvement on

those of the second, in that the area of choanocytes is increased by the

pouching of the chamber-layer without corresponding increase in the size of

the sponge. However, the area of choanocytes represents expenditure of

energy, and the next problem to be solved is how to retain the improved

current and at the same time to cut down expense. The first step is to

change the form of the chamber from tubular to spherical. Now the energy of

all the choanocytes is concentrated on the same small volume of water. The

area of choanocytes is less, but the end result is as good as before. At

the wide mouth of the spherical chamber there is nevertheless still a cause

of loss of energy in the form of eddies, and it is as an obviation of these

that one must regard the aphodi and prosodi with which higher members of

the Demospongiae are provided. The correctness of this view receives

support, apart from mechanical principles, from the fact that the mass of

the body of any one of these sponges is greater relatively to the total

flagellated area than in those sponges with eurypylous chambers; that is to

say, a few {237}aphodal and diplodal chambers are as efficient as many of

the eurypylous type.

It is manifest that the current is the bearer of the supply of FOOD; but it

requires more care to discover (1) what is the nature of the food; (2) by

which of the cells bathed by the current the food is captured and by which

digested. The answer to the latter question has long been sought by

experimenters,[275] who supplied the living sponge with finely powdered

coloured matters, such as carmine, indigo, charcoal, suspended in water.

The results received conflicting interpretations until it became recognised

that it was essential to take into account the length of time during which

the sponge had been fed before its tissues were subjected to microscopic

examination. Vosmaer and Pekelharing obtained the following facts:

_Spongilla lacustris_ and _Sycon ciliatum_, when killed after feeding for

from half an hour to two hours with milk or carmine, contain these

substances in abundance in the bodies of the choanocytes and to a slight

degree in the deeper cells of the dermal tissue; after feeding for

twenty-four hours the proportions are reversed, and if a period of

existence in water uncharged with carmine intervenes between the long feed

and death then the chambers are completely free from carmine. These are

perhaps the most conclusive experiments yet described, and they show that

the choanocytes ingest solid particles and that the amoeboid cells of the

dermal layer receive the ingested matter from them. In all probability it

is fair to argue from these facts that solid particles of matter suitable

to form food for the sponge are similarly dealt with by it and undergo

digestion in the dermal cells.

Choanocytes are the feeding organs _par excellence_; but the pinacocytes

perform a small share of the function of ingestion, and in the higher

sponges where the dermal tissue has acquired a great bulk the share is

perhaps increased.

In the above experiments is implied the tacit assumption that sponges take

their food in the form of finely divided solids. Haeckel[276] states his

opinion that they feed on solid particles derived from decaying organisms,

but that possibly decaying substances in solution may eke out their diet.

Loisel, in 1898,[277] {238}made a new departure in the field of experiment

by feeding sponges with coloured solutions, and obtained valuable results.

Thus solutions, if presented to the sponge in a state of extreme dilution,

are subjected to choice, some being absorbed, some rejected. When absorbed

they are accumulated in vacuoles within both dermal and gastral cells,

mixed solutions are separated into their constituents and collected into

separate vacuoles. In the vacuoles the solutions may undergo change; Congo

red becomes violet, the colour which it assumes when treated with acid, and

similarly blue litmus turns red. The contents of the vacuoles, sometimes

modified, sometimes not, are poured out into the intercellular gelatinous

matrix of the dermal layer, whence they are removed partly by amoeboid

cells, partly, so Loisel thinks, by the action of the matrix itself. It

adds to the value of these observations to learn that Loisel kept a

_Spongilla_ supplied with filtered spring-water, to which was added the

filtered juice obtained from another crushed sponge. This _Spongilla_ lived

and budded, and was in good health at the end of ten days.

MOVEMENT.—Sponges are capable of locomotion only in the young stage; in the

adult the only signs of movement are the exhalant current, and in some

cases movements of contraction sufficiently marked to be visible to the

naked eye. Meresjkowsky was one of the early observers of these movements.

He mentions that he stimulated a certain corticate Monaxonid sponge by

means of a needle point: a definite response to each prick inside the

oscular rim was given by the speedy contraction of the osculum.[278]

PIGMENTS AND SPICULES.—Various reasons lead one to conclude that the

spicules have some function other than that of support and defence,

probably connected with metabolism. For the spicules are cast off,

sometimes in large numbers, to be replaced rapidly by new ones, a process

for which it is difficult to find an adequate explanation if the spicules

are regarded as merely skeletal and defensive.[279] Potts remarks upon the

striking profusion with which spicules are secreted by developing

Spongillids from water in which the percentage of silica present must have

been exceedingly small. The young sponges climbed {239}up the strands of

spicules as they formed them, leaving the lower parts behind and adding to

the upper ends.

Of the physiology of the pigments of sponges not much is yet known: a

useful summary of facts will be found in Von Fürth's text-book.[280]

SPONGIN.—Von Fürth[280] points out that this term is really a collective

one, seeing that the identity of the organic skeletal substance of all

sponge species is hardly to be assumed. Spongin is remarkable for

containing iodine. The amount of iodine present in different sponges varies

widely, reaching in certain tropical species of the Aplysinidae and

Spongidae the high figure 8 to 14 per cent. Seaweeds which are specially

rich in iodine contain only 1.5 to 1.6 per cent.

In view of the fact that iodine is a specific for croup, it is of interest

to observe that the old herb doctors for many centuries recognised the bath

sponge as a cure for that disease.

[Illustration: FIG. 122.—The ordinals measure (i.) the number of species,

_a-f_, and (ii.) the number of stations, _a'-f'_, at which successful hauls

were made. The abscissae measure the depth: thus at I. the depth is from 0

to 50 fathoms; at II. from 51 to 200; at III. from 201 to 1000; at IV. from

1001 upwards. _a_, _a'_, are the curves for Sponges generally; _b_, _b'_,

for Monaxonida; _c_, _c'_, for Hexactinellida; _d_, _d'_, for

Tetractinellida; _e_, _e'_, for Calcarea; _f_, _f'_, for Ceratosa.]

DISTRIBUTION IN SPACE.—All the larger groups of Sponges are cosmopolitan.

Each group has, however, its characteristic bathymetrical range: the facts

are best displayed by means of curves, as in Fig. 122, which is based

wholly on the results obtained by the "Challenger" Expedition. The

information as to littoral species is consequently inadequate, and we have

not the data requisite for their discussion.

Sponges generally (_a_) and Monaxonida in particular (_b_) are more

generally distributed in water of depths of 51 to 200 fathoms than in

depths of less than 50 fathoms; but localities in shallow water are

{240}richer, for the station curve (_a'_) rises abruptly from I. to II.,

while the species curve (_a_) in the same region is almost horizontal.

The Hexactinellid curve (_c_) culminates on III., showing that the group is

characteristically deep water. That for Tetractinellida (_d_) reaches its

greatest height on II., _i.e._ between 51 and 200 fathoms. Even here, in

their characteristic depths, the Tetractinellida fall below the

Hexactinellida, and far below the Monaxonida in numbers. Again, the

Monaxonida are commoner than Hexactinellida in deep water of 201 to 1000

fathoms, and it is not till depths of 1000 fathoms are passed that

Hexactinellida prevail, finally preponderating over the Monaxonida in the

ratio of 2:1.

The Calcarea and Ceratosa are strictly shallow-water forms. It is a fact

well worth consideration that the stations at which sponges have been found

are situated, quite irrespective of depth, more or less in the

neighbourhood of land. In the case of Calcarea and Ceratosa this is to be

expected, seeing that shallow water is commonest near land, but it is

surprising that it should be true also of the Hexactinellida and of the

deep-water species of Tetractinellida and of Monaxonida.

While the family groups are cosmopolitan, this is not true of genera and

species. The distribution of genera and species makes it possible to define

certain geographical provinces for sponges as for other animals. That this

is so, is due to the existence of ocean tracts bare of islands; for ocean

currents, can act as distributing agents with success only if they flow

along a coast or across an ocean studded with islands. It is, of course,

the larval forms which will be transported; whether they will ever develop

to the adult condition depends on whether the current carrying them passes

over a bottom suitable to their species before metamorphosis occurs and the

young sponge sinks. If such a bottom is passed over, and if the depth is

one which can be supported by the particular species in question, then a

new station may thus be established for that species.

The distance over which a larva may be carried depends on the speed of the

current by which it is borne, and on the length of time occupied by its

metamorphosis. Certain of the ocean currents accomplish 500 miles in six

days; this gives some idea of the distance which may intervene between the

birthplace and {241}the final station of a sponge; for six days is not an

excessive interval to allow for the larval period of at any rate some

species.

DISTRIBUTION IN TIME.—All that space permits us to say on the palaeontology

of sponges has been said under the headings of the respective classes. We

can here merely refer to the chronological table shown in Fig. 123:[281]—

[Illustration: FIG. 123.—Table to indicate distribution of Sponges in

time.]

FLINTS.—The ultimate source of all the silica in the sea and fresh-water

areas is to be found in the decomposition of igneous rocks such as granite.

The quantity of silica present in solution in sea water is exceedingly

small, amounting to about one-and-a-half parts in 100,000; it certainly is

not much more in average fresh water. This is no doubt due to its

extraction by diatoms, which begin to extract it almost as soon as it is

set free from the parent rock. It is from this small quantity that the

siliceous sponges derive the supply from which they form their spicules.

Hence it would appear that for the formation of one {242}ounce of spicules

at least one ton of sea water must pass through the body of the sponge.

Obviously from such a weak solution the deposition of silica will not occur

by ordinary physical agencies; it requires the unexplained action of living

organisms. This may account for the fact that deposits of flint and chert

are always associated with organic remains, such as Sponges and Radiolaria.

By some process, the details of which are not yet understood, the silica of

the skeleton passes into solution. In Calcareous deposits, a replacement of

the carbonate of lime by the silica takes place, so that in the case of

chalk the shells of Foraminifera, such as _Globigerina_ and _Textularia_

and those of Coccoliths, are converted into a siliceous chalk. Thus a

siliceous chalk is the first stage in the formation of a flint.

A further deposition of silica then follows, cementing this pulverulent

material into a hard white porous flint. It is white for the same reason

that snow is white. The deposition of silica continues, and the flint

becomes at first grey and at last apparently black (black as ice is black

on a pond). Frequently flints are found in all stages of formation:

siliceous chalk with the corroded remains of sponge spicules may be found

in the interior, black flint blotched with grey forming the mass of the

nodule, while the exterior is completed by a thin layer of white porous

flint. This layer must not be confused with the white layer which is

frequently met with on the surface of weathered flints, which is due to a

subsequent solution of some of the silica, so that by a process of

unbuilding, the flint is brought back to the incompleted flint in its

second stage. In the chalk adjacent to the flints, hollow casts of large

sponge spicules may sometimes be observed, proving the fact, which is

however unexplained, of the solution of the spicular silica. The formation

of the flints appears to have taken place, to some extent at least, long

after the death of the sponge, and even subsequent to the elevation of the

chalk far above the sea-level, as is shown by the occurrence of layers of

flints in the joints of the solid chalk.[282]

COELENTERATA AND CTENOPHORA

BY

S. J. HICKSON, M.A., F.R.S.

Formerly Fellow and now Honorary Fellow of Downing College, Beyer Professor

of Zoology in the Victoria University of Manchester.

{245}CHAPTER X

COELENTERATA

INTRODUCTION—CLASSIFICATION—HYDROZOA—ELEUTHEROBLASTEA—MILLEPORINA—

GYMNOBLASTEA—CALYPTOBLASTEA—GRAPTOLITOIDEA—STYLASTERINA

The great division of the animal kingdom called COELENTERATA was

constituted in 1847 by E. Leuckart for those animals which are commonly

known as polyps and jelly-fishes. Cuvier had previously included these

forms in his division Radiata or Zoophyta, when they were associated with

the Starfishes, Brittle-stars, and the other Echinodermata.

The splitting up of the Cuvierian division was rendered necessary by the

progress of anatomical discovery, for whereas the Echinodermata possess an

alimentary canal distinct from the other cavities of the body, in the

polyps and jelly-fishes there is only one cavity to serve the purposes of

digestion and the circulation of fluids. The name Coelenterata (κοῖλος =

hollow, ἔντερον = the alimentary canal) was therefore introduced, and it

may be taken to signify the important anatomical feature that the

body-cavity (or coelom) and the cavity of the alimentary canal (or enteron)

of these animals are not separate and distinct as they are in Echinoderms

and most other animals.

Many Coelenterata have a pronounced radial symmetry, the body being

star-like, with the organs arranged symmetrically on lines radiating from a

common centre. In this respect they have a superficial resemblance to many

of the Echinodermata, which are also radially symmetrical in the adult

stage. But it cannot be insisted upon too strongly that this superficial

resemblance of the Coelenterata and Echinodermata has no genetic

significance. {246}The radial symmetry has been acquired in the two

divisions along different lines of descent, and has no further significance

than the adaptation of different animals to somewhat similar conditions of

life. It is not only in the animals formerly classed by Cuvier as Radiata,

but in sedentary worms, Polyzoa, Brachiopoda, and even Cephalopoda among

the Mollusca, that we find a radial arrangement of some of the organs. It

is interesting in this connexion to note that the word "polyp," so

frequently applied to the individual Coelenterate animal or zooid, was

originally introduced on a fancied resemblance of a _Hydra_ to a small

Cuttle-fish (_Fr._ Poulpe, _Lat._ Polypus).

The body of the Coelenterate, then, consists of a body-wall enclosing a

single cavity ("coelenteron"). The body-wall consists of an inner and an

outer layer of cells, originally called by Allman the "endoderm" and

"ectoderm" respectively. Between the two layers there is a substance

chemically allied to mucin and usually of a jelly-like consistency, for

which the convenient term "mesogloea," introduced by G. C. Bourne, is used

(Fig. 125).

The mesogloea may be very thin and inconspicuous, as it is in _Hydra_ and

many other sedentary forms, or it may become very thick, as in the

jelly-fishes and some of the sedentary Alcyonaria. When it is very thick it

is penetrated by wandering isolated cells from the ectoderm or endoderm, by

strings of cells or by cell-lined canals; but even when it is cellular it

must not be confounded with the third germinal layer or mesoblast which

characterises the higher groups of animals, from which it differs

essentially in origin and other characters. The Coelenterata are

two-layered animals (DIPLOBLASTICA), in contrast to the Metazoa with three

layers of cells (TRIPLOBLASTICA). The growth of the mesogloea in many

Coelenterata leads to modifications of the shape of the coelenteric cavity

in various directions. In the Anthozoa, for example, the growth of vertical

bands of mesogloea covered by endoderm divides the peripheral parts of the

cavity into a series of intermesenterial compartments in open communication

with the axial part of the cavity; and in the jelly-fishes the growth of

the mesogloea reduces the cavity of the outer regions of the disc to a

series of vessel-like canals.

Another character, of great importance, possessed by all Coelenterata is

the "nematocyst" or "thread-cell" (Fig. 124). {247}This is an organ

produced within the body of a cell called the "cnidoblast," and it consists

of a vesicular wall or capsule, surrounding a cavity filled with fluid

containing a long and usually spirally coiled thread continuous with the

wall of the vesicle. When the nematocyst is fully developed and receives a

stimulus of a certain character, the thread is shot out with great velocity

and causes a sting on any part of an animal that is sufficiently delicate

to be wounded by it.

The morphology and physiology of the nematocysts are subjects of very great

difficulty and complication, and cannot be discussed in these pages. It

may, however, be said that by some authorities the cnidoblast is supposed

to be an extremely modified form of mucous or gland cell, and that the

discharge of the nematocyst is subject to the control of a primitive

nervous system that is continuous through the body of the zooid.

There is a considerable range of structure in the nematocysts of the

Coelenterata. In _Alcyonium_ and in many other Alcyonaria they are very

small (in _Alcyonium_ the nematocyst is 0.0075 mm. in length previous to

discharge), and when discharged exhibit a simple oval capsule with a plain

thread attached to it. In _Hydra_ (Fig. 124) there are at least two kinds

of nematocysts, and in the larger kind (0.02 mm. in length previous to

discharge) the base of the thread is beset with a series of recurved hooks,

which during the act of discharge probably assist in making a wound in the

organism attacked for the injection of the irritant fluid, and possibly

hold the structure in position while the thread is being discharged. In the

large kind of nematocyst of _Millepora_ and of _Cerianthus_ there is a band

of spirally arranged but very minute thorns in the middle of the thread,

but none at the base. In some of the Siphonophora the undischarged

nematocysts reach their maximum size, nearly 0.05 mm. in length.

[Illustration: FIG. 124.—Nematocyst (_Nem_) of _Hydra grisea_, enclosed

within the cnidoblast. _CNC_, Cnidocil; _f_, thread of nematocyst; _Mf_,

myophan threads in cnidoblast; _N_, nucleus of cnidoblast. (After

Schneider.)]

When a nematocyst has once been discharged it is usually {248}rejected from

the body, and its place in the tissue is taken by a new nematocyst formed

by a new cnidoblast; but in the thread of the large kind of nematocyst of

_Millepora_ there is a very delicate band, which appears to be similar to

the myophan thread in the stalk of a _Vorticella_. Dr. Willey[283] has made

the important observation that in this coral the nematocyst threads can be

withdrawn after discharge, the retraction being effected with great

rapidity. The "cnidoblast" is a specially modified cell. It sometimes bears

at its free extremity a delicate process, the "cnidocil," which is supposed

to be adapted to the reception of the special stimuli that determine the

discharge of the nematocyst. In many species delicate contractile fibres

(Fig. 124, _Mf_) can be seen in the substance of the cnidoblast, and in

others its basal part is drawn out into a long and probably contractile

stalk ("cnidopod"), attached to the mesogloea below.

There can be little doubt that new nematocysts are constantly formed during

life to replace those that have been discharged and lost. Each nematocyst

is developed within the cell-substance of a cnidoblast which is derived

from the undifferentiated interstitial cell-groups. During this process the

cnidoblast does not necessarily remain stationary, but may wander some

considerable distance from its place of origin.[284] This habit of

migration of the cnidoblast renders it difficult to determine whether the

ectoderm alone, or both ectoderm and endoderm, can give rise to

nematocysts. In the majority of Coelenterates the nematocysts are confined

to the ectoderm, but in many Anthozoa, Scyphozoa, and Siphonophora they are

found in tissues that are certainly or probably endodermic in origin. It

has not been definitely proved in any case that the cnidoblast cells that

form these nematocysts have originally been formed in the endoderm, and it

is possible that they are always derived from ectoderm cells which migrate

into the endoderm.

It is probably true that all Coelenterata have nematocysts, and that, in

the few cases in which it has been stated that they are absent (e.g.

_Sarcophytum_), they have been overlooked. It cannot, however, be

definitely stated that similar structures do not occur in other animals.

The nematocysts of the Mollusc _Aeolis_ are not the product of its own

tissues, but are introduced {249}into the body with its food.[285] The

nematocysts that occur in the Infusorian _Epistylis umbellaria_ and in the

Dinoflagellate _Polykrikos_ (p. 131) require reinvestigation, but if it

should prove that they are the product of the Protozoa they cannot be

regarded as strictly homologous with those of Coelenterata. In many of the

Turbellaria, however, and in some of the Nemertine worms, nematocysts occur

in the epidermis which appear to be undoubtedly the products of these

animals.

The Coelenterata are divided into three classes:—

1. HYDROZOA.—Without stomodaeum and mesenteries. Sexual cells discharged

directly to the exterior.

2. SCYPHOZOA.—Without stomodaeum and mesenteries. Sexual cells discharged

into the coelenteric cavity.

3. ANTHOZOA = ACTINOZOA.—With stomodaeum and mesenteries. Sexual cells

discharged into the coelenteric cavity.

The full meaning of the brief statements concerning the structure of the

three classes given above cannot be explained until the general anatomy of

the classes has been described. It may be stated, however, in this place

that many authors believe that structures corresponding with the stomodaeum

and mesenteries of Anthozoa do occur in the Scyphozoa, which they therefore

include in the class Anthozoa.

Among the more familiar animals included in the class Hydrozoa may be

mentioned the fresh-water polyp _Hydra_, the Hydroid zoophytes, many of the

smaller Medusae or jelly-fish, the Portuguese Man-of-war (_Physalia_), and

a few of the corals.

Included in the Scyphozoa are the large jelly-fish found floating on the

sea or cast up on the beach on the British shores.

The Anthozoa include the Sea-anemones, nearly all the Stony Corals, the

Sea-fans, the Black Corals, the Dead-men's fingers (_Alcyonium_), the

Sea-pens, and the Precious Coral of commerce.

CLASS I. HYDROZOA

In this Class of Coelenterata two types of body-form may be found. In such

a genus as _Obelia_ there is a fixed branching colony of zooids, and each

zooid consists of a simple tubular body-wall composed of the two layers of

cells, the ectoderm and the {250}endoderm (Fig. 125), terminating distally

in a conical mound—the "hypostome"—which is perforated by the mouth and

surrounded by a crown of tentacles. This fixed colony, the "hydrosome,"

feeds and increases in size by gemmation, but does not produce sexual

cells. The hydrosome produces at a certain season of the year a number of

buds, which develop into small bell-like jelly-fish called the "Medusae,"

which swim away from the parent stock and produce the sexual cells. The

Medusa (Fig. 126) consists of a delicate dome-shaped contractile bell,

perforated by radial canals and fringed with tentacles; and from its centre

there depends, like the clapper of a bell, a tubular process, the

manubrium, which bears the mouth at its extremity. This free-swimming

sexual stage in the life-history of _Obelia_ is called the "medusome."

It is difficult to determine whether, in the evolution of the Hydrozoa, the

hydrosome preceded the medusome or _vice versâ_. By some authors the

medusome is regarded as a specially modified sexual individual of the

hydrosome colony. By others the medusome is regarded as the typical adult

Hydrozoon form, and the zooids of the hydrosome as nutritive individuals

arrested in their development to give support to it. Whatever may be the

right interpretation of the facts, however, it is found that in some forms

the medusome stage is more or less degenerate and the hydrosome is

predominant, whereas in others the hydrosome is degenerate or inconspicuous

and the medusome is predominant. Finally, in some cases there are no

traces, even in development, of a medusome stage, and the life-history is

completed in the hydrosome, while in others the hydrosome stages are lost

and the life-history is completed in the medusome.

If a conspicuous hydrosome stage is represented by H, a conspicuous

medusome stage by M, an inconspicuous or degenerate hydrosome stage by h,

an inconspicuous or degenerate medusome stage by m, and the fertilised ovum

by O, the life-histories of the Hydrozoa may be represented by the

following formulæ:—

1. O — H — O (_Hydra_)

2. O — H — m — O (_Sertularia_)

3. O — H — M — O (_Obelia_)

4. O — h — M — O (_Liriope_)

5. O — M — O (_Geryonia_)

The structure of the HYDROSOME is usually very simple. It {251}consists of

a branched tube opening by mouths at the ends of the branches and closed at

the base. The body-wall is built up of ectoderm and endoderm. Between these

layers there is a thin non-cellular lamella, the mesogloea.

In a great many Hydrozoa the ectoderm secretes a chitinous protective tube

called the "perisarc." The mouth is usually a small round aperture situated

on the summit of the hypostome, and at the base of the hypostome there may

be one or two crowns of tentacles or an area bearing irregularly scattered

tentacles. The tentacles may be hollow, containing a cavity continuous with

the coelenteric cavity of the body; or solid, the endoderm cells arranged

in a single row forming an axial support for the ectoderm. The ectoderm of

the tentacles is provided with numerous nematocysts, usually arranged in

groups or clusters on the distal two-thirds of their length, but sometimes

confined to a cap-like swelling at the extremity (capitate tentacles). The

hydrosome may be a single zooid producing others asexually by gemmation (or

more rarely by fission), which become free from the parent, or it may be a

colony of zooids in organic connexion with one another formed by the

continuous gemmation of the original zooid derived from the fertilised ovum

and its asexually produced offspring. When the hydrosome is a colony of

zooids, specialisation of certain individuals for particular functions may

occur, and the colony becomes dimorphic or polymorphic.

[Illustration: FIG. 125.—Diagram of a vertical section through a hydrosome.

_Coel_, Coelenteron; _Ect_, ectoderm; _End_, endoderm. Between the ectoderm

and the endoderm there is a thin mesogloea not represented in the diagram.

_M_, mouth; _T_, tentacle.]

The MEDUSOME is more complicated in structure than the hydrosome, as it is

adapted to the more varied conditions of a free-swimming existence. The

body is expanded to form a disc, "umbrella," or bell, which bears at the

edge or margin a number of tentacles. The mouth is situated on the end of a

hypostome, called the "manubrium," situated in the centre of the radially

symmetrical body. The surface that bears the manubrium is {252}called oral,

and the opposite surface is called aboral. The cavity partly enclosed by

the oral aspect of the body when it is cup- or bell-shaped is called the

"sub-umbrellar cavity."

In the medusome of nearly all Hydrozoa there is a narrow shelf projecting

inwards from the margin of the disc and guarding the opening of the

sub-umbrellar cavity, called the "velum."

The mouth leads through the manubrium into a flattened part of the

coelenteric cavity, which is usually called the gastric cavity, and from

this a number of canals pass radially through the mesogloea to join a

circular canal or ring-canal at the margin of the umbrella.

A special and important feature of the medusome is the presence of

sense-organs called the "ocelli" and "statocysts," situated at the margin

of the umbrella or at the base of the tentacles.

[Illustration: FIG. 126.—Diagram of a vertical section through a medusome.

_coel_, Coelenteron; _M_, mouth; _Man_, manubrium; _R_, radial canal; _r_,

ring or circular canal; _T_, tentacle; _v_, velum.]

The ocelli may usually be recognised as opaque red or blue spots on the

bases of the tentacles, in marked contrast to their transparent

surroundings. The ocellus may consist simply of a cluster of pigmented

cells, or may be further differentiated as a cup of pigmented cells filled

with a spherical thickening of the cuticle to form a lens. The exact

function of the ocelli may not be fully understood, but there can be little

doubt that they are light-perceiving organs.

The function of the sense-organs known as statocysts, however, has not yet

been so satisfactorily determined. They were formerly thought to be

auditory organs, and were called "otocysts," but it appears now that it is

impossible on physical grounds for these organs to be used for the

perception of the waves of sound in water. It is more probable that they

are organs of the static function, that is, the function of the perception

of the position of the body in space, and they are consequently called

statocysts. In the Leptomedusae each statocyst consists of a small vesicle

in the mesogloea at the margin of the umbrella, containing a hard, stony

body called the "statolith." In _Geryonia_ and some other Trachomedusae the

statolith is carried by a short tentacular process, the "statorhab,"

{253}projecting into the vesicle; in other Trachomedusae, however, the

vesicle is open, but forms a hood for the protection of the statorhab; and

in others, but especially in the younger stages of development, the

statorhab is not sunk into the margin of the umbrella, and resembles a

short but loaded tentacle. Recent researches have shown that there is a

complete series of connecting links between the vesiculate statocyst of the

Leptomedusae and the free tentaculate statorhab of the Trachomedusae, and

there can be little doubt of their general homology.

In the free-swimming or "Phanerocodonic" medusome the sexual cells are

borne by the ectoderm of the sub-umbrellar cavity either on the walls of

the manubrium or subjacent to the course of the radial canals.

ORDER I. ELEUTHEROBLASTEA.

This order is constituted mainly for the well-known genus _Hydra_. By some

authors _Hydra_ is regarded as an aberrant member of the order

Gymnoblastea, to which it is undoubtedly in many respects allied, but it

presents so many features of special interest that it is better to keep it

in a distinct group.

_Hydra_ is one of the few examples of exclusively fresh-water

Coelenterates, and like so many of the smaller fresh-water animals its

distribution is almost cosmopolitan. It occurs not only in Europe and North

America, but in New Zealand, Australia, tropical central Africa, and

tropical central America.

_Hydra_ is found in this country in clear, still fresh water attached to

the stalks or leaves of weeds. When fully expanded it may be 25 mm. in

length, but when completely retracted the same individual may be not more

than 3 mm. long. The tubular body-wall is built up of ectoderm and

endoderm, enclosing a simple undivided coelenteric cavity. The mouth is

situated on the summit of the conical hypostome, and at the base of this

there is a crown of long, delicate, but hollow tentacles. The number of

tentacles is usually six in _H. vulgaris_ and _H. oligactis_,[286] and

eight in _H. viridis_, but it is variable in all species.

During the greater part of the summer the number of individuals is rapidly

increased by gemmation. The young Hydras produced by gemmation are usually

detached from their parents {254}before they themselves produce buds, but

in _H. oligactis_ the buds often remain attached to the parent after they

themselves have formed buds, and thus a small colony is produced. Sexual

reproduction usually commences in this country in the summer and autumn,

but as the statements of trustworthy authors are conflicting, it is

probable that the time of appearance of the sexual organs varies according

to the conditions of the environment.

Individual specimens may be male, female, or hermaphrodite. Nussbaum[287]

has published the interesting observation that when the Hydras have been

well fed the majority become female, when the food supply has been greatly

restricted the majority become male, and when the food-supply is moderate

in amount the majority become hermaphrodite. The gonads are simply clusters

of sexual cells situated in the ectoderm. There is no evidence, derived

from either their structure or their development, to show that they

represent reduced medusiform gonophores. The testis produces a number of

minute spermatozoa. In the ovary, however, only one large yolk-laden

egg-cell reaches maturity by the absorption of the other eggs. The ovum is

fertilised while still within the gonad, and undergoes the early stages of

its development in that position. With the differentiation of an outer

layer of cells a chitinous protecting membrane is formed, and the escape

from the parent takes place.[288] It seems probable that at this stage,

namely, that of a protected embryo, there is often a prolonged period of

rest, during which it may be carried by wind and other agencies for long

distances without injury.

The remarkable power that _Hydra_ possesses of recovery from injury and of

regenerating lost parts was first pointed out by Trembley in his classical

memoir.[289]

A _Hydra_ can be cut into a considerable number of pieces, and each piece,

provided both ectoderm and endoderm are represented in it, will give rise

by growth and regeneration to a complete zooid. There is, however, a limit

of size below which fragments of _Hydra_ will not regenerate, even if they

contain {255}cells of both layers. The statement made by Trembley, that

when a _Hydra_ is turned inside out it will continue to live in the

introverted condition has not been confirmed, and it seems probable that

after the experiment has been made the polyp remains in a paralysed

condition for some time, and later reverts, somewhat suddenly, to the

normal condition by a reversal of the process. There is certainly no

substantial reason to believe that under any circumstances the ectoderm can

undertake the function of the endoderm or the endoderm the functions of the

ectoderm.

[Illustration: FIG. 127.—A series of drawings of _Hydra_, showing the

attitudes it assumes during one of the more rapid movements from place to

place. 1, The _Hydra_ bending over to one side; 2, attaching itself to the

support by the mouth and tentacles; 3, drawing the sucker up to the mouth;

4, inverted; 5, refixing the sucker; 6, reassuming the erect posture.

(After Trembley.)]

One of the characteristic features of _Hydra_ is the slightly expanded,

disc-shaped aboral extremity usually called the "foot," an unfortunate term

for which the word "sucker" should be substituted. There are no root-like

tendrils or processes for attachment to the support such as are found in

most of the solitary Gymnoblastea. The attachment of the body to the stem

or weed or surface-film by this sucker enables the animal to change its

position at will. It may either progress slowly by gliding along its

support without the assistance of the tentacles, in a manner similar to

that observed in many Sea-anemones; or more rapidly by a series of

somersaults, as originally described by Trembley. The latter mode of

locomotion has been recently described as follows:—"The body, expanded and

with expanded tentacles, bends over to one side. As soon as the tentacles

touch the bottom they attach themselves and contract. Now one of two things

happens. The foot may loosen its hold on the bottom and the body contract.

In this manner the animal comes to stand on its tentacles with the foot

pointing upward. The body now bends over again until the foot attaches

itself close to the attached tentacles. These loosen in their turn, and so

the _Hydra_ is again {256}in its normal position. In the other case the

foot is not detached, but glides along the support until it stands close to

the tentacles, which now loosen their hold."[290]

_Hydra_ appears to be purely carnivorous. It will seize and swallow

Entomostraca of relatively great size, so that the body-wall bulges to more

than twice its normal diameter. But smaller Crustacea, Annelid worms, and

pieces of flesh are readily seized and swallowed by a hungry _Hydra_. In

_H. viridis_ the chlorophyll corpuscles[291] of the endoderm may possibly

assist in the nourishment of the body by the formation of starch in direct

sunlight.

Three species of _Hydra_ are usually recognised, but others which may be

merely local varieties or are comparatively rare have been named.[292]

_H. viridis._—Colour, grass-green. Average number of tentacles, eight.

Tentacles shorter than the body. Embryonic chitinous membrane spherical and

almost smooth.

_H. vulgaris_, Pallas (_H. grisea_, Linn.).—Colour, orange-brown. Tentacles

rather longer than the body, average number, six. Embryonic chitinous

membrane spherical, and covered with numerous pointed branched spines.

_H. oligactis_, Pallas (_H. fusca_, Linn.).—Colour, brown. Tentacles

capable of great extension; sometimes, when fully expanded, several times

the length of the body. Average number, six. Embryonic chitinous membrane

plano-convex, its convex side only covered with spines.

The genera _Microhydra_ (Ryder) and _Protohydra_ (Greeff) are probably

allied to _Hydra_, but as their sexual organs have not been observed their

real affinities are not yet determined. _Microhydra_ resembles _Hydra_ in

its general form and habits, and in its method of reproduction by

gemmation, but it has no tentacles. It was found in fresh water in North

America.

_Protohydra_[293] was found in the oyster-beds off Ostend, and resembles

_Microhydra_ in the absence of tentacles. It multiplies by transverse

fission, but neither gemmation nor sexual reproduction has been observed.

_Haleremita_ is a minute hydriform zooid which is also marine. {257}It was

found by Schaudinn[294] in the marine aquarium at Berlin in water from

Rovigno, on the Adriatic. It reproduces by gemmation, but sexual organs

have not been found.

Another very remarkable genus usually associated with the Eleutheroblastea

is _Polypodium_. At one stage of its life-history it has the form of a

spiral ribbon or stolon which is parasitic on the eggs of the sturgeon

(_Acipenser ruthenus_) in the river Volga.[295] This stolon gives rise to a

number of small _Hydra_-like zooids with twenty tentacles, of which sixteen

are filamentous and eight club-shaped. These zooids multiply by

longitudinal fission, and feed independently on Infusoria, Rotifers, and

other minute organisms. The stages between these hydriform individuals and

the parasitic stolon have not been discovered.

ORDER II. MILLEPORINA.

_Millepora_ was formerly united with the Stylasterina to form the order

Hydrocorallina; but the increase of our knowledge of these Hydroid corals

tends rather to emphasise than to minimise the distinction of _Millepora_

from the Stylasterina.

_Millepora_ resembles the Stylasterina in the production of a massive

calcareous skeleton and in the dimorphism of the zooids, but in the

characters of the sexual reproduction and in many minor anatomical and

histological peculiarities it is distinct. As there is only one genus,

_Millepora_, the account of its anatomy will serve as a description of the

order.

The skeleton (Fig. 128) consists of large lobate, plicate, ramified, or

encrusting masses of calcium carbonate, reaching a size of one or two or

more feet in height and breadth. The surface is perforated by numerous

pores of two distinct sizes; the larger—"gastropores"—are about 0.25 mm. in

diameter, and the smaller and more numerous "dactylopores" about 0.15 mm.

in diameter. In many specimens the pores are arranged in definite cycles,

each gastropore being surrounded by a circle of 5-7 dactylopores; but more

generally the two kinds appear to be irregularly scattered on the surface.

When a branch or lobe of a Millepore is broken across and examined in

section, it is found that each pore is continued as a {258}vertical tube

divided into sections by horizontal calcareous plates (Fig. 129, _Tab_).

These plates are the "tabulae," and constitute the character upon which

_Millepora_ was formerly placed in the now discarded group of Tabulate

corals.

The coral skeleton is also perforated by a very fine reticulum of canals,

by which the pore-tubes are brought into communication with one another. In

the axis of the larger branches and in the centre of the larger plates a

considerable quantity of the skeleton is of an irregular spongy character,

caused by the disintegrating influence of a boring filamentous Alga.[296]

[Illustration: FIG. 128.—A portion of a dried colony of _Millepora_,

showing the larger pores (gastropores) surrounded by cycles of smaller

pores (dactylopores). At the edges the cycles are not well defined.]

The discovery that _Millepora_ belongs to the Hydrozoa was made by

Agassiz[297] in 1859, but Moseley[298] was the first to give {259}an

adequate account of the general anatomy. The colony consists of two kinds

of zooids—the short, thick gastrozooids (Fig. 129, _G_) provided with a

mouth and digestive endoderm, and the longer and more slender mouthless

dactylozooids (_D_)—united together by a network of canals running in the

porous channels of the superficial layer of the corallum. The living

tissues of the zooids extend down the pore-tubes as far as the first

tabulae, and below this level the canal-system is degenerate and

functionless. It is only a very thin superficial stratum of the coral,

therefore, that contains living tissues.

The zooids of _Millepora_ are very contractile, and can be withdrawn below

the general surface of the coral into the shelter of the pore-tubes. When a

specimen is examined in its natural position on the reef, the zooids are

usually found to be thus contracted; but several observers have seen the

zooids expanded in the living condition. It is probable that, as is the

case with other corals, the expansion occurs principally during the night.

The colony is provided with two kinds of nematocysts—the small kind and the

large. In some colonies they are powerful enough to penetrate the human

skin, and _Millepora_ has therefore received locally the name of "stinging

coral." On each of the dactylozooids there are six or seven short capitate

tentacles (Fig. 129, _t_), each head being packed with nematocysts of the

small kind; similar batteries of these nematocysts are found in the four

short capitate tentacles of the gastrozooids. The nematocysts of the larger

kind are found in the superficial ectoderm, some distributed irregularly on

the surface, others in clusters round the pores. The small nematocysts are

about 0.013 mm. in length before they are exploded, and exhibit four spines

at the base of the thread; the large kind are oval in outline, 0.02 × 0.025

mm. in size, and exhibit no spines at the base, but a spiral band of minute

spines in the middle of the filament. There is some reason to believe that

the filament of the large kind of nematocysts can be retracted.[299]

At certain seasons the colonies of _Millepora_ produce a great number of

male or female Medusae. The genus is probably dioecious, no instances of

hermaphrodite colonies having yet been found. Each Medusa is formed in a

cavity situated above the last-formed tabula in a pore-tube, and this

cavity, the "ampulla," having a greater diameter than that of the

gastrozooid tubes, can be recognised even in the dried skeleton.

{260}[Illustration: FIG. 129.—Diagrammatic sketch to show the structure of

_Millepora_. _Amp_, an ampulla containing a medusa; _Can.1_, canal system

at the surface; _Can.2_, canal system degenerating in the lower layers of

the corallum; _Cor_, corallum; _D_, an expanded dactylozooid with its

capitate tentacles; _Ect_, the continuous sheet of ectoderm covering the

corallum (_Cor_); _G_, a gastrozooid, seen in vertical section; _Med_,

free-swimming Medusae; _t_, tentacle; _Tab_, tabula in the pore-tubes.

(Partly after Moseley.)]

It is not known how frequently the sexual seasons occur, but from the

rarity in the {261}collections of our museums of Millepore skeletons which

exhibit the ampullae, it may be inferred that the intervals between

successive seasons are of considerable duration.

The Medusae of _Millepora_ are extremely simple in character. There is a

short mouthless manubrium bearing the sexual cells, an umbrella without

radial canals, while four or five knobs at the margin, each supporting a

battery of nematocysts, represent all that there is of the marginal

tentacles. The male Medusae have not yet been observed to escape from the

parent, but from the fact that the spermatozoa are not ripe while they are

in the ampullae, it may be assumed that the Medusae are set free. Duerden,

however, has observed the escape of the female Medusae, and it seems

probable from his observations that their independent life is a short one,

the ova being discharged very soon after liberation.

_Millepora_ appears to be essentially a shallow-water reef coral. It may be

found on the coral reefs of the Western Atlantic extending as far north as

Bermuda, in the Red Sea, the Indian and Pacific Oceans. The greatest depth

at which it has hitherto been found is 15 fathoms on the Macclesfield Bank,

and it flourishes at a depth of 7 fathoms off Funafuti in the Pacific

Ocean.

_Millepora_, like many other corals, bears in its canals and zooids a great

number of the symbiotic unicellular "Algae" (Chrysomonadaceae, see pp. 86,

125) known as Zooxanthellae. All specimens that have been examined contain

these organisms in abundance, and it has been suggested that the coral is

largely dependent upon the activity of the "Algae" for its supply of

nourishment. There can be no doubt that the dactylozooids do paralyse and

catch living animals, which are ingested and digested by the gastrozooids,

but this normal food-supply may require to be supplemented by the

carbohydrates formed by the plant-cells. But as the carbohydrates can only

be formed by the "Algae" in sunlight, this supplementary food-supply can

only be provided in corals that live in shallow water. It must not be

supposed that this is the only cause that limits the distribution of

_Millepora_ in depth, but it may be an important one.

The generic name _Millepora_ has been applied to a great many fossils from

different strata, but a critical examination of their structure fails to

show any sufficient reason for including many of them in the genus or even

in the order. Fossils that are {262}undoubtedly _Millepora_ occur in the

raised coral reefs of relatively recent date, but do not extend back into

Tertiary times. There seems to be no doubt, therefore, that the genus is of

comparatively recent origin. Among the extinct fossils the genus that comes

nearest to it is _Axopora_ from the Eocene of France, but this genus

differs from _Millepora_ in having monomorphic, not dimorphic, pores, and

in the presence of a minute spine or columella in the centre of each tube.

The resemblances are to be observed in the general disposition of the canal

system and of the tabulation. Whether _Axopora_ is or is not a true

Milleporine, however, cannot at present be determined, but it is the only

extinct coral that merits consideration in this place.

ORDER III. GYMNOBLASTEA—ANTHOMEDUSAE.

This order was formerly united with the Calyptoblastea to form the order

Hydromedusae, but the differences between the two are sufficiently

pronounced to merit their treatment as distinct orders.

In many of the Gymnoblastea the sexual cells are borne by free Medusae,

which may be recognised as the Medusae of Gymnoblastea by the possession of

certain distinct characters. The name given to such Medusae, whether their

hydrosome stage is known or not, is Anthomedusae. The Gymnoblastea are

solitary or colonial Hydrozoa, in which the free (oral) extremity of the

zooids, including the crown of tentacles, is not protected by a skeletal

cup. The sexual cells may be borne by free Anthomedusae, or by more or less

degenerate Anthomedusae that are never detached from the parent hydrosome.

The Anthomedusae are small or minute Medusae provided with a velum, with

the ovaries or sperm-sacs borne by the manubrium and with sense-organs in

the form of ocelli or pigment-spots situated on the margin of the umbrella.

The solitary Gymnoblastea present so many important differences in

anatomical structure that they cannot be united in a single family. They

are usually fixed to some solid object by root-like processes from the

aboral extremity, the "hydrorhiza," or are partly embedded in the sand

(_Corymorpha_), into which long filamentous processes project for the

support of the zooid. The remarkable species _Hypolytus peregrinus_[300]

from Wood's Holl, {263}however, has no aboral processes, and appears to be

only temporarily attached to foreign objects by the secretion of the

perisarc. Among the solitary Gymnoblastea several species reach a gigantic

size. _Corymorpha_ is 50-75 mm. in length, but _Monocaulus_ from deep water

in the Pacific and Atlantic Oceans is nearly 8 feet in length. Among the

solitary forms attention must be called to the interesting pelagic

_Pelagohydra_ (see p. 274).

The method of colony formation in the Gymnoblastea is very varied. In some

cases (_Clava squamata_) a number of zooids arise from a plexus of canals

which corresponds with the system of root-like processes of the solitary

forms. In _Hydractinia_ this plexus is very dense, and the ectoderm forms a

continuous sheet of tissue both above and below. The colony is increased in

size in these cases by the gemmation of zooids from the hydrorhiza. In

other forms, such as _Tubularia larynx_, new zooids arise not only from the

canals of the hydrorhiza, but also from the body-walls of the upstanding

zooids, and thus a bushy or shrubby colony is formed.

In another group the first-formed zooid produces a hydrorhiza of

considerable proportions, which fixes the colony firmly to a stone or shell

and increases in size with the growth of the colony. This zooid itself by

considerable growth in length forms the axis of the colony, and by

gemmation gives rise to lateral zooids, which in their turn grow to form

the lateral branches and give rise to the secondary branches, and these to

the tertiary branches, and so one; each branch terminating in a mouth,

hypostome and crown of tentacles. Such a method of colony formation is seen

in _Bougainvillia_ (Fig. 130). A still more complicated form of colony

formation is seen in _Ceratella_, in which not a single but a considerable

number of zooids form the axis of the colony and of its branches. As each

axis is covered with a continuous coat of ectoderm, and each zooid of such

an axis secretes a chitinous fenestrated tube, the whole colony is far more

rigid and compact than is usual in the Gymnoblastea, and has a certain

superficial resemblance to a Gorgoniid Alcyonarian (Fig. 133, p. 271).

The branches of the colony and a considerable portion of the body-wall of

each zooid in the Gymnoblastea are usually protected by a thin, unjointed

"perisarc" of chitin secreted by the ectoderm; but this skeletal structure

does not expand distally to {264}form a cup-like receptacle in which the

oral extremity of the zooid can be retracted for protection.

The zooids of the Gymnoblastea present considerable diversity of form and

structure. The tentacles may be reduced to one (in _Monobrachium_) or two

(in _Lar sabellarum_), but usually the number is variable in each

individual colony. In many cases, such as _Cordylophora_, _Clava_, and many

others, the tentacles are irregularly scattered on the sides of the zooids.

In others there may be a single circlet of about ten or twelve tentacles

round the base of the hypostome. In some genera the tentacles are arranged

in two series (_Tubularia_, _Corymorpha_, _Monocaulus_), a distal series

round the margin of the mouth which may be arranged in a single circlet or

scattered irregularly on the hypostome, and a proximal series arranged in a

single circlet some little distance from the mouth. In _Branchiocerianthus

imperator_ the number of tentacles is very great, each of the two circlets

consisting of about two hundred tentacles.

[Illustration: FIG. 130.—Diagrammatic sketch to show the method of

branching of _Bougainvillia_. _gon_, Gonophores; _Hr_, hydrorhiza; _t.z_,

terminal zooid.]

The zooids of the hydrosome are usually monomorphic, but there are cases in

which different forms of zooid occur in the same colony. In _Hydractinia_,

for example, no less than four different kinds of zooids have been

described. These are called gastrozooids, dactylozooids, tentaculozooids,

and blastostyles respectively. The "gastrozooids" are provided with a

conical hypostome bearing the mouth and two closely-set circlets of some

ten to thirty tentacles. The "dactylozooids" are longer than the

gastrozooids and have the habit of actively coiling and {265}uncoiling

themselves; they have a small mouth and a single circlet of rudimentary

tentacles. The "tentaculozooids" are situated at the outskirts of the

colony, and are very long and slender, with rudimentary tentacles and no

mouth. The "blastostyles," usually shorter than the gastrozooids, have two

circlets of rudimentary tentacles and a mouth. They bear on their sides the

spherical or oval gonophores.

The medusome stage in the life-history of these Hydrozoa is produced by

gemmation from the hydrosome, or, in some cases, by gemmation from the

medusome as well as from the hydrosome. In many genera and species the

medusome is set free as a minute jelly-fish or Medusa, which grows and

develops as an independent organism until the time when the sexual cells

are ripe, and then apparently it dies. In other Gymnoblastea the medusome

either in the female or the male or in both sexes does not become detached

from the parent hydrosome, but bears the ripe sexual cells, discharges them

into the water, and degenerates without leading an independent life at all.

In these cases the principal organs of the medusome are almost or entirely

functionless, and they exhibit more or less imperfect development, or they

may be so rudimentary that the medusoid characters are no longer obvious.

Both the free and the undetached medusomes are gonophores, that is to say,

the bearers of the sexual cells, but the former were described by Allman as

the "phanerocodonic" gonophores, _i.e._ "with manifest bells," and the

latter as the "adelocodonic" gonophores. The gonophores may arise either

from an ordinary zooid of the colony (_Syncoryne_), from a specially

modified zooid—the blastostyle—as in _Hydractinia_, or from the hydrorhiza

as in certain species of _Perigonimus_. The free-swimming Medusa may itself

produce Medusae by gemmation from the manubrium (_Sarsia_, _Lizzia_,

_Rathkea_, and others), from the base of the tentacles (_Sarsia_,

_Corymorpha_, _Hybocodon_), or from the margin of the umbrella

(_Eleutheria_).

The free-swimming Medusae or phanerocodonic gonophores of the Gymnoblastea

are usually of small size (1 or 2 mm. in diameter) when first liberated,

and rarely attain a great size even when fully mature. They consist of a

circular, bell-shaped or flattened disc—the umbrella—provided at its margin

with a few or numerous tentacles, and a tubular manubrium bearing the mouth

depending from the exact centre of the under (oral) {266}side of the

umbrella (Fig. 132, A). The mouth leads into a shallow digestive cavity,

from which radial canals pass through the substance of the umbrella to join

a ring-canal at the margin (Fig. 131).

The sense-organs of the Medusae of the Gymnoblastea are in the form of

pigment-spots or very simple eyes (ocelli), situated at the bases of the

tentacles. The orifice of the umbrella is guarded by a thin shelf or

membrane, as in the Calyptoblastea, called the velum. The sexual cells are

borne by the manubrium (Figs. 131 and 132, A).

There are many modifications observed in the different genera as regards

the number of tentacles, the number and character of the radial canals, the

minute structure of the sense-organs, and some other characters, but they

agree in having a velum, ocellar sense-organs, and manubrial sexual organs.

The tentacles are rudimentary in _Amalthea_; in _Corymorpha_ there is only

one tentacle; in _Perigonimus_ there are two; and in _Bougainvillia_ they

are numerous; but the usual number is four or six. The radial canals are

usually simple and four in number, but there are six in _Lar sabellarum_,

which branch twice or three times before reaching the margin of the

umbrella (Fig. 132, B).

[Illustration: FIG. 131.—Medusa of _Cladonema_, from the Bahamas, showing

peculiar tentacular processes on the tentacles, the ocelli at the base of

the tentacles, the swellings on the manubrium that mark the position of the

gonads, and the radial and ring-canals of the umbrella. (After Perkins.)]