So little is known of _Epithetosoma_ that it is difficult to discuss its

affinities. The presence of the hollow proboscis and the pores leading into

the body-cavity undoubtedly justify its being placed in a separate Order,

but beyond the presence of a terminal anus, in which it resembles the

Echiuroidea, there is nothing in its structure which connects it more

nearly with one than with the other of the three larger Orders of Gephyrea.

LIST OF GEPHYREA FOUND IN THE BRITISH AREA AS DEFINED BY CANON NORMAN.

_Phascolosoma vulgare_ Blainv. English Channel and North Sea.

" _elongatum_ Kef. English Channel.

" _papillosum_ Thom. English coast.

" _eremita_ Sars North Sea.

" _procerum_ Moeb. Bass Rock.

_Phascolion strombi_ Mont. English coast (Plymouth).

_Sipunculus nudus_ L. North Sea, English Channel (Paignton,

Teignmouth).

_Golfingia macintoshii_ Lank. East coast of Scotland (St. Andrews Bay).

_Petalostoma minutum_ Kef. English Channel (Plymouth).

_Priapulus caudatus_ Lam. Scarborough, Outer Hebrides.

_Echiurus pallasii_ Guérin Coast of Scotland, English Channel.

_Thalassema neptuni_ Gaertner English Channel (Coast of Devonshire).

{450}CHAPTER XVI

PHORONIS

HISTORY—HABITS—STRUCTURE—REPRODUCTION—LARVA—METAMORPHOSIS—LIST OF SPECIES

AND LOCALITIES—SYSTEMATIC POSITION.

This interesting genus was discovered and first described by Dr. Strethill

Wright of Edinburgh, who in the year 1856 found specimens of it living on a

stone with _Caryophyllia_ sent to him from Ilfracombe. He christened the

form _Phoronis hippocrepia_,[490] the generic name being apparently taken

from an epithet applied to Io, the specific name having reference to the

beautiful horseshoe shape of its tentacular crown. Two years later a

closely allied or identical form was described by Professor P. J. van

Beneden under the name of _Crepina gracilis_.[491]

_Phoronis_ is a sedentary animal living in "colonies," but each member of

the colony is distinct, and has no organic connexion with the others, from

which it is isolated by the presence of a tube in which it lives, and into

which it can be completely withdrawn. The tube is formed from a secretion

which probably has its origin from the anterior end of the body-wall. The

secretion hardens and forms at first a transparent coating, but it soon

becomes opaque, and numerous sand particles, small pieces of shell, sponge

spicules, and other marine objects adhere to the outside of the tubes,

giving them a very characteristic appearance, and doubtless serving to

protect the inhabitants from predatory animals.

{451}What little we know about the habits of _Phoronis_ is in the main due

to the observations of Cori,[492] who studied _Ph. psammophila_ at Faro, an

inlet of the sea near Messina. The least disturbance causes the animal to

withdraw its head with lightning rapidity into the tube, from which after a

time it re-emerges very slowly, and does not expand its tentacular crown

until its body is completely extended. Cori states that not unfrequently

individuals are found either without the crown of tentacles or with the

latter in process of regeneration. These may have been bitten off by fish,

etc.; but, on the other hand, van Beneden describes in _Crepina gracilis_

(_Ph. hippocrepia_) the throwing off and regeneration of the crown of

tentacles; and Cori confirms his observation, at any rate as far as

concerns those individuals kept in captivity, and whose surroundings were

presumably somewhat unfavourable. He further observed the interesting fact

that the cast-off crown of tentacles continued to live, and suggests that

possibly it may develop a new body, in which case the phenomenon would be

an interesting case of binary fission producing two new animals.

[Illustration: FIG. 226.—A piece of a matted colony of _Ph. kowalevskii_

Cald. Slightly magnified. In most cases the tentacular head is protruding

from the tube.]

With regard to the habitation of _Ph. australis_, the largest species

known, some discrepancies have crept into the literature of the genus, and

to prevent their recurring again it may be worth while to quote the

statements of its discoverer, Mr. Haswell.[493] He says: "_Phoronis

australis_ occurs in communities of twenty to thirty, in spaces in the

substance of the wall of the tube inhabited and formed by a species of

_Cerianthus_. Each worm has a tube of its own, very delicate and

transparent, made up of several layers, the mouth opening on the outer

surface of the tube of the _Cerianthus_. The _Cerianthus_ tubes sometimes

come up empty, as we should naturally expect, the animal having dropped

out; but a sufficient number of {452}occupied tubes are found to show that,

under ordinary circumstances, a living _Cerianthus_ occupies the interior

of the tube and a community of _Phoronis_ live in its wall. This species of

_Phoronis_ is never found anywhere else, and the species of _Cerianthus_ is

very rarely found without the _Phoronis_."

_Ph. australis_ is sluggish in its movements, but other species are capable

of very active movement, and withdraw their heads in a moment at the

approach of danger. A Neapolitan species, _Ph. kowalevskii_—known to the

fishermen of that place as "Ficchetelli bianchi" or "Vermi di ceppa"—lives

chiefly on submarine posts and piles; its tubes, closely interlacing, form

a dense feltwork, upon which Ascidians and Sea-anemones often settle, and

over which Ophiurids and Polychaets creep. The tubes of this species are

rendered opaque by the excreta ejected from the body, and they do not

attach foreign substances to the outside to anything like the same degree

as _Ph. psammophila_, which live in sandy places, and are termed by the

Sicilian fishermen "Tubi di sabbia." The feltwork of _Ph. kowalevskii_

attains a thickness of 5 to 8 cm. In each case the tube is much longer than

the animal it shelters, and is so entangled with its neighbours, to which

it frequently adheres, that it is a matter of considerable difficulty to

isolate it.

[Illustration: FIG. 227.—A piece of a colony of _Ph. psammophila_ Cori.

Slightly magnified. The tubes are covered by particles of sand, small

shells, etc.]

The various species of _Phoronis_ differ a good deal in size; Cori gives

the average length as varying from 1.5 to 7.9 mm. in _Ph. hippocrepia_ and

up to 127 mm. (6 inches) in _Ph. australis_. Probably the very short

individuals of the first-named species had not attained their adult

stature. _Ph. australis_ has recently formed the subject of a memoir by Dr.

W. B. Benham,[494] from whom the following account is mainly taken.

The length of the individuals varied from three to six inches, and their

diameter, which is not very uniform, averaged one-eighth of an inch. At one

end, which, since it bears the mouth, we may call the oral end, is the very

characteristic tentacular {453}crown surrounding the mouth on all sides but

one, where there is a slight break in its continuity. The crown of

tentacles or lophophore is flattened, and the two ends drawn out, and each

is coiled into a spiral (Fig. 229); between the bases of these two spirals

three ridges can be seen, each ending in a pore; the median opening is the

anus, the two lateral are the openings of the nephridia or kidneys, which

also serve as ducts for the reproductive organs. The anus is thus

approximated to the mouth, and since the continuity of the tentacular crown

is broken at a spot just between the two, there would be nothing to

separate these orifices if it were not for the presence of the epistome, a

projection or flap of the body-wall which overhangs the mouth between it

and part of the crown of tentacles.

The extent to which the ridge bearing the tentacles is incurved at each

side varies in different species. In _Ph. kowalevskii_ and _Ph.

psammophila_ the ends are only slightly turned in, so that the crown of

tentacles is truly horse-shoe shaped; but in _Ph. australis_ they are

turned in and form three coils on each side. The number of tentacles also

varies, being very numerous in _Ph. australis_ and _Ph. buskii_—the latter

having as many as 300, whilst the other species as a rule have from 60 to

90. The bases of the tentacles are fused for a short distance with one

another, forming a thin membrane.

[Illustration: FIG. 228.—A specimen of _Ph. buskii_ M‘Int. removed from its

tube and seen from behind, × about 2. (After M‘Intosh.)]

The rest of each tentacle is free, and its inner surface, or that turned

towards the mouth, is covered with long cilia, which, by the currents they

set up, doubtless serve to bring food to the mouth. The tentacles are

hollow, and their cavity is kept open by a stiffening of the tissue, which

almost resembles an internal skeleton; the cavity communicates with the

anterior part of the general body-cavity, and up it runs a single

blood-vessel containing red blood. A single nerve is also distributed to

each tentacle.

{454}At the base of the two spirals of the tentacular crown lie two

ciliated pits, regarded by Caldwell and M‘Intosh[495] as sensory organs,

but Benham looks upon them as glandular in structure and function. Perhaps

they secrete the substance from which the tubes are formed.

The skin is covered by a delicate cuticle secreted by the underlying

epidermis; within the latter is a well-marked basement membrane, and

beneath this a layer of circular muscle fibres; these surround a layer of

longitudinally-arranged fibres, which do not form a continuous sheet but

are arranged in bundles. In both layers the fibres are unstriated. The

longitudinal fibres are covered on their inner side by a layer of flat

pavement cells, which line the general cavity of the body.

This space, the body-cavity, is divided into two parts by the presence of a

diaphragm or septum which runs across from one side of the body to the

other about the level of the ridge bearing the tentacular crown. The

anterior space is continuous with the cavities of the tentacles and of the

epistome. The partition is pierced by the blood-vessels and the oesophagus,

but the rest of the alimentary canal, including the anus, the kidneys, and

the reproductive organs, all lie in the posterior half of the body-cavity

behind the diaphragm. This portion of the body-cavity is further subdivided

by the presence of three longitudinal mesenteries supporting the alimentary

canal and running between it and the body-wall. One of these mesenteries

runs along the outside of the alimentary canal throughout its whole length,

attaching both the descending and ascending limbs of the U-shaped tube to

the body-wall. The other two are lateral mesenteries, which pass from the

body-wall to the sides of the oesophagus. These mesenteries therefore

divide the body-cavity into three spaces—one in which the rectum lies,

which may be called the rectal, and two lateral; owing to the fact that the

lateral mesenteries end before they reach the bend of the alimentary canal,

the three chambers are in free communication one with another. The

body-cavity is further traversed by irregular strands of tissue which run

from the body-wall to the various organs. It contains a corpusculated

fluid.

The alimentary canal (Fig. 230) consists of a U-shaped tube {455}which may

be divided into four regions. The mouth (_m_) leads into the oesophagus

(_oe_), which gradually enlarges into the stomach (_st_) situated just

before the bend; a constriction just at the bend separates the stomach from

the intestine (_int_), and this leads into the rectum (_r_), which

terminates in the anus (_an_). The first three divisions of the alimentary

canal are ciliated, but the rectum is not; the walls of the stomach also

contain glandular cells, but there are no special glands opening into any

part of the tract.

[Illustration: FIG. 229.—The dorsal surface of _Ph. australis_ Has.,

looking down on the head. The tentacles are cut away on the left side, and

the innermost are shortened on the right side to show the arrangement; in

reality they are of the same length throughout. _a_, Mouth; _b_, anus; _c_,

pore of left nephridium; _d_, epistome; _e_, break in the inner series of

tentacles. The drawing is to some extent diagrammatic, and is considerably

enlarged. (After Benham.)]

One of the most interesting features of _Phoronis_ is the presence of a

closed system of blood-vessels containing red blood. There are two main

blood-vessels; one, lying in the rectal chamber between the two limbs of

the U-shaped alimentary canal, has been named the afferent vessel. Just

below the diaphragm this splits into two, and each branch, after piercing

this partition, runs in a spiral course along the base of the crown of

tentacles, giving off a single blood-vessel into each tentacle. At its base

each tentacular vessel opens not only into the above-mentioned

{456}"distributing" vessel, but also into a "recipient" vessel which takes

a course parallel with the former. The two recipient vessels pierce the

diaphragm, and after running for some distance apart, fuse to form the

efferent vessel, which continues down the body on the left side of the

oesophagus. At the aboral end of the body the efferent vessel turns forward

and becomes the afferent. Both the main vessels give off numerous blood

diverticula, which are developed into plexiform sinuses on the walls of the

stomach, and in this region they are covered with the reproductive cells.

All the vessels are contractile, and Strethill Wright counted about fifteen

pulsations a minute. The blood contains numerous nucleated, disc-shaped

corpuscles differing in appearance from those of the fluid in the

body-cavity. The corpuscles contain haemoglobin, which gives the red colour

to the blood.

The two nephridia or kidneys are essentially tubes which open on the one

side into the body-cavity, and on the other to the exterior. The position

of the external pores has already been described, one being on each side of

the anus. Each pore leads into a tube which passes into that part of the

body-cavity situated below the diaphragm, where it divides, and each of the

two branches terminates in a ciliated funnel-shaped opening. The smaller of

these two funnels pierces the lateral mesentery and opens into the lateral

chamber, whilst the larger, whose opening is very much drawn out

longitudinally, opens into the rectal chamber. The whole organ is ciliated

internally.

The nervous system lies in the skin immediately below the epidermis. This

position is very primitive, and forms one of the most interesting

anatomical peculiarities of the genus. The nervous tissue is probably

diffused all over the body, but there is a special concentration or

thickening in the form of a ring which surrounds the mouth, following the

base of the tentacular spirals and giving off a nerve to each tentacle. The

ring lies at the outside of the base of the tentacles, the anus is not

included in it. Caldwell[496] has described in _Ph. kowalevskii_ an

asymmetrical nerve-cord given off from the ring and running along the left

side of the body; associated with which is a tubular structure of unknown

function. In _Ph. australis_ Benham mentions two such tubes, one on each

side of the body; their precise value is obscure.

{457}The epithelium covering the nerve-ring is slightly modified in the

neighbourhood of the kidney pore, and may have some special sensory

function; no other organs of sense are known (but see p. 454).

[Illustration: FIG. 230.—A schematic view of the interior of the body of

_Phoronis_. The middle seven-eighths of the body are omitted, _af_,

Afferent blood-vessel; _an_, anus; _ef_, efferent blood-vessel; _ep_,

epistome; _gl_, glandular pit; _int_, intestine or "second stomach"; _k_,

large funnel of the left nephridium; _ko_, opening of right nephridium, the

opening of the left is seen immediately below in section; _m_, mouth; _n_,

nerve concentration; _oe_, oesophagus; _oem_, oesophageal mesentery; _ov_,

ovary; _r_, rectum; _rm_, rectal mesentery; _rv_, right recipient

blood-vessel; _s_, septum; _st_, stomach; _t_, testis; _tm_, right lateral

mesentery. (From Benham.)]

_Phoronis_ is hermaphrodite, male and female reproductive cells being

formed in the same individual. The testes and ovaries form two white masses

lying on the left side of the stomach, one on one side and the other on the

other side of the efferent blood-vessel. The glands are traversed in all

directions by the diverticula given off from this trunk, and are thus well

supplied with blood; in fact both the ovary and the testis are formed by

the {458}multiplication and growth of the epithelial cells which cover

these diverticula. When ripe the ova and spermatozoa drop off into the

body-cavity and make their way to the exterior through the duct of the

kidney.

[Illustration: FIG. 231.—Three stages in the metamorphosis of the

Actinotrocha into _Phoronis_. A, Actinotrocha larva with the invagination

(_c_), which will form the trunk of the _Phoronis_ larva beginning to

appear. B, Stage with the invagination partly extruded. C, Stage when the

extrusion is complete and the alimentary canal has passed into it. C is

after Metschnikoff. _a_, Mouth; _b_, anus; _c_, invagination which

ultimately forms the greater part of the body of the adult.]

The ova are probably fertilised in the sea-water; they undergo the early

stages of their development whilst entangled amongst the tentacles of the

parent. The larval form to which they give rise was known long before its

connexion with the adult was demonstrated by Kowalevsky.[497] It is known

as the Actinotrocha (Fig. 231, A), and according to Caldwell has the

following structure in _Ph. kowalevskii_. The mouth is anterior, and the

anus terminal and posterior; the mouth is overhung by an immense prae-oral

lobe, which bears a special larval nerve {459}ganglion, and in some species

four eye-spots; at the base of this, but behind the mouth, is a ring of

larval tentacles. The prae-oral lobe and the tentacles are ciliated; the

margin of the lobe bears, however, specially long cilia, and there is also

a ring of long cilia around the anus.

Before the Actinotrocha stage has been reached the larva has forsaken the

shelter of its parent's tentacles, and swims actively about in the open

sea. As it grows older a finger-like involution of the skin (_c_) arises

just behind the tentacles on the ventral surface and grows into the body,

increasing greatly in length and becoming much folded. The larva now sinks

to the bottom of the sea, and after swimming round many times on its axis,

undergoes a very astonishing metamorphosis (Fig. 231, B, C). The

finger-like involution is suddenly turned inside out, and forms a large

projection on the ventral surface, into which the alimentary canal passes,

assuming a U-shape, as in the adult. This ventral process in fact forms all

the body of the adult behind the line of tentacles, and subsequently

contains, not only the alimentary canal, but the kidneys, the reproductive

organs, and a large part of the vascular system. At the same time the

prae-oral lobe breaks off, and, together with its ganglia and eye-spots,

passes into the mouth and is digested in the stomach; the larval tentacles

follow the prae-oral lobe, and are similarly digested. Their place is taken

by a ring of adult tentacles which commence to appear just behind the

larval tentacles before they fall off. The animal is now practically

adult.[498]

It is obvious that this astonishing metamorphosis is accompanied by the

rotation of the axes of the animal. The adult practically lives at right

angles to the larva. In the latter the anus marked the posterior end, and

the prae-oral lobe the anterior. The prae-oral lobe has disappeared in the

adult, but its position is marked by the mouth. The ventral surface has

enormously increased, and corresponds with the whole surface of the trunk.

To be consistent we must therefore regard the mouth of the {460}adult as

marking the anterior end of the animal, the anus the posterior. The short

line between the mouth and anus across the centre of the tentacular crown

marks the dorsal surface; and the line running all round the trunk from

anus to mouth, the ventral. In fact, in its usual position in its tube

_Phoronis_ is lying on its ventral surface, its back faces upwards, and the

anterior and posterior ends lie on one side or the other.

SPECIES AND AFFINITIES.—In his exhaustive memoir on the anatomy and

histology of _Phoronis_, Cori enumerates seven different species, and

quotes the characters of each as enumerated by eight different authors. He,

however, reserves his opinion as to the identity or distinctness of some of

these species. Benham in his account of _Ph. australis_ enumerates five

species, including amongst them _Ph. ovalis_, which, however, he regards as

probably a young form, an opinion in which Cori coincides. The latter

regards it as possibly a young form of _Ph. hippocrepia_.

Without comparing specimens of each of the alleged species, it is difficult

to come to any very satisfactory solution of the problem of how many

distinct species are at present known, but it seems probable that there are

at least six.

(i.) _Phoronis hippocrepia_ Wright.—Under this name is included the first

form, described and named by Wright in 1856; also _Ph. ovalis_, described

two years later by the same observer as a distinct form, though it now

seems probable that it is but a young form of _Ph. hippocrepia_. The

_Crepina gracilis_ of van Beneden is probably identical with this

species.

This species occurs in membranous tubes embedded in limestone, corals, or

oyster shells. Its length varies from 1.5 to 15 mm. The number of

tentacles varies from 16 to 86. It has been found off the coast of

Devonshire and in the Firth of Forth.

(ii.) _Phoronis kowalevskii_ Caldwell.—This name is given by Benham to

the species from Naples described by Caldwell, and replaces the name _Ph.

caespitosa_, which was given by Cori. This species is found in the Bay of

Naples, living in considerable colonies on submarine piles and posts. It

is not firmly attached to its substratum. The tube may be coated with

sand or other foreign particles. The length of the individuals varies

from 3 to 39 mm. The lophophore is simple, with from 50 to 100 tentacles.

(iii.) _Phoronis australis_ Haswell.—This is the giant of the genus, the

length of the individuals being from 3 to 5 (76-127 mm.) or rarely 6

inches. It lives in delicate transparent tubes, interlacing the walls of

the tube of a sea-anemone, _Cerianthus_. The arms of the lophophore coil

into two spirals. The colour is reddish or purple. Found in Port Jackson.

(iv.) _Phoronis buskii_ M‘Intosh.—This species was dredged by the

_Challenger_ {461}from a sandy bottom at a depth of 10 to 20 fathoms off

the Philippines. Its tube is covered with particles of sand, sponge

spicules, etc. Its length is 52 mm. or more (more than two inches). The

anatomy of this species closely resembles that of _Ph. australis_, and

Benham thinks that, in spite of the difference in their habitat, they may

belong to the same species.

(v.) _Phoronis architecta_ Andrews.—A species recently described by

Andrews from Beaufort, N.C. Its distinctive features are: "the formation

of isolated tubes covered by definite collections of sand grains; the

presence of special prostomial organs, possibly of use in the formation

of these tubes; the great development of the longitudinal muscles; the

presence of a ciliated groove in the digestive tract; the apparent

separation of the sexes."

(vi.) _Phoronis psammophila_ Cori.—Found in Faro, near Messina. The tube

is hyaline, and is covered by numerous grains of sand, some of

considerable size. The length of the individuals is 25 to 50 mm. There

are 60 to 90 tentacles. The colour is a fleshy red. A second species

discovered by Haswell in Port Jackson had no points of importance to

distinguish it from _Ph. psammophila_, except that no sand adheres to its

tube and the number of tentacles is slightly greater.

In addition to the various species of _Phoronis_, several distinct forms

of its larva, Actinotrocha, are known, and have been named without having

been traced into their corresponding adult form.

The position of _Phoronis_ in the animal kingdom has formed the matter of

considerable divergence of opinion amongst the naturalists who have studied

it. The earlier writers regarded _Phoronis_ as allied to the Gephyrea, and

it was for a long time classed with these animals, but placed in a separate

sub-Order, the _Gephyrea tubicola_, which was opposed to the _Gephyrea

nuda_, which comprised the true Gephyrea.

Caldwell referred _Phoronis_, the Brachiopoda, the Polyzoa, and the

Gephyrea to the same type of body structure, and Lankester subsequently

suggested the provisional name Podaxonia for this miscellaneous collection

of animals. Lankester divided his phylum Podaxonia into three classes: (i.)

the Sipunculoidea (Gephyrea), (ii.) the Brachiopoda, and (iii.) the

Polyzoa. The last-named class he divided into three sections: (_a_) the

Vermiformia, this includes the single genus _Phoronis_; (_b_) the

Pterobranchia, including the forms _Cephalodiscus_ and _Rhabdopleura_,

whose affinities with Balanoglossus were subsequently demonstrated; and

(_c_) the Eupolyzoa, including the forms treated as Polyzoa in the

following pages.

Masterman's recent researches[499] on _Phoronis_ seem to indicate {462}that

the Vermiformia, like the Pterobranchia, must in future be grouped with the

Hemichordata. He finds three well-defined coelomic spaces corresponding

with the epistome, the collar, and the trunk, and also representatives of

the collar pores, and is further inclined to believe that structures

representing the notochord exist in Actinotrocha.

Should Masterman's researches be confirmed, _Phoronis_ will be removed from

its present isolated and enigmatical position, and placed with

_Cephalodiscus_ and _Rhabdopleura_ amongst the Hemichordata, which will be

described in Vol. VII. of this work.

POLYZOA

BY

SIDNEY F. HARMER, M.A.

Fellow of King's College, Cambridge

{465}CHAPTER XVII

POLYZOA

INTRODUCTION—GENERAL CHARACTERS AND TERMINOLOGY—BROWN BODIES—HISTORY—

OUTLINES OF CLASSIFICATION—MARINE POLYZOA—OCCURRENCE—FORMS OF COLONY AND OF

ZOOECIA—OVICELLS—AVICULARIA—VIBRACULA—ENTOPROCTA.

The following pages[500] deal with animals whose very existence is hardly

known to those who are not professed naturalists. There are but few Polyzoa

which have earned the distinction of possessing a popular name, and most of

such names as do exist cannot be found outside treatises on Natural

History. It is true that many of the members of this group have been

vaguely termed "Zoophytes"; but this term implies no more than that they

possess a superficial resemblance to certain plants, and it must be

remembered that this habit of growth is assumed by many animals which have

nothing to do with the Polyzoa. The term "Coralline" is sometimes applied

to those calcareous Polyzoa which grow into coral-like forms; and the

Tertiary deposit known as the "Coralline Crag" is so called from the large

number of fossil Polyzoa which it contains.

The Polyzoa are none the less a most attractive group. Let any one examine

a dry piece of a brown paper-like substance (Fig. 232, A), which may be

found thrown up on the beach on many parts of our coasts. Of this species

(_Flustra foliacea_), the {466}so-called "sea-mat," an old writer says:

"For curiosity and beauty, I have not, among all the plants or vegetables I

have yet observed, seen any one comparable to this seaweed."[501] Viewed

with the microscope, the frond is seen to consist of two layers, placed

back to back, of oblong chambers, each of which is the dried body-wall of a

single individual. The whole is obviously a _colony_, and to this fact the

term Polyzoa refers.

The chambers just noticed are termed "zooecia." Each is rounded at one end,

near which is the "orifice," through which the tentacles of the living

animal can be pushed out. Two short, stiff spines usually occur on each

side of the orifice; and the symmetry of this forest of spines fully

justifies the above-quoted remark.

[Illustration: FIG. 232.—_Flustra foliacea_ L., Cromer. A, Natural size, B'

indicating the portion magnified in B (× 30): _a_, avicularium with closed

mouth, to the left of which are seen two avicularia with open months; _o_,

ovicell, forming the upper part of a zooecium. Ovicells are seen on three

consecutive zooecia. The operculum, which closes the orifice of the

zooecium, is seen in different positions in the individuals figured.]

The upper part of some of the zooecia is somewhat swollen, these swellings

representing the conspicuous "ovicells" of many other genera. In the early

part of the year each ovicell protects an orange-coloured egg or embryo,

and the larvae are readily liberated if the fresh colony be placed in clean

sea-water. "At least ten thousand" were hatched out in three hours from a

colony placed in a glass by Sir John Dalyell.[502] The larva swims freely

in the water for a short time, and should it find a {467}suitable

resting-place, it fixes itself and forms the starting-point of a colony,

the number of whose individuals is continually increased by the production

of buds at the growing edge. The "avicularia" of this species will be

alluded to later (see p. 482).

_F. foliacea_ has long been known to possess in the fresh state a

remarkable odour, which is described, according to the fancy of the

observer, as a strong odour of fish, or as the smell of violets after a

shower. Others have compared it to that of the orange or verbena, or to

that of a mixture of roses and geranium.

_Flustrella hispida_, another of our commonest Polyzoa, which may be found

between tide-marks on the stalks of _Fucus_, consists of a softish brown

encrustation, about one-sixteenth of an inch thick, covered by numerous

spines. If examined undisturbed in a rock-pool, or transferred to a glass

of sea-water, the brown mass will be seen to become surrounded by a

delicate bluish halo, which is about as thick as the encrusting mass

itself, and consists of the tentacles of the numerous individuals of the

colony. The microscope shows that each individual is provided with a

circlet of some thirty or more long, delicate tentacles, which together

form a graceful funnel (as in Fig. 233). At the bottom of the funnel is the

mouth, to which Diatoms or other minute organic particles are conveyed by

the cilia which fringe the tentacles. If the tentacles be touched with a

needle, the whole funnel is retracted with great rapidity, and in this

retracted condition we see no more than the body-walls of the animals.

After an interval the tips of the tentacles are cautiously protruded; the

tentacles are _gradually_ pushed out, at first in a close bundle, but

finally separating from one another to form the funnel which we have

already noticed.

There is hardly a more surprising spectacle in the whole animal kingdom

than a living fragment of the genus _Bugula_. The colony grows in the shape

of a small tree, whose height may amount to several inches; and is

characterised, in many species, by a spiral arrangement of the branches,

which makes the genus easy to recognise at first sight (Fig. 233, A). The

stem and branches are composed of a single layer of zooecia, arranged two

or more abreast. Each zooecium bears, on its outer side, a most singular

body termed an avicularium, from its resemblance to a bird's head. Imagine

a minute eagle's head attached by a short but flexible neck to the

zooecium. Suppose further {468}that this structure moves backwards and

forwards in a deliberate but determined fashion, its lower jaw usually

widely open so as to be nearly 180° distant from its position when closed.

Suppose that the lower jaw is moved by powerful muscles which can be

distinctly seen inside the transparent head of the avicularium, and that

every now and then it closes with a snap, seizing any unfortunate worm

which may happen to be within reach with a grasp of iron. The above gives a

very faint idea of the appearance of a living _Bugula_ colony, with its

hundreds of swaying avicularia, and with its tentacular funnels protruding

from their zooecia, and withdrawing themselves capriciously from time to

time.

[Illustration: FIG. 233.—_Bugula turbinata_ Alder, Plymouth. A, A small

colony (natural size); B, portion of a branch (× 50): _a_, _a'_,

avicularia, in different positions; _ap_, "aperture" (see p. 524); _b_,

polypide-bud, attached by its stomach to _b.b_, brown body; _m_, mouth,

surrounded by the circle of tentacles; two individuals to the right show

the tentacles partially expanded; _o_, ovicell; _s_, marginal spine. The

avicularia of some of the zooecia have been omitted in B.]

GENERAL CHARACTERS.—The Polyzoa are colonies, leaf-like or tree-like in

form, and often strongly resembling seaweeds, or forming encrustations on

the surface of stones and water-plants, or taking on other shapes. The

units of the colony are complete individuals (Fig. 234). The zooecium or

body-wall encloses a body-cavity, in which lies a digestive canal, with

which are closely connected the central nervous system and the retractile,

ciliated tentacles. The structures other than the zooecium constitute the

"polypide." The mouth (_m_) leads into the ciliated pharynx (_ph_) which is

followed by the oesophagus (_oe_) which again passes into the stomach

(_s_), whose walls are coloured by a {469}characteristic yellowish pigment.

The stomach gives off the intestine (_in_), which is lined by strong cilia,

by means of which a rotatory movement is given to the faeces contained in

it. This communicates by a narrow passage with the rectum (_r_), which

opens by means of the anus (_a_).

[Illustration: FIG. 234.—_Alcyonidium albidum_ Alder, Banyuls-sur-Mer.

Diagram showing the structure of a single zooecium with its polypide

retracted: _a_, anus; _d_, diaphragm; _e_, ectocyst; _em_, ectoderm; _f_,

funiculus; _g_, ganglion; _i_, intertentacular organ; _in_, intestine; _m_,

mouth; _mm_, mesoderm of body-wall; _o_, orifice; _oe_, oesophagus; _ov_,

ovary; _p_, parietal muscles; _ph_, pharynx; _p.v_, parieto-vaginal

muscles; _r_, rectum; _r.m_, retractor muscles (contracted); _s_, stomach;

_t_, testis; _tn_, tentacles; _t.s_, tentacle-sheath or kamptoderm. (After

Prouho.[503])]

In the retracted condition the tentacles (_tn_) lie in a cavity {470}which

opens to the exterior by the orifice (_o_). The cavity is bounded by a thin

membrane termed the "tentacle-sheath" (_ts_), and it is incompletely

subdivided, near its upper end, by a diaphragm (_d_), perforated by a

circular hole through which the tentacles can be protruded. The diaphragm

bears the thin folded collar characteristic of the Ctenostomata, the group

to which the species figured belongs (see p. 477).

Fig. 238, B, shows the tentacles of _Bowerbankia_ in their fully expanded

and partially expanded condition. Comparing this with Fig. 234, it will be

clear that when protrusion is taking place, the tentacles are forced in a

bundle, tips first, through the diaphragm and next through the orifice of

the zooecium, the alimentary canal offering no resistance to this movement,

owing to the length of the oesophagus. A moment's consideration will show

that the bases of the tentacles, in passing through the orifice, will carry

with them that part of the flexible tentacle-sheath to which they are

attached; and it will further be clear that so much of the tentacle-sheath

as is thus protruded will be turned inside out. This process of

"evagination" continues until its further progress is stopped by the

retractor-muscles (_r.m_), and by the parieto-vaginal muscles (_p.v_),

which pass from the interior of the body-wall to the upper part of the

tentacle-sheath. The latter has now become the delicate layer which

connects the expanded tentacles with the zooecium; and the anus (Fig. 238,

C, _a_) opens directly to the exterior. Since the name "tentacle-sheath" is

thus descriptive of the condition of retraction only, the term

"kamptoderm"[504] has been suggested as an alternative name.

The presence of a complete digestive canal and the ciliation of the

tentacles in Polyzoa are conspicuous differences between these animals and

the Hydroids, with some of which the Polyzoa may have a marked external

similarity.

The outermost[505] layer of the body-wall is known as the "ectocyst" (Fig.

234, _e_). This may be densely calcareous, in which case the dried Polyzoon

differs little in appearance from the living animal with its tentacles

retracted; or it may be partially calcified, or it may consist entirely of

a flexible cuticle, {471}as in Fig. 234. The ectocyst is prolonged through

the orifice (_o_) as far as the diaphragm (_d_).

Forms with a calcareous ectocyst are commonly ornamented with ridges or

other patterns, which are often of great beauty. The ectocyst in these

cases is commonly interrupted at intervals by pores (Fig. 239, C), into

which processes of the "endocyst"—the living, internal part of the

body-wall—extend. These may appear as superficial pores, which apparently

open to the exterior in the dried condition, or they may perforate the

septa between adjacent individuals. This may be strikingly demonstrated by

decalcifying a branch of _Crisia_ (Fig. 237), in which the zooecia then

appear connected by numerous strands of tissue. In many marine forms the

communications between the individuals are in the form of small sieve-like

plates known as "rosette-plates."

The endocyst may consist of definite layers of ectoderm (_em_) and mesoderm

(_mm_), as in Fig. 234, but the mesoderm is commonly in the form of a loose

network, some of which is attached to the body-wall, some to the alimentary

canal, some forming connecting strands between these two layers, and other

cells floating about freely in the body-cavity. These mesodermic structures

are often spoken of as the "funicular tissue," since one or more strands of

it commonly take on the form of a definite "funiculus" (_f_). This

structure may bear the ovary (_ov_), while the testes (_t_) are found,

commonly in the same zooecium, attached to various parts of the body-wall.

The eggs and spermatozoa, when ripe, break off and float freely in the

body-cavity.

The funicular tissue was at one time described as a "colonial nervous

system." The idea expressed by this term must be considered erroneous from

the fact that no nervous co-ordination of the individuals is known to

exist, in the vast majority of cases. The actual nervous system consists of

a ganglion (_g_) placed between the mouth and anus of each polypide, and

lying in a small circular canal (not shown in Fig. 234) which immediately

surrounds the oesophagus. This canal is developed in the bud as a part of

the body-cavity, from which it becomes completely separated in marine

forms. The Polyzoa have no vascular system.

BROWN BODIES.—In the majority of cases, an extraordinary process of

regeneration takes place periodically during the life of each zooecium. The

tentacles, alimentary canal, and nervous system break down, and the

tentacles cease to be capable of being {472}protruded (Fig. 235, 1). The

degenerating organs become compacted into a rounded mass (Fig. 235, 2 and

3, _b.b_), known from its colour as the "brown body." This structure may

readily be seen in a large proportion of the zooecia of transparent

species. In active parts of the colony the body-wall next develops an

internal bud-like structure (Fig. 235, 1, _b_), which rapidly acquires the

form of a new polypide (Fig. 235, 2 and 3). This takes the place originally

occupied by the old polypide, while the latter may either remain in the

zooecium in the permanent form of a "brown body," or pass to the exterior.

In _Flustra_ the young polypide-bud becomes connected with the "brown body"

by a funiculus (Fig. 235, 1, 2). The apex of the blind pouch or "caecum" of

the young stomach is guided by this strand to the "brown body," which it

partially surrounds (3). The "brown body" then breaks up, and its fragments

pass into the cavity of the stomach, from which they reach the exterior by

means of the anus.

[Illustration: FIG. 235.—_Flustra papyrea_ Pall. Naples. × 50. Illustrating

the development of a new polypide after the formation of a "brown body." In

1, _a_, two masses formed from the alimentary canal; _b_, young

polypide-bud; _b.b_, degenerating tentacles; _c_, connective tissue: 2,

another zooecium, later stage; _b.b_, brown body; _r.m_, retractor muscles;

_s_, stomach; _t_, tentacles of new polypide; _t.s_, tentacle-sheath: 3,

the same zooecium, 191 hours later; letters as in 2. 1 and 2 are seen from

the front, 3 from the back.[506]]

There is some reason to believe[507] that these remarkable processes are

connected with the removal of waste nitrogenous matters. The Marine Polyzoa

are not known to be, in most cases, provided with definite excretory

organs, although it is {473}possible that the intertentacular organ (Fig.

234, _i_) described on p. 508 may in some cases perform excretory

functions. There can, however, be little doubt that some kind of excretion

takes place in the Polyzoa; and in considering what organs could possibly

perform this work, our attention is arrested by the alimentary canal. The

digestive organs of the young bud are perfectly colourless. As growth

proceeds, certain parts acquire a yellowish, and later a brown colour. The

degeneration of the polypide is followed by the grouping of large numbers

of the free cells of the body-cavity into a mass which closely surrounds

the incipient "brown body." Under their action, the latter becomes

considerably smaller, probably as the result of the absorption of matters

of nutritive value into other tissues. The final result is the formation of

the compact "brown body," whose colour is principally derived from the

pigment formerly present in the alimentary canal. Experiments made by

introducing into the tissues of the Polyzoa certain artificial pigments

which are known to be excreted by the _kidneys_ when injected into the

bodies of other animals, have given some reason for believing that the

appearance of the brown pigment in the wall of the digestive organs is, in

part, a normal process of excretion; although that process is not entirely

carried out by the organs in question.

Little is known with regard to the duration of life of a single polypide;

but some information bearing on this question may be obtained from a set of

observations made on _Flustra papyrea_.[508] The table gives the number of

days from the time at which the polypides were noticed to commence their

degeneration:—

Days.

2 "Brown body" partially formed, the parts of the polypide being still

easily recognisable.

5 Tentacles still just recognisable: appearance of new polypide-bud.

8 Stage shown in Fig. 235, 2.

11 Union of apex of stomach with "brown body."

16 "Brown body" half surrounded by stomach, and preparing to break up

(Fig. 235, 3).

21 "Brown body" broken up into numerous fragments, contained in the

alimentary canal of the new polypide.

35 "Brown body" almost completely absorbed.[509]

{474}These results did not hold good for all the zooecia in a single

colony. In some, the "brown body" was not completely got rid of at the end

of sixty-eight days, the conclusion of the experiment.

So striking are the facts relating to the "brown bodies" that it has been

believed[510] that what we have above described as the individual really

consists of two kinds of individuals: firstly, the "polypide" or complex of

tentacles and digestive organs; and secondly, the "zooecium," or house of

the zooid or polypide, corresponding with what has been described above as

the body-wall. The one individual, the zooecium, is on this view provided

with successive generations of the second kind of individual, the polypide;

and these latter function as the digestive organs of the two-fold organism.

This view, though fascinating at first sight, is not borne out by an

examination of all the facts of the case, especially when the Entoprocta

are taken into account.

HISTORY.—The history of the Polyzoa, as far as 1856, has been fully treated

by Allman in his great work on the Fresh-water Polyzoa;[511] but a few

words may be said on this subject.

The Polyzoa attracted comparatively little attention before the beginning

of the present century. Originally passed over as seaweeds, their real

nature was established in connexion with the discovery of the animal nature

of corals. So great a revolution could hardly be accepted without a

struggle, and even Linnaeus went no further in this direction than to place

them in a kind of half-way group of "zoophytes," whose nature was partly

animal and partly vegetable. It is hardly necessary to point out that this

view has now been abandoned by common consent; and indeed there is no more

reason for regarding an animal as showing an approach to the plants because

it grows in the external semblance of a seaweed than there would be for

supposing a bee-orchid to be allied to the animal kingdom because of the

form of its flowers.

But the claims of the Polyzoa to rank as a separate class were by no means

admitted with the discovery that they were animals. They were still

confounded with Hydroids, Alcyonarians, or Corals until their possession of

a complete alimentary canal was recognised as a feature distinguishing them

from those {475}animals. This was principally due to the observations of J.

V. Thompson[512] in Ireland, who introduced the term POLYZOA; and of C. G.

Ehrenberg[513] in Germany, who proposed the class-name BRYOZOA, or

moss-like animals.

It is impossible to avoid all mention of the controversy which has raged

with regard to these two rival terms. The controversy is for the present at

rest, the name Polyzoa being employed by the majority of English writers,

amongst whom must be mentioned Allman, Busk, Hincks, and Norman, admittedly

authorities of the first rank; while Bryozoa is employed by practically all

the Continental writers.

The priority of Thompson's name is unquestioned. While Ehrenberg, however,

definitely introduced Bryozoa _as the name of a group_, Thompson was less

precise in this respect, although he states[514] that his discovery "must

be the cause of extensive alterations and dismemberments in the class with

which they [the Polyzoa] have hitherto been associated." Thompson, in fact,

clearly understood that the Polyzoa could no longer rank with the Hydroids.

The controversy has been summarised by Hincks, in his _History of the

British Marine Polyzoa_,[515] where references to other papers on the same

subject are given.

The Polyzoa were associated by H. Milne-Edwards with the Tunicata in the

group Molluscoidea (Molluscoïdes[516]), to which the Brachiopoda were

afterwards added by Huxley.[517] A knowledge of the development of the

Tunicata has, however, shown that these animals must be withdrawn from any

association with the other two groups; while there is little real evidence

that even the Brachiopods have anything to do with the Polyzoa.

CLASSIFICATION.—The Polyzoa are divided into two sub-classes:—I, the

ENTOPROCTA; and II, the ECTOPROCTA.[518] Although the character referred to

by these terms is merely the position of the anus with relation to the

tentacles,[519] there can be no doubt that the two groups differ widely

from one another in {476}many important respects. I do not, however, accept

the view, maintained by some authors, that the Entoprocta and the

Ectoprocta are two separate classes which are not nearly related.

The base from which the whole set of tentacles springs is known as the

"lophophore."[520] In the Entoprocta (Fig. 236, 1) the lophophore is

circular; the mouth is situated near the margin of the area surrounded by

the tentacles; and the anus is found _within_ the circlet, near the end

opposite to the mouth.

In (2) and (3), representing the Ectoprocta, the anus is _outside_ the

series of tentacles. In the majority of cases, including all the marine

Ectoprocta and one or two of the fresh-water forms, the lophophore is

circular (2), the mouth occurring at the centre of the circle, and not

being provided with a lip. These forms of Ectoprocta constitute the Order

GYMNOLAEMATA,[521] the dominant group of the Polyzoa in respect of number

of genera and species. The remaining Ectoprocta belong to the exclusively

fresh-water Order PHYLACTOLAEMATA,[522] in which the mouth is protected by

an overhanging lip or "epistome"; the ground-plan of the tentacles is,

except in _Fredericella_, horse-shoe shaped (Fig. 236, 3), and the

tentacles themselves are usually much more numerous than in the other

cases.

[Illustration: FIG. 236.—Ground-plan of the lophophore in (1) Entoprocta,

(2) Gymnolaemata, (3) Phylactolaemata: _a_, anus; _ep_, epistome; _m_,

mouth. The tentacles are represented by shaded circles.]

The general characters of these divisions will be more easily understood by

referring to the figures given of living representatives of the groups. The

Entoprocta are illustrated by Figs. 243-245; the Gymnolaemata by Figs. 238,

240; and the Phylactolaemata by Figs. 247, 248.

{477}The Gymnolaemata include three Sub-Orders:—

1. CYCLOSTOMATA.[523]—Body-wall densely calcareous, the zooecia being

more or less tubular, usually with a _circular_ orifice (Fig. 237).

2. CHEILOSTOMATA.[524]—Body-wall of varying consistency. The orifice is

closed, in the retracted state of the polypide, by a chitinous _lip_ or

"operculum," which is more or less semicircular (Figs. 239, 241).

3. CTENOSTOMATA.[525]—Body-wall always soft. The cavity into which the

tentacles are retracted is closed by a frill-like membrane, the edges of

whose folds have some resemblance to the teeth of a _comb._ This

membrane, the "collar," is seen in different conditions of protrusion or

retraction in Figs. 234, 238. The stomach may, in this group, be preceded

by a muscular gizzard (Fig. 238, C, _g_).

OCCURRENCE.—By far the larger number of the Polyzoa are inhabitants of the

sea. A recently published catalogue[526] of marine Polyzoa includes nearly

1700 living species; and of these, the great majority belong to the

Gymnolaemata. This group is further known to include an enormous number of

fossil forms. Not only do we find that in living Polyzoa the members of a

single Order largely outnumber the remainder of the Polyzoa, but we may

further notice that the Cheilostomata, one of the sub-Orders of the

dominant group, are at present largely in excess of the whole of the rest

of the Polyzoa taken together.

Polyzoa may be collected with ease on almost any part of our coasts. The

fronds of the "sea-mat" (_Flustra foliacea_) are thrown up by the waves in

thousands in places where the bottom is shallow and sandy. The bases of the

larger seaweeds growing on rocks between tide-marks are nearly always

thickly covered with encrustations of _Flustrella hispida_ or of species of

_Alcyonidium_, in places where they are kept moist by being covered with a

sufficiently thick layer of other algae. Rocks which are protected from the

sun may be coated with calcareous Cheilostomes; and these are also found,

in company with branching Polyzoa of various kinds, on the bases of the

_Laminaria_ thrown up by gales or exposed at spring tides. The graceful

spirals of _Bugula turbinata_ (Fig. 233, A) may be found hanging from the

rocks at extreme low water; while colonies of _Scrupocellaria_, remarkable

for their vibracula (see p. 484), are common in many places between

tide-marks. Certain species affect the mouths of estuaries.

{478}_Membranipora membranacea_ commonly covers many square inches of the

frond of _Laminaria_ with its delicate lace-like encrustation. Nitsche[527]

has shown that this species has its calcareous matter deposited in plates,

separated by intervals of uncalcified ectocyst. The effect of this

arrangement is to make the colony flexible, and to enable it to adapt its

shape to the movements of the _Laminaria_, which is swayed to and fro by

the action of the waves. Many of the calcareous forms growing on

_Laminaria_ have no special arrangement of this kind, and they accordingly

grow in colonies whose area is so small that the greatest movements to

which the seaweed is liable are not sufficient to crack or break the

colony.

Many species show a decided, or even exclusive, preference for particular

situations; as, for instance, species of _Triticella_, which are only found

on certain Crustacea. Many encrusting forms prefer the inside of dead

shells of _Pecten_, _Cyprina_, etc., to any other habitat.

_Terebripora_[528] excavates tubular cavities in the substance of the

shells of Molluscs. _Hypophorella_[529] inhabits passages which it forms in

the walls of the tubes of the Polychaets, _Lanice_ and _Chaetopterus_.

_Lepralia foliacea_, one of the Cheilostomata, forms masses which may reach

a circumference of several feet, simulating a small coral-reef. Its

contorted plates are a regular museum of Polyzoa, so numerous are the

species which delight to find shelter in the quiet interstices of the

colony. The exquisite little colonies of _Crisia eburnea_ are commonly

found on red seaweeds, or on the branches of the Hydroid _Sertularia_.

The Polyzoa are found at all depths, certain Cheilostomes having been

recorded from 3000 fathoms. The Cyclostomes dredged by the "Challenger"

were all found in depths of 1600 fathoms or less, while the Ctenostomes are

a distinctly shallow water group, most having been found at less than 40

fathoms, and only three at so great a depth as 150 fathoms.[530]

A few forms (_Membranipora pilosa_, _Scrupocellaria reptans_, etc.) are

known to be phosphorescent;[531] but it is not known what is the purpose of

this phenomenon.

{479}EXTERNAL FORM.—The Polyzoa may be roughly divided into (1) encrusting

forms, usually calcareous, but sometimes soft; and (2) erect forms, which

are either rigid or flexible. This flexibility can coexist with a highly

calcified ectocyst, as in _Crisia_ (Fig. 237), _Cellaria_, and others in

which the branches are interrupted at intervals by chitinous joints. The

coral-like forms may assume the most exquisite shapes, pre-eminent among

which are the lovely net-like colonies of _Retepora_. Polyzoa of this type

are seldom found between tide-marks, where their brittle branches would be

liable to be snapped off by the waves. The _erect_ species which occur in

such positions are flexible, although flexible species are by no means

restricted to the zone between tide-marks.

[Illustration: FIG. 237.—_Crisia ramosa_ Harmer, Plymouth. A, End of a

branch, × 1; B, another branch, × 20, showing the chitinous joints, the

tubular zooecia characteristic of Cyclostomata, and the pear-shaped ovicell

with a funnel-shaped orifice at its upper end.]

Although the form of the colony is very different in different Polyzoa, a

pocket-lens will usually show whether a given specimen belongs to the group

or not. The surface is nearly always more or less distinctly composed of

zooecia, or at least shows their orifices. The entire colony may be built

up of these zooecia; and this is by far the commonest arrangement, both in

encrusting and in erect forms. In certain genera, however, and particularly

in some Ctenostomes (Fig. 238), and in most of the Entoprocta, the

{480}individuals grow out at intervals from a cylindrical stem or "stolon"

(_st_), which is not composed of zooecia.

The Cyclostomata may assume an erect or encrusting habit. Their zooecia are

always more or less cylindrical; the upper ends being often completely

free, although in many cases the whole zooecium is closely adnate to its

neighbours. In the breeding season the forms which belong to this group are

provided with curious "ovicells," which contain the embryos. These may

either be pear-shaped swellings on the branches (_Crisia_, Fig. 237), or

they may form inflations of the surface, between the zooecia. The mature

ovicell is provided with one or more openings, through which the larvae

escape.

[Illustration: FIG. 238.—_Bowerbankia pustulosa_ Ell. and Sol., Plymouth.

A, Fragment of a colony, natural size, showing the branching stem, bearing

tufts of zooecia: B, one of these tufts, with the growing apex of the stem

(_st_), × 27; _b_, young zooecia (buds); _c_, the "collar" characteristic

of Ctenostomata; _t_, tentacles; C, a single zooecium, with expanded

tentacles, more highly magnified; _a_, anus; _c_, collar; _g_, gizzard;

_i_, intestine; _o_, oesophagus; _s_, stomach.]

The Ctenostomata rarely have even the slightest trace of calcareous matter.

_Alcyonidium_ and its allies form soft encrustations, or may even grow into

erect masses six inches or more in height (_A. gelatinosum_). In this type

the zooecia are often so closely united that it may be difficult or

impossible to make out their limits in the living colony. Many of the

dendritic or branching {481}Ctenostomes (Fig. 238) are characterised by an

extreme delicacy of habit. The zooecia in these cases are sharply marked

off from the stem. They are either cylindrical or ovoid, being commonly

attached by a very narrow base, so that in some species they readily fall

off, and may thus be completely absent in certain parts of the colony. In

such forms as _Vesicularia spinosa_, it requires considerable experience to

recognise a stem which has lost its zooecia as being part of a Polyzoon. In

_Mimosella_ the zooecia possess a remarkable power of movement on the stem,

similar to that possessed by the leaflets of the Sensitive Plant.[532] In

certain forms (_Bowerbankia_, _Amathia_) the zooecia occur in groups

separated by intervals which are devoid of zooecia, but in other cases they

may have a more irregular arrangement. The collar to which this group owes

its name is by no means a conspicuous feature. Its position when retracted

has been shown in Fig. 234, while Fig. 238 further illustrates its

relations.

The Cheilostomata grow in a great variety of forms, and also show a wide

range of character in their zooecia. The orifice is commonly surrounded by

stiff spines (Fig. 257, p. 524), which perhaps have the function of

protecting the delicate polypides from the sudden impact of foreign bodies.

These spines may attain an enormous development, as in _Bicellaria

ciliata_, and some forms of _Electra_ (_Membranipora_) _pilosa_ (Fig. 256,

A).

The operculum is usually, though by no means always, a conspicuous feature

of the Cheilostome zooecium. It is invariably of chitinous consistency, and

is more or less semicircular in outline, the straight edge forming a hinge

on which the operculum opens. In some cases the orifice is surrounded by a

raised margin or "peristome" (Fig. 255, B, C); the operculum is then

situated at the bottom of a depression of the surface, and may be concealed

from view. In others, in which the front wall of the zooecium is membranous

(_Bugula_, Fig. 233), the operculum is merely a part of this membrane, and

so is quite inconspicuous; and in cases of this kind the membranous wall

may be protected by an arched spine, the "fornix," developed from one side

of the zooecium (Fig. 254, _f_). The ovicells are commonly a conspicuous

feature of this group, although they are believed to differ fundamentally

from those of Cyclostomata. They have the form of a helmet-like covering

overhanging the orifice (Figs. 240, 241), {482}and may be either prominent

or more or less concealed by the growth of adjacent parts of the zooecia.

The presence of ovicells of this description is perfectly distinctive of

the Cheilostomata.

AVICULARIA AND VIBRACULA.—Most singular of the external appendages of the

Cheilostomata are the extraordinary "avicularia" and "vibracula" of some

genera.[533] By the comparison of a carefully selected series of genera, it

has been established that the avicularium is a special modification of a

zooecium. One of its least modified forms is found in _Flustra foliacea_

(Fig. 232), where the avicularia (_a_) are small zooecia with a

conspicuously large operculum ("mandible"). Avicularia of a similar type

occur in _Cellaria_ (Fig. 239, A), _Schizotheca_, etc., the avicularium

occupying the place of an ordinary zooecium. These are the "vicarious"

avicularia of Mr. Busk.[534]

[Illustration: FIG. 239.—Forms of avicularia. A, _Cellaria fistulosa_ L.,

Plymouth, × 43; _a.z_, avicularian zooecium, with closed mandible; _o_,

operculum of zooecium: B, _Schizoporella unicornis_ Johnst., Scilly Is., ×

43; zooecium bearing two avicularia; _m_, opened mandible of avicularium;

_s_, sinus of orifice: C, zooecium of _Smittia landsborovii_ Johnst.,

Plymouth, × 43; the operculum is seen at the bottom of a depression

surrounded by a thin collar or "peristome," in an emargination of which is

seen an avicularian zooecium (_a.z_); _m_, mandible (opened); _p_, pores;

_t_, tooth.]

In the next stage (Figs. 239, B, 256, B) the avicularian zooecium is

further reduced; it has in most cases lost its place in the series of

individuals, and is found instead seated on some part of an ordinary

zooecium ("adventitious" avicularia). The avicularium now consists of a

much reduced zooecium, bearing the well-developed operculum or mandible.

{483}Having arrived at this point, the avicularia seem to lose all sense of

the propriety of remaining in the positions once occupied by zooecia. They

have become degraded to the rank of appendages of the zooecia, and as such

they may occur in an astonishing variety of positions. Sometimes one occurs

on each zooecium in the middle line, or asymmetrically, or even on the top

of the ovicell; in other cases the orifice is flanked by an avicularium on

each side (Fig. 239, B). Sometimes (_Cellepora_) the avicularia are of more

than one kind, some being large and some small, some having a pointed

mandible and others a mandible with a rounded spoon-like end.

In the cases so far considered, the body of the avicularium is fixed. The

highest differentiation acquired by these structures occurs in cases like

_Bugula_, where they are borne on flexible stalks, which may even exceed

the avicularia in length.[535]

[Illustration: FIG. 240.—_Bugula turbinata_, showing avicularia (_a_,

_a'_). The figure is explained on p. 468.]

In _Bugula turbinata_ (Fig. 240) each zooecium is provided with one of

these appendages, attached to the base of the outer of the two spines which

border its orifice. The avicularia of the two edges of the flattened branch

are much larger than those of the more internal zooecia. The upper jaw is

strengthened by a kind of buttress, or thickening of the ectocyst, which

passes on each side across the avicularium to the hinge-line of its

mandible. The upper part of the beak is strongly hooked, while the tip of

the mandible bears a {484}prominent spike, which fits inside the upper beak

when the jaw snaps. A great part of the head is filled with a strong

muscle, whose fibres exhibit a distinct transverse striation, and converge

into a median tendon. The latter is inserted into the middle of the

mandible. The muscle serves to close the jaws, and is the representative of

the muscles by which the operculum is closed in an ordinary zooecium. The

lower jaw is opened by means of a pair of muscles which are situated

immediately under the ectocyst of the avicularium, and pass into the

mandible close to its hinge.

[Illustration: FIG. 241.—Illustrating the transition from avicularia to

vibracula. A, _Microporella ciliata_ Pall., Scilly Is., × 62; _a_,

avicularium with short mandible (closed); _a'_, avicularium with

vibraculoid mandible (open); _m.p_, median pore; _o_, ovicell: B,

_Mastigophora dutertrei_ Aud., Shetland Is., × 47; _s_, sinus of orifice;

_v_, seta of vibraculum (or vibraculoid avicularium).]

Within the jaws, in the region which we may term the palate, is a rounded

knob, which bears a tuft of delicate sensory hairs, which doubtless enable

the avicularium to recognise the presence of any foreign body. The closure

of the mouth may, indeed, be instantaneously induced by touching it with

the point of a needle. It has been suggested that a small mass of cells

which bears these hairs may represent the rudiment of the polypide.

The "vibraculum" (Fig. 242) is regarded as an avicularium in which the

mandible has become elongated, so as to form a {485}thin, chitinous "seta,"

which from time to time moves through the water. The part of the vibraculum

which represents the zooecium commonly bears a tubular rootlet, used for

attaching the colony to the substance on which it is growing (Fig. 254, p.

517).

In _Microporella ciliata_ (Fig. 241, A) the avicularia are very variable,

and in some cases take on a "vibraculoid" character. But in the

fully-developed vibraculum (Fig. 242) there is usually no such compromise

of characters. It may, however, be noted that _Scrupocellaria scabra_ (Fig.

254), which belongs to a genus characterised by its highly differentiated

vibracula, possesses structures (_v.z_) which could hardly be distinguished

from avicularia were it not for the presence of the rootlet (_r_).

In the course of some observations which I had the opportunity of making on

_Bugula calathus_ at Naples, a fine hair offered to a small colony was

seized with such force by the avicularia that the entire colony was lifted

out of the water by the hair. The same colony had captured (1) a small

_Nereis_, which it held with several of its avicularia; (2) an Anisopod

Crustacean, 2½ mm. long; and (3) a small Amphipod, which was held by one of

its antennae. The Anisopod was held by the tip of one leg with one

avicularium, and by the penultimate joint of one of its chelae with an

avicularium of another branch. It was captured in such a way that its

chela, the "hand" of which was about half as long as the avicularium,

actually closed on to the avicularium without being able to effect its

escape. A little later the other chela was caught by another avicularium.

Curiously enough, however, an avicularium did not necessarily close even

when part of a captured animal was actually in its mouth. The avicularia

made no attempt to place themselves in an advantageous position for

catching fresh parts of the _Nereis_, which they might easily have done.

The avicularia which had captured prey remained motionless. The others

moved backwards and forwards (cf. the various positions of the avicularia

shown in Fig. 240) ten times in ¾ to 1 minute, snapping their jaws perhaps

once in that time. The two Crustacea were still retained by the avicularia

two days later. On the next day they had both disappeared; but the colony

had again caught the _Nereis_, which had previously effected its escape

with the loss of nearly all its tentacular cirri.

These observations, and others which have been recorded, do not,

unfortunately, give any information as to the purpose of the {486}movements

of the avicularia and vibracula. It is obvious that they may be defensive

in character; and it cannot be doubted that the avicularia can prevent

inquisitive worms from straying at will over the surface of the colony.

There is no evidence to show that animals are discouraged from interfering

with a _Bugula_ owing to the presence of its defensive weapons.

[Illustration: FIG. 242.—_Caberea ellisii_ Flem., Norway. × 40. Back view

of part of a branch. The large vibracular zooecia (_v.z_) occupy nearly the

whole of the surface. _s_, Seta of vibraculum; _z_, zooecia.]

It is not, indeed, certain what are the enemies against which the Polyzoa

have specially to guard. Sea-urchins and certain Molluscs are known to

browse on Polyzoa. Fresh-water Polyzoa, in which avicularia and vibracula

are absent, are attacked by the larvae of Insects, and by Triclad

Planarians. I have found the latter with their long pharynx everted and

completely buried in a _Cristatella_ colony. It is possible that some

marine Cheilostomes may be saved from attacks of this kind owing to the

existence of their armoury of avicularia and vibracula. It is also possible

that these structures are of service by removing foreign particles which

might otherwise settle on the colony, and tend to block up its orifices. It

has further been suggested that animals seized by the avicularia may be

held until they die, and that their disintegrating particles may then be

carried to the mouths of the polypides by the ciliary currents of the

tentacles; but proofs of this suggestion are {487}wanting, and it must be

admitted that the subject needs further elucidation.

The vibracula ordinarily remain stationary for some little time, every now

and then giving a sweep through the water. In the majority of cases these

structures, like the avicularia, act perfectly independently of one

another, so far as can be made out; but in _Caberea_ (Fig. 242) the

vibracula move in unison, the simultaneous action of the whole series,

after a period of quiet, being described as "positively startling."[536]

It has been stated by Busk[537] that the entire colony in _Selenaria_ and

_Lunulites_ may be moved from place to place by the large vibracula which

these forms possess.

[Illustration: FIG. 243.—_Pedicellina cernua_ Pall., Guernsey. Entire

colony. × 27. The colony has three growing ends, _a_; 1-8, individuals of

colony; 1 and 8 are quite immature; and 7 (tentacles retracted) is still

young; 2, is seen in longitudinal section; _g_, generative organ, and below

it the ganglion; _m_, mouth; _r_, rectum; _s_, stomach; between _g_ and _r_

are three embryos in the brood-pouch; the tentacles are retracted; in 5 and

6 the tentacles are expanded; in 6 two embryos are seen within the circle

of the tentacles, to the left of them is the rectum, and to the right the

mouth; 3 is in the act of losing its calyx, and has already developed the

beginning of a new polypide-bud; in 4 the primary calyx has been lost, and

the new calyx is clearly marked off from the stalk.]

ENTOPROCTA.—The Entoprocta, although a very small sub-class, deserve

special consideration, if for no other reason, from the fact that many

writers regard them as the most primitive group of Polyzoa, and

consequently as the forms which show most affinity to other classes of

animals.

Their most obvious characteristic is, as we have already seen,[538] the

position of the anus within the circle of tentacles. The individuals formed

by budding always remain more separate from one another than those of most

Ectoprocta.

{488}The commonest Entoproctous genus is _Pedicellina_, a graceful little

animal, which occurs on many parts of our coast. It may often be discovered

by looking carefully on the pink, jointed, calcareous alga, _Corallina_,

which may be found growing at the edges of deep and cool rock-pools not too

far above low-water mark. Its creeping stem or "stolon" is firmly attached

to the surface of the seaweed, and sends off vertical stems here and

there.[539] Each stem bears a "calyx," which is practically an individual

of the colony. The stolon terminates, at one or both ends, in a

growing-point (_a_), from which new individuals are budded off. The stalks

bend from time to time in a curious spasmodic manner, by which means the

calyces are moved about with an irritable and angry air. A good idea of the

way in which the tentacles are folded away when the animal is disturbed may

be obtained by putting the two wrists together, with the fingers spread out

to represent the tentacles, the retraction of which would be represented by

turning the tips of the fingers down into the space, the "vestibule,"

between the two palms. A delicate fold of skin growing from the edge of the

calyx closes over the retracted tentacles, owing to the contraction of a

sphincter muscle present in its circular edge. The body-wall is not

separated from the alimentary canal by a definite body-cavity, so that

there is no obvious distinction between the polypide and the zooecium. The

existence of the Entoprocta is in fact a strong reason for refusing to

admit that these two terms correspond with two different kinds of

individuals.

Let us now imagine the condition we should have if a large and continuous

cavity were developed between the alimentary canal and the body-wall. The

body-wall would clearly have the general relations of a zooecium, while the

alimentary canal and tentacles would obviously correspond with the

polypide. The existence of the body-cavity would make it possible for the

animal to _retract_ its tentacles instead of merely turning them in.

Regarded in this way, there is but little difficulty in comparing the

Ectoprocta with the Entoprocta.

The calyces are deciduous, _i.e._ they are lost from time to time, the end

of the stalk then producing a polypide-bud, which {489}forms the vestibule

and alimentary canal of a new calyx. Hence the phenomenon which may so

commonly be noticed in _Pedicellina_ of a "young head on old shoulders."

The loss of the calyces may have some relation to the formation of the

"brown bodies" in the Ectoprocta.

Another Entoproct, _Loxosoma_ (Fig. 245) is remarkable for being the only

Polyzoon which is not colonial. The buds, which are formed in two lateral

series, break off as soon as they are mature, and at once begin to lead an

independent existence. _Loxosoma_ is further remarkable for being almost

invariably found commensally with other animals, where it may occur in

enormous numbers. _L. phascolosomatum_, common in the Channel Islands, is

only found on the tip of the tail of _Phascolosoma_ (see p. 428), which

inhabits the mud of _Zostera_-beds. Other species are found on the external

surface of certain sponges (_Tethya_, _Euspongia_, _Cacospongia_); or on

the outside of a compound Ascidian, _Leptoclinum_, which may itself be

carried about as a detachable covering on the back of a crab (_Dromia_).

Another species is found on the ventral surface of the Polychaet

_Aphrodite_, and of its ally _Hermione_.

[Illustration: FIG. 244.—Side view of _Loxosoma annelidicola_ Van Ben. and

Hesse. × 50. (From Prouho.)]

_L. annelidicola_, an interesting species recently investigated by

Prouho,[540] was originally described in 1863 as a Trematode, under the

name of _Cyclatella_. It escaped further notice until it was again found in

the neighbourhood of Roscoff, in Brittany, on certain Polychaets belonging

to the family Maldanidae (see p. 332). The calyx has a very flattened form,

and is borne on a short stalk, which terminates in a large attaching disc,

formerly mistaken for the sucker of a Trematode. The features in which this

species differs from other members of the genus are shown by M. Prouho to

be correlated with its mode of life. The animal has the habit of lying flat

on its back, the disc at the end of its stalk being firmly attached to the

skin of the worm, and its short stalk being bent round into a curve so

{490}as to bring the calyx into a supine position, with its lophophore

directed upwards. This habit, together with its flattened form, prevents it

from being crushed between the worm and its tube. But without some further

provision its position might be merely a source of danger. For supposing

the calyx to be directed backwards in relation to the worm, a sudden

backward movement of the latter into its tube might bring the _Loxosoma_

into fatal contact with the inner surface of the tube. There would

obviously not be sufficient room to turn round in a _vertical_ plane, so as

to bring the body into a position of safety, _i.e._ into a position in

which it moves stalk first. But by a beautiful arrangement of the muscles

of its stalk this movement is effected in a horizontal plane; on touching

the _Loxosoma_ with the point of a needle it would swing round in this way

through 180° with "une rapidité qui étonne."

_Urnatella_[541] is a beautiful form with a segmented stalk, the stalks

usually arising in pairs from a common base. It has at present only been

found in fresh water in the United States.

[Illustration: FIG. 245.—Diagram of the structure of _Loxosoma_, seen from

the oesophageal side. _×_ about 70. _a_, Anus; _b_, buds; _e_, excretory

organ; _f_, foot-gland; _g_, ganglion; _gn_, generative organs; _o_,

orifice of vestibule; _oe_, oesophagus; _s_, stomach; _t_, retracted

tentacles.]

In _Pedicellina_ the plane of the lophophore is at right angles to the

stalk, which is separated from its calyx by a marked constriction. In

_Loxosoma_ the lophophore is set obliquely,[542] and there is no

constriction at the base of the calyx. In _Urnatella_ we find an

intermediate condition, the lophophore resembling that of _Loxosoma_, while

the constriction at the base of the calyx is similar to that of

_Pedicellina_. Since the latter is known to pass in its development[543]

through a stage with an oblique lophophore, it may be presumed that

_Loxosoma_ is a more archaic form than _Pedicellina_. In other respects,

the structure of the Entoprocta is very constant, whatever the genus.

{491}A pair of ciliated excretory tubes open into the vestibule. These are

similar in structure to the "head-kidneys" of the larvae of Polychaet

worms, or to the excretory organs of adult Rotifers. Flame-cells have been

described by Davenport in the stalk of _Urnatella_, but it is not known

whether they are connected with the excretory tubes of the calyx. The

animals are either hermaphrodite or have separate sexes, and the generative

organs open by ducts of their own into the vestibule. The nervous system

consists of a ganglion placed between the mouth and the anus, giving off a

set of nerves, many of which end in delicate tactile hairs placed on the

tentacles or other parts of the body.[544]

{492}CHAPTER XVIII

POLYZOA (_continued_)

FRESH-WATER POLYZOA—PHYLACTOLAEMATA—OCCURRENCE—STRUCTURE OF _CRISTATELLA_—

DIVISION OF COLONY—MOVEMENTS OF COLONY—RETRACTION AND PROTRUSION OF

POLYPIDES IN POLYZOA—STATOBLASTS—TABLE FOR DETERMINATION OF GENERA OF

FRESH-WATER POLYZOA—REPRODUCTIVE PROCESSES OF POLYZOA—DEVELOPMENT—

AFFINITIES—METAMORPHOSIS—BUDDING.

FRESH-WATER POLYZOA.—Although the Gymnolaemata are ordinarily marine

animals, fresh-water examples from this Order are not altogether wanting.

The Ctenostomata among the typically marine groups show the most tendency

to stray into fresh-water.

_Alcyonidium_ and _Bowerbankia_ (Fig. 238) flourish in estuaries, while

_Victorella_ and _Paludicella_ (Fig. 250) are only known as fresh or

brackish water forms. _Victorella_ was named after the Victoria Docks in

London, where it was first found; more recently it has also been discovered

in other parts of England and on the Continent.[545]

The systematic position of the genera _Hislopia_ and _Norodonia_,[546]

which have been described from fresh water of India and China respectively,

is at present uncertain. The undoubted Cheilostome _Membranipora_ has,

however, a British representative (_M. monostachys_), which occurs in

brackish water, in ditches on the coast of East Anglia. It is there known

to form "friable, irregularly-shaped, sponge-like masses," which grow on

water-plants.[547]

{493}The Entoprocta, as we have seen, are represented in fresh water by the

genus _Urnatella_.

The PHYLACTOLAEMATA are an exclusively fresh-water group, and they are

believed by Kraepelin[548] to have been derived from the Ctenostomata. Many

of their special peculiarities can, with great probability, be regarded as

adaptations to a fresh-water existence. This is particularly clear in the

all but universal habit of dying down in the winter, and in the occurrence

of the so-called statoblasts (Fig. 251), which are hard-shelled

reproductive bodies, absolutely restricted to the Phylactolaemata, and

capable of resisting the winter's cold and even a certain amount of drying

up. Phylactolaemata have indeed been recorded from the tropics; but it is

not yet sufficiently clear how they there behave in these respects. F.

Müller[549] has found these animals in Brazil, where they are said to be

more common at certain periods of the year than at others. Stuhlmann has

found them in Tropical Africa (Victoria Nyanza, etc.);[550] and

Meissner[551] has discovered the sessile statoblasts of _Plumatella_ on the

shells preserved in the Berlin Museum, of species of the Mollusc _Aetheria_

from various localities in Africa. Fresh-water representatives of a

considerable number of other groups of animals agree with the

Phylactolaemata in the possession of reproductive bodies which are

protected by hard coats. Such, for instance, are the ephippian ova of

_Daphnia_—bodies which have an extraordinary external similarity to

statoblasts—the gemmules of Spongillidae, the winter-eggs of Rhabdocoels

and Rotifers, and the cysts of Protozoa. The evolution of these bodies in

so many widely different cases may have been due to the selection of

variations calculated to minimise the dangers attendant on the drying up of

the water in summer, or on its freezing in winter.

The Phylactolaemata are by no means uncommon, although they can seldom be

found without a careful search. Their presence may often be detected by

taking advantage of the property of the free statoblasts of rising to the

top of the water, where they can be discovered by skimming the surface with

a fine hand-net.

The colonies themselves are usually found attached to water-plants, roots

of trees or stones. Most of them flourish best in {494}a zone not more than

two feet below the surface. Certain species show a preference for floating

leaves, such as those of water-lilies, where they are not liable to be

dried up by alterations in the level of the water. Some forms (e.g.

_Plumatella_, Fig. 246) are, however, able to withstand being dried for

some time. Most species prefer shady places, and accordingly settle on the

lower sides of leaves and sticks. Others (e.g. _Cristatella_, Fig. 247)

have no objection to the direct rays of the sun. Most forms prefer still

water, but one or two are found in running water.

_Fredericella_ is a common constituent of the deep-water fauna of Swiss

Lakes (down to over forty fathoms); and reaches there a size considerably

larger than the shallow-water form of the same species. _Paludicella_ is

common at thirteen fathoms. These two genera, with _Plumatella_, have been

found in absolute darkness, under a pressure of 2½-5½ atmospheres, in the

Hamburg aqueduct. The Polyzoa and other organisms growing in the

water-supply of Hamburg were accused of being concerned in the spreading of

cholera, during the recent epidemic, by choking up the water-pipes, and

creating obstructions which formed a favourable nidus for the development

of cholera-germs.

The colony may take the form of a series of delicate, branching tubes

(_Plumatella_, _Fredericella_), of more massive aggregations of parallel

tubes (as in the Alcyonelloid forms of _Plumatella_), or of gelatinous

masses of varying size (_Lophopus_, _Cristatella_).

[Illustration: FIG. 246.—A, _Plumatella_ (_Alcyonella_) _fungosa_ Pall.,

Naples (fresh water), small part of a mass, natural size; B, _Plumatella

repens_ L., R. Yare, on the leaf of a water-lily, natural size.]

_Cristatella mucedo_ (Fig. 247) is remarkable for its power of moving from

place to place; it consists of an elongated mass of greenish, gelatinous

substance, which, in its fully developed state, may reach a length of eight

inches or more, with a transverse diameter of three-eighths of an inch. It

has a flattened sole on which it crawls, while the graceful plumes of its

numerous polypides protrude as a delicate fringe from its upper side.

{495}The tentacles are about eighty to ninety in number, and they are, as

in other Phylactolaemata, united at their bases by a delicate web. The

lophophore is horse-shoe-shaped (Fig. 236, 3) throughout the group, with

the exception of _Fredericella_, in which genus it is circular.

In some Phylactolaemata the polypide has been observed to interlace its

tentacles, so that the plume becomes a kind of cage, in which the more

active Infusoria are imprisoned until their struggles have so far weakened

them that they are swept into the mouth by the action of the cilia of the

tentacles.[552]

[Illustration: FIG. 247.—_Cristatella mucedo_ Cuv. (a small colony), R.

Yare, above Norwich, × 24.]

Around the edge of the _Cristatella_ is found a zone of budding tissue,

which gives rise continuously to new individuals. Now, whereas in

Gymnolaemata the growing edge gives rise to zooecia, whose cavities become

completely cut off from that of the older ones; in Phylactolaemata the

partitions between the zooecia are never completed. The body-cavity of

_Cristatella_ is thus a continuous space, interrupted at the margin only by

vertical septa (see Fig. 247), which represent the partitions between the

zooecia of other forms.

The body-wall consists of two epithelial layers of ectoderm and mesoderm,

between which is a layer of muscular fibres. {496}Parts of the epithelium

lining the body-cavity are ciliated. Into the common body-cavity hang the

polypide-buds at the edge of the colony, and the mature polypides in the

more central regions. There are usually three rows of polypides on either

side of the middle line, in the neighbourhood of which is an area devoid of

polypides, but containing "brown bodies" and statoblasts. The polypides

nearest to the middle line pass in succession into the condition of "brown

bodies," while young buds near the margin grow up coincidently to form new

polypides.

The movement of the colony is in the direction of the long axis, although

either end may go first. Sir John Dalyell records an observation[553] on a

specimen (about one inch long) which was artificially divided into two

halves. The two halves "receded from each other as if by common consent,"

and were nearly an inch apart in twenty hours.

An observation made at Cambridge on a small colony of about 7 mm. in

greatest length gave the following results. The colony moved 13 mm. (nearly

twice its own length) in 8¼ hours: in the next 40 hours it moved 20 mm. (⅘

inch); while in the following 24 hours it moved only 6 mm. Large colonies

change their place only with reluctance.

The locomotive power possessed by _Cristatella_ is not unique among

Phylactolaemata. _Lophopus_, the first fresh-water Polyzoon of which any

description was published, was originally described by Trembley in 1744

under the name of the "Polype à pannache." Trembley observed the

spontaneous division of the colony, _followed by the gradual separation

from one another of the daughter-colonies_.[554] The power of dividing

spontaneously is also possessed by colonies of _Cristatella_ and of

_Pectinatella_.

The colonies of _Lophopus_ are surrounded by an excessively hyaline

ectocyst, and are usually triangular, as shown by Fig. 248. When division

is about to occur, the base of the triangle becomes indented, and the

indentation travels towards the apex in such a way as to bisect the

triangle. The two halves diverge from one another during the process, so

that before division is complete, they are looking, in some cases, in

opposite directions. {497}After a time the narrow connection breaks, and

two new colonies are formed.

Fig. 248 shows a colony shortly after division has taken place. The colony

had moved forwards, in a direction away from its apex, for three days in a

nearly straight line, the distances moved in each day being respectively 6,

8½, 8½ mm. These observations, for which I am indebted to Mr. Lister, show

a considerably higher speed than in those recorded by Trembley, who

observed no colony which moved more than half an inch (12.5 mm.) in eight

days.

The genus _Pectinatella_ also has some power of locomotion. This

magnificent Polyzoon occurs in masses several feet in length (as much as

six feet in _P. gelatinosa_ from Japan[555]), and four to eight inches in

thickness. The greater part of _P. magnifica_[556] consists of a thick,

opaline, and gelatinous ectocyst, the upper surface of which is covered by

hundreds of rosette-like colonies, which increase in number by division.

The masses are thus aggregations of colonies, which secrete a common basal

ectocyst. The latter decays in the autumn; and the separate rosettes, or

groups of them, may thus be set free, being found as floating masses, which

may again attach themselves to a solid object till the time of their death.

_Pectinatella_ has not yet been recorded in England, although, considering

the ease with which statoblasts are transported, it is by no means

improbable that it will eventually be recorded as a British genus. It is at

present known to inhabit America, Japan, and Hamburg.

[Illustration: FIG. 248.—_Lophopus crystallinus_ Pall., Cambridge, showing

the rate of movement. The colony and the distances moved are × 2.]

It is by no means certain what is the mechanism by which {498}movement

takes place in the above cases. The ectocyst of _Cristatella_ is confined

to the base of the colony, and there forms a thin slimy film, which

lubricates the surface over which the animal moves. It has been stated[557]

that progression is produced in the following way. The polypides are

withdrawn by means of retractor muscles, which originate from the septa and

inner surface of the sole. Thus at each retraction of any polypide, the

muscle pulls on a portion of the sole. Should the expanded polypides place

themselves in a suitable position, the movement will be in the direction of

the resultant of the forces due to the separate retractor muscles; while it

is probable that their cilia assist in the onward movement. It should be

noted that it is definitely stated that a colony in which all the polypides

are retracted can alter its position,[558] although even then the retractor

muscles might still contract to some extent.

The movement probably depends on several causes. It must probably be

conceded that the sole itself has some effect on this process. Its outer

cells are contractile, and have the power of raising themselves from the

underlying ectocyst. They may then again attach themselves, and this new

attachment does not always take place in exactly the same place as the

former one. Any movement of the muscles of the sole, or of the retractor

muscles, will thus shift the skin to a new place.[559]

PROTRUSION OF THE POLYPIDE.—While it is perfectly clear that retraction is

principally performed by the great retractor muscles acting directly on the

polypide, it is less easy to explain the converse movement. There can,

however, be little doubt that protrusion is effected by the pressure of the

fluid of the body-cavity, caused in large part by contractions of the

common body-wall.

Now since, in _Cristatella_, the body-cavity is a continuous space, any

pressure on the fluid must act uniformly on all its contents. The cause

which determines the protrusion of a polypide is thus to a large extent the

relaxation of the sphincter-muscle which surrounds its orifice, aided by

special muscles which dilate the orifice. Any polypide which is retracted

while the pressure of the fluid in the body-cavity is sufficient to keep

other polypides protruded, must therefore keep either its

{499}retractor-muscles or its sphincter in a state of contraction in order

to remain in that position. And as a matter of fact, _Cristatella_ and

_Lophopus_ differ from most other Polyzoa in the readiness with which they

expand their tentacles, after they have been induced to retract themselves

by mechanical irritation.

_Plumatella_ and other forms have a chitinous ectocyst, which, however, is

sticky when it is first formed. By virtue of this property, the branches

become attached to the leaf on which the colony is growing, and may have

their natural transparency obscured by taking up foreign bodies. The

stiffness of the ectocyst naturally involves some modification of the

process by which the polypides are protruded. In some cases, this is

effected by the separation of the endocyst from the ectocyst in the lower

parts of the tube. The muscles of the body-wall can thus press on the fluid

of the body-cavity without being restrained by the inflexible ectocyst. In

other cases, the tube of ectocyst is rendered flexible by the presence of a

thin line along one side where the chitin is deficient.

[Illustration: FIG. 249.—_Plumatella repens_ L., R. Yare, × 30. _a_, Anus;

_b_, polypide-bud; _c_, caecum of stomach; _d_, duplicature; _e_, epistome

(see p. 476); _f_, funiculus; _g_, ganglion; _m_, retractor muscle; _p_,

parieto-vaginal muscles; _ph_, pharynx; _s_, statoblasts attached to _f_.]

The upper end of the retracted tentacle-sheath is connected with the

body-wall by bands known as the parieto-vaginal muscles (Fig. 249, _p_).

These {500}serve not only to dilate the orifice when protrusion is

commencing, but also to prevent the polypide from being forced out too far.

They are arranged in such a way that a circular fold, the duplicature

(_d_), is never turned inside out, even in the state of complete protrusion

of the polypide.

The mechanism of the protrusion of the polypide in the Gymnolaemata is in

many cases obscure. The body-wall is not muscular in this group, in some

forms of which, however, short strands known as the parietal muscles (Fig.

234, _p_) pass across the body-cavity from one point to another of the

zooecium. As doubts have been thrown on the function of these muscles in

causing protrusion, it will be worth while to refer to the detailed and

convincing statements of Farre,[560] relating to this point.

Farre's observations were made on certain transparent Ctenostomes

(_Bowerbankia_ and _Farrella_). He states that the parietal muscles "were

distinctly seen to contract whenever the protrusion of the animal took

place, and to become relaxed again upon its retiring into its cell." Their

contraction may indent the outline of the ectocyst, or may cause the

separation of the endocyst from the ectocyst. The endocyst is then drawn

into longitudinal lines at the origin and insertion of these fibres. It is

further suggested that some part is played in the process by the muscular

walls of the alimentary canal, which is a good deal bent in the retracted

condition. The effort to straighten itself is believed to have some share

in forcing out the polypide. The flexible, membranous character of the

"aperture" (see p. 524) in _Membranipora_ (Fig. 256, A) is said by

Nitsche[561] to be an arrangement for the protrusion of the polypides; the

parietal muscles passing from the lateral walls of the zooecium to the

upper membranous wall, which is accordingly depressed by their contraction.

Although it is hardly possible to doubt the accuracy of Farre's

observations, which have, moreover, been confirmed by Hincks, it is by no

means certain that this is the whole explanation in all cases. Oka,[562]

for instance, states that protrusion of the polypide in Phylactolaemata can

be effected in a branch whose body-wall has been cut open. Pergens[563]

believes that the diaphragm (Fig. {501}234, _d_) acts as a pump,

introducing water from the tentacle-sheath into the body-cavity, into which

it is said by him to open, and so forcing out the polypide. It is probable

that many of the forms which have a stiff, unyielding ectocyst possess

special arrangements for introducing water in some way into the space

bounded by the ectocyst,[564] and so forcing out the polypide. Such, for

instance, may be the median pore which occurs beneath the orifice in

_Microporella_ (Fig. 241, A, _mp_), and in certain other cases.

REPRODUCTION OF PHYLACTOLAEMATA.—Sexual reproduction takes place in

_Cristatella_ from June to August. The spermatozoa are ordinarily produced

on the funiculus. The ovaries usually occur on the inner side of the common

wall of the colony, not far below the orifice of a polypide. Each ovary

matures a single egg, which develops _in situ_, the free larva leaving the

colony by the orifice of one of the degenerated polypides.

A second method of reproduction takes place by means of the statoblasts,

which are developed on the funiculus (Fig. 249). According to Verworn,[565]

each statoblast arises from a single cell of the funiculus; and on this

view, the statoblast is, as supposed by the earlier observers, a special

kind of winter-egg. According to more recent researches,[566] the funiculus

consists of a central axis, formed from the ectoderm, and of an outer

sheath of mesoderm-cells; the statoblast is developed from the two kinds of

cells of which the funiculus is composed, and is consequently comparable in

its mode of origin to an ordinary bud. Its special peculiarities are: its

origin as an internal bud, its possession of a chitinous shell, and the

fact that it is destined to leave the parent colony, and to develop, after

a period of rest, into a new colony. Germination takes place by the

formation of a polypide-bud inside the statoblast, which finally splits

along its equator into two halves. The contents emerge as a young colony

which possesses at least one fully-formed polypide.

Remarkable structures known as "hibernacula" occur in the fresh-water

Ctenostomes, _Paludicella_ and _Victorella_. These bodies are in the former

(Fig. 250, B) specially modified _external_ buds, which persist through the

winter when the rest of the colony dies down. At the close of winter the

shell splits into two {502}halves, exactly as takes place in the

statoblasts, and a young colony emerges. It is possible that the

statoblasts may have been evolved from a hibernaculum, which was at first

produced externally, but has become modified in such a way as to acquire an

internal mode of origin.[567]

The simplest known statoblast is that of _Fredericella_ (Fig. 251, A),

which differs from that of other Phylactolaemata in having no ring of

air-cells. In _Plumatella_, the statoblast (Fig. 251, B) has a broad

equatorial ring of air-cells, which enable it to float at the surface of

the water on the decay of the parent tubes. In some species, certain

statoblasts which are produced in the adherent parts of the colony remain

attached to the substratum. These "sessile statoblasts" may have no trace

of the ring of air-cells; but the fact that many sessile statoblasts have

rudiments of this structure suggests that they are a secondary modification

of the floating statoblast. In _Lophopus_ (Fig. 251, C) the ring of

air-cells is very broad, and is pointed at each end; while in _Cristatella_

(Fig. 251, D) and in _Pectinatella_ the statoblast is circular, and

possesses an armature of hooked spines. That of _Cristatella_, measures

about .75 mm. in its greatest length.

[Illustration: FIG. 250.—_Paludicella ehrenbergi_ van Beneden, × about 3.

A, Part of a colony with expanded polypides; B, remains of part of a colony

which has produced hibernacula or winter-buds (_h_); _z_, zooecium. (From

Kraepelin.)]

Kraepelin has suggested that the above order of increasing complexity of

the statoblasts corresponds with the order in which the genera to which

they respectively belong would be placed, on the assumption that the

Phylactolaemata have been derived from the Ctenostomata. Thus, in

_Fredericella_, the form of the lophophore is circular, as in the

Gymnolaemata. The number of the tentacles is comparatively small (20-24).

The arborescent form of the colony resembles that of many Ctenostomes, and

the zooecia are more or less cut off from one another by incomplete septa.

{503}In _Plumatella_, the lophophore has become horse-shoe-shaped, and the

tentacles are more numerous (38-60). In general form and in the arrangement

of the septa this genus resembles _Fredericella_, with which it may easily

be confused.

In _Cristatella_ we have the most highly modified of all the

Phylactolaemata. The individuality of the zooecium is here subordinated to

that of the colony as a whole. The branched arrangement of the zooecia is

greatly obscured. The body-cavities have become completely confluent,

although rudiments of the septa still exist. The ectocyst has been lost,

with the exception of the basal layer of the colony. The tentacles are more

numerous (80-90); and in accordance with the increase in the elaboration of

the genus, its statoblasts belong to the most complicated type known.

[Illustration: FIG. 251.—Statoblasts of Phylactolaemata. A, _Fredericella

sultana_ Blum., × 38; B, _Plumatella repens_ L., × 38; C, _Lophopus

crystallinus_ Pall., × 28; D, _Cristatella mucedo_ Cuv., × 28. (A, from

Allman; B-D, from Kraepelin.)]

The production of _floating_ statoblasts may seem a strange adaptation to

the conditions of fresh-water life, since it might be assumed, _a priori_,

that these structures would be specially liable to be frozen during the

winter. The following experiments made by Braem[568] show, however, that

the germinating power of the statoblasts is improved by a certain amount of

frost. A number of statoblasts were taken; half of these were placed in

water, which was then frozen; and these were found to germinate readily

when afterwards exposed to suitable conditions. The other half were not

subjected to the action of frost; and these could not be made to germinate,

even although the water had been cooled to a point slightly above the

freezing point. It thus appears that the buoyancy, so far from being a

risk, is a means of exposing the statoblast to the conditions which are

most favourable to its later development.

{504}Braem supposes that the beneficial action of frost is due to a

lowering of the vital energy of the statoblast. As in the case of

reproductive bodies known in many other fresh-water organisms, the

statoblast germinates only after a period of rest. Although this period is

often shortened by a lowering of the temperature, it can also be induced by

the exclusion of air, as in an experiment during which the statoblasts were

enclosed in airtight tubes. The respiratory processes were thereby

lessened, and the germinating power was materially improved.

Since the development of the statoblasts depends largely on the

temperature, the first warm weather in early spring will probably induce

the germination of those which are floating; and the young colony, leaving

the protection of the statoblast, will become susceptible to frost. But

even if the first-formed colonies are killed off by a subsequent frost,

other statoblasts which have remained in the mud during the winter are

disentangled from time to time, and germinate on reaching the surface.

DISTRIBUTION.—The protective value of the shell is also shown by the fact

that the statoblast may be kept for some months in a dry condition without

losing its power of germination. There can be little doubt that the

capability of withstanding desiccation enables the species to enlarge its

area of distribution. It is asserted that fresh-water Polyzoa decrease in

abundance in proportion to the distance from the mouth of the river in

which they are found. The current will naturally tend to bring together the

statoblasts from the Polyzoa growing in the upper waters.

Nothing is more surprising than the wide geographical distribution of the

Phylactolaemata. The European genera are all recorded from North America.

_Fredericella_, _Plumatella_, and _Lophopus_ are further recorded from

Australia; while _Plumatella_ is known to occur also in Malacca, the

Philippine Islands, India, Japan, Africa, and South America, It is even

stated that some of the Australian _species_ are identical with those found

in Europe.

Some of the fresh-water Polyzoa are extremely variable, and observers are

by no means agreed in deciding whether certain well-known forms are to be

regarded as varieties or as species. While certain genera, such as

_Cristatella_ and _Lophopus_, are comparatively constant in their form,

_Plumatella_ is excessively variable. _Plumatella_ has a number of species

greater than that of any other form, and the genus has a wider distribution

than any {505}other. This greater variation of species of the dominant

genus is in complete accordance with the general law enunciated by Darwin

that "wide-ranging, much diffused, and common species vary most."

While the ordinary forms of _Plumatella_ consist of branching colonies,

which are either completely adherent to their substratum, or grow in a more

or less erect manner, another habit which is assumed by this genus is so

different from the first that it has been considered to mark a distinct

genus, _Alcyonella_. The Alcyonelloid form (Fig. 246, A) consists of

closely packed tubes which stand more or less at right angles to their

substratum, which they may cover with a dense mass an inch thick, and with

a superficial area of several square inches. But in spite of this

difference, it is possible that _A. fungosa_ is only a variety of an

ordinary _Plumatella_ form. Whether this is so or not, a typical

_Plumatella_ may in places take on an Alcyonelloid habit; and parts of an

_Alcyonella_ may become so lax in growth as to resemble a _Plumatella_.

The British genera of fresh-water Polyzoa may be distinguished from one

another by means of the following table:—

{ Zooecia perfectly distinct from one another. Lophophore circular.

{ Statoblasts absent 2

1. { Colony formed of branching tubes composed of confluent zooecia 3

{ Colony gelatinous, not obviously formed of branching tubes.

{ Lophophore horse-shoe shaped 4

{ Colony consisting of a stolon from which new zooecia originate.

{ These may give rise to new stolons, or directly to new zooecia

{ _Victorella_

2. { Branches composed entirely of club-shaped zooecia, each of which

{ may give off two zooecia near its upper end

{ _Paludicella_ (Fig. 250)

{ Tubes hyaline or opaque, usually containing numerous oval

{ statoblasts (Fig. 251, B), most of which have a ring of

{ air-cells. Lophophore horse-shoe shaped.

{ (_a_) Tubes divergent _Plumatella_ (Fig. 246, B)

3. { (_b_) Tubes parallel with one another _Alcyonella_ form of

{ _Plumatella_ (Fig. 246, A)

{ Tubes cylindrical, usually dark brown. Statoblasts (Fig. 251, A)

{ few, without air-cells. Lophophore circular _Fredericella_

{ Colony hyaline, usually divided into three or four short lobes.

{ Ectocyst thick. Statoblasts (Fig. 251, C) pointed at each end,

{ with a broad ring of air-cells _Lophopus_ (Fig. 248)

{ Colony slug-shaped, crawling on a flattened sole. Ectocyst

4. { rudimentary. Statoblasts (Fig. 251, D) circular, with

{ marginal hooks _Cristatella_ (Fig. 247)

{ Colonies consisting of small rosettes, many of which are attached

{ to a thick basal layer of hyaline ectocyst. Statoblasts circular,

{ with marginal hooks. (Not recorded as British) _Pectinatella_

{506}REPRODUCTIVE PROCESSES OF POLYZOA IN GENERAL.

In studying the reproductive processes of Polyzoa, we have to deal with two

very distinct phenomena; firstly, with the development of eggs; and

secondly, with the formation of buds.

The process of budding usually does no more than increase the number of

individuals in a colony which already exists, and is seldom responsible for

the commencement of a new colony. In _Loxosoma_, however, the buds break

off and lead an independent existence; and in the Phylactolaemata a large

proportion of the colonies have their origin in the statoblasts. In certain

cases, again, new colonies may be formed by the detachment of parts of an

old one, as by the fission of _Cristatella_ and _Lophopus_, or by the

breaking up of a richly-branched species into several colonies by the decay

of the proximal parts.

We may then in the majority of cases look to an embryo for the foundation

of a new colony. The embryo develops into a larva, which, after a period in

which it swims freely, settles down, and is metamorphosed into the first

zooecium. This primary individual forms the starting-point of a colony, and

often differs to a considerable extent from the other zooecia which arise

from it. In Cyclostomata, for instance, the proximal end of the primary

zooecium permanently retains the disc-like shape assumed by the young larva

when it first fixed itself. The primary zooecium may be recognised with

equal ease in many Cheilostomata, and may differ from its successors by

possessing a richer development of marginal spines, or in other respects.

REPRODUCTIVE ORGANS.—Eggs and spermatozoa are commonly found in the same

colony, either in different individuals, or else in the same zooecium (see

Fig. 234, p. 469). In some cases, the zooecium first develops spermatozoa,

and later eggs. The Entoprocta have a more marked separation of the sexes

than obtains in other Polyzoa. The genus _Loxosoma_ is perhaps always

dioecious (_i.e._ with separate sexes). _Pedicellina_ is sometimes found

with ovaries and testes in the same individual, sometimes with these organs

in different individuals; and it is not clear whether a given species

always behaves alike in these respects.

The reproductive organs of the Entoprocta open by ducts of their own into

the vestibule. In the Ectoprocta they are developed in the body-cavity, and

they have no ducts.

{507}The fate of the ripe egg differs widely in different cases. In the

Entoprocta it develops in a kind of brood-pouch formed from part of the

vestibule. The fact that in _Pedicellina_ (Fig. 243) the embryos grow

largely during their development, shows that nutritive material must be

supplied to them from the parent. There is reason to believe that the

epithelium of the brood-pouch is responsible for this process. The eggs are

also known to develop at the expense of nutritive substances prepared by

the parent in the ovicells of the Cyclostomata. In other cases, as in some

species of _Alcyonidium_, the egg is large, and its copious yolk doubtless

supplies a large part of the material required for development.

In the Ectoprocta, development takes place in a variety of places. In most

Cheilostomata a single egg passes into the globular ovicell, which is

formed above the orifice of many of the zooecia. In certain

Ctenostomata,[569] Phylactolaemata,[570] and Cyclostomata,[571] the ripe

egg is taken up by a rudimentary polypide-bud, which is specially formed

for the purpose. In the Ctenostomata and in the fresh-water Polyzoa these

buds, if present, are found in ordinary zooecia which do not become

modified externally in any special way. In the Cyclostomata (_Crisia_), on

the contrary, the formation of the polypide-bud is intimately bound up with

the development of the ovicell. The number of the zooecia which produce

eggs that are capable of development is greatly restricted in this group.

The ovicell, which contains numerous embryos, is not merely a portion of a

zooecium, as in the Cheilostomata; but it is probably to be regarded as a

modification of the entire fertile zooecium or zooecia. These take on an

appearance widely differing from that of the ordinary zooecia, and in

course of time give rise to the ovicells (see Fig. 237).

In all these cases the egg develops inside the parent, and it was hardly

known, before the publication of the interesting researches of M.

Prouho,[572] that some of the Polyzoa lay eggs which develop externally. In

these cases a considerable number of eggs are produced simultaneously by a

single zooecium. {508}M. Prouho further throws light on a much contested

subject; namely, the nature of the so-called "intertentacular organ" (_i_,

Fig. 234, p. 469), described so long ago as 1837 by Farre,[573] but looked

for in vain by the majority of later observers.

The failure to find this organ, even in species which possess it, _in

certain individuals_, according to Farre's statements, is now

satisfactorily explained by M. Prouho, who shows that while it is absent in

a large number of polypides, it is normally present in those individuals

which possess an ovary, and in those only; and that its primary function is

that of an oviduct.

The intertentacular organ is an unpaired ciliated tube, which is situated

between the two tentacles which are nearest to the ganglion. In the

retracted condition of the polypide, it opens from the body-cavity into the

tentacle-sheath; and in the expanded condition, directly to the exterior.

In the remarkable case of _Alcyonidium duplex_, each zooecium normally

possesses two sexual polypides. The first of these produces a testis and

then becomes a "brown body." The second is meanwhile developed, and

produces an ovary and an intertentacular organ, a structure which was not

present in the male polypide. The eggs pass through the intertentacular

organ into the tentacle-sheath, and attach themselves to the diaphragm

(_d_, Fig. 234), where they remain during their development.

Although the intertentacular organ has been found by Prouho in female

polypides only, it would perhaps be going too far to assert that it is

confined to polypides of that sex. Hincks[574] has observed the passage of

spermatozoa in enormous numbers through the organ, although it may be noted

that there is no sufficient proof that eggs were not present as well in

these zooecia. It further appears that in some cases waste matters may be

removed from the body-cavity through the same passage.

It may be presumed that the egg is normally fertilised by a spermatozoon,

although this is at present largely a matter of inference. It is believed

by Joliet[575] that fertilisation is reciprocal, although Prouho has come

to the opposite conclusion. Joliet has, however, very justly pointed out

that the enormous number of spermatozoa developed by a single individual

would be disproportionately large, if their function were merely to

fertilise the {509}ovum in the same zooecium. According to his view, the

egg is fertilised by a spermatozoon after it has passed into the

tentacle-sheath or ovicell, or some other place where it is in free

communication with the outside water.

DEVELOPMENT AND AFFINITIES.—Few parts of the history of the Polyzoa are

more fascinating than that which deals with their development; and it is

probable that no other is capable of giving so much insight into the

affinities of the several groups to one another and to other groups of the

animal kingdom.

[Illustration: FIG. 252.—Diagrams of larvae. A, _Loxosoma_, × 208; _a_,

anus; _b_, brain, with left eye and ciliated pit; _c_, ciliated ring; _ep_,

epistome; _m_, mouth; _o_, oesophagus; _st_, stomach; _x_, aboral adhesive

organ: B, _Cyphonautes_ larva of _Membranipora_ (_Electra_) _pilosa_, ×

about 90; _a_, _m_, _o_, _st_ as in A; _c_, anterior part, and _c'_,

posterior part of the ciliated ring; _e_, epidermis; _ms_, adductor muscle

of shells; _p_, pyriform organ, of unknown function; _sh_, shell; _v_,

vestibule; the "internal sac" or sucker, by which fixation is effected, is

seen between _a_ and _ms_. (B, after Prouho.)]

The comparative study of the larvae of the Polyzoa may be said to date from

1877, when J. Barrois published an elaborate Monograph[576] on this

subject. Although some of Barrois' earlier opinions have been subsequently

modified, this work still gives the best figures of the external form of

the beautiful larvae of many genera. A detailed account of the larval forms

of Polyzoa must be omitted from want of space; and the general conclusions

only can be given.

{510}The larvae of the Entoprocta (Fig. 252, A) resemble the so-called

"Trochosphere" of Polychaeta (see p. 274). The common characters shared by

the larvae of Chaetopoda, Echiuroid Gephyrea, Mollusca, and Polyzoa, and by

adult Rotifera, may well point to the derivation of these groups from a

common ancestor. On this assumption, it is possible that the Polyzoa have

been derived from forms which existed long ages ago, which combined the

common characters of these groups, and the structure of which we can

picture to ourselves only so far as the "Trochosphere" larva can be taken

to represent it in a much simplified condition. Such a view harmonises well

with the great antiquity of the Polyzoa. Certain Ectoproct forms have a

larva, known as _Cyphonautes_ (Fig. 252, B), which closely resembles the

larval form of the Entoprocta; and it is a fact which probably has

considerable significance that this type of larva is known to occur only in

those species of _Membranipora_ (_Electra_), _Alcyonidium_, and

_Hypophorella_, which lay eggs.[577] This may perhaps be regarded as a

primitive form of development which has been lost in species in which

development takes place inside the parent. _Cyphonautes compressus_ (Fig.

252, B), one of the commonest objects taken in the surface-net off our own

coasts, is the larva of _Membranipora_ (_Electra_) _pilosa_. Whilst this

larva is provided with a well-developed alimentary canal, those of most

other Ectoprocta possess a mere rudiment of this structure, and depend for

their nutrition either on yolk present in the egg or on material supplied

by the parent. In most cases the mature larva has no recognisable trace of

a digestive system; and, although it has a free-swimming period, it does

not become truly pelagic.

The alimentary canal of the larva of _Pedicellina_ is known to persist in

the primary individual of the colony. In all other known cases, even in

that of _Cyphonautes_, the larva at fixation loses practically all its

internal organs, and becomes a mere body-wall containing a mass of

degenerated larval tissues. It is in fact a zooecium containing a "brown

body." A polypide-bud is now developed, the body-cavity appears as the

result of the shrinkage of the "brown body," and the primary individual of

the colony is thereby established.

The larvae of the Ectoprocta form a tolerably complete series, starting

from _Cyphonautes_, itself allied to the larva of the {511}Entoprocta, and

ending with the Phylactolaemata. _Alcyonidium_ (Fig. 253, B) possesses a

rudimentary alimentary canal,[578] although the most conspicuous structures

are those connected with the fixation and other phenomena of larval life.

The larvae of many of the encrusting Cheilostomes (Fig. 253, A) resemble

that of _Alcyonidium_, while those of _Bugula_, _Scrupocellaria_, etc.,

belong to a type easily derivable from that of the encrusting forms. The

branching Ctenostomes (_Bowerbankia_, etc.) have a larva which may be

regarded as derived, along slightly different lines, from that of

_Alcyonidium_. The Cyclostomata and the Phylactolaemata have the most

modified forms of larva. That of the former group may owe some of its

peculiarities to the occurrence of a remarkable process of embryonic

fission, which takes place in the ovicell, and as the result of which each

egg gives rise to a large number of larvae.[579] The Phylactolaemata have a

larva which is not unlike that of _Bowerbankia_.

[Illustration: FIG. 253.—A, Aboral view of free larva of _Lepralia

foliacea_ Ell. and Sol.; _a_, long cilia of pyriform organ; _g_, aboral

groove: B, longitudinal section of embryo of _Alcyonidium_, × 135; _c_,

ciliated ring; _g_, aboral groove; _m_, mouth; _n_, nervous system; _p_,

"pyriform organ," of unknown function; _s_, "internal sac" or "sucker," by

which fixation is effected; _st_, stomach.]

We have seen that the larva at fixation becomes a zooecium, {512}which in

the Gymnolaemata forms a polypide-bud after fixation. The peculiarities of

the Phylactolaematous larva may be explained by assuming that it becomes a

zooecium while it is still free-swimming. Thus the larva of _Plumatella_

develops one or sometimes two polypides, which actually reach maturity

before fixation takes place. That of _Cristatella_ develops from two to

twenty[580] polypides or polypide-buds at the corresponding period, and it

is in fact a young colony while still free-swimming.

Now in most colonial animals, such as Coelenterates and Ascidians, the

larva metamorphoses itself into a temporarily solitary animal, which then

gives rise to the remainder of the colony by budding. The majority of the

Gymnolaemata behave in this way; while the Phylactolaemata may not only

develop a multiplicity of polypides in their larval stage, but the

individuality of the zooecia is then just as much obscured as in the adult

state. These facts are more easily explained if we assume that

_Cristatella_ is the end-point in a series than if we suppose it to be a

starting-point.

On the view maintained by many authorities, that the Polyzoa are related,

through _Phoronis_, with the Gephyrea and the Brachiopoda, we should expect

to find in those Polyzoa which most closely resemble _Phoronis_ in their

adult state—that is to say in the Phylactolaemata—some indications of

affinity to that animal in their development. This is emphatically not the

case. The hypothesis that the Phylactolaemata are related to _Phoronis_

leads, moreover, to the improbable conclusion that the similarities between

the Entoproct-larva and _Cyphonautes_, on the one hand, and the

Trochosphere larva of Polychaeta, on the other hand, is entirely

superficial and meaningless. In spite, therefore, of the similarity between

_Phoronis_ and a single individual of the Phylactolaemata, and in spite of

the marked resemblance between its nephridia and structures which have been

described in _Cristatella_[581] and _Pectinatella_[582] the comparative

study of the development appears to indicate that the resemblances between

_Phoronis_ and the Phylactolaemata are the result of a coincidence rather

than of any close relationship.

A few points connected with the metamorphosis of the {513}Polyzoa deserve

more special notice. There is generally great difficulty in persuading

larvae to fix themselves when kept in a small quantity of water, which

becomes over-heated in the air of a laboratory. The difficulty may be

surmounted by placing colonies containing embryos, together with some clean

pieces of the seaweed on which the adults are habitually found, in a vessel

closed by a piece of fine muslin, and by leaving the vessel attached to a

buoy or in a deep tide-pool. The larvae being without an alimentary canal,

fix themselves, after a very short free life, on the seaweed.

It is probable that a great struggle for existence normally takes place at

the commencement of the metamorphosis. Any one who will examine, in June or

July, rocks covered by _Fucus_ on which _Flustrella hispida_ is growing,

will probably find numerous young fronds of _Fucus_, from half an inch to

an inch or two in length, growing under the shelter of the older fronds.

The bivalve larvae of _Flustrella_ show a marked preference for fixing on

these young fronds—perhaps in order that the duration of life of the colony

may coincide with that of the _Fucus_—and these young fronds are commonly

covered by very numerous recently-fixed larvae, and by young colonies of

various ages. Or, it is easy to observe, by placing pregnant colonies of

_Bowerbankia_ in a vessel of water, that the larvae, which are hatched out

in thousands, fix themselves in dense masses on certain parts of the wall

of the vessel. It is clear that but a small proportion of these larvae will

find room for further development.

Next with regard to the mode of fixation. Attachment always takes place by

the surface on which the mouth or its rudiment is situated, and the

permanent alimentary canal opens on the opposite surface. In _Pedicellina_,

the one case in which the larval digestive organs are known to become those

of the first adult individual, this presupposes a rotation of the

alimentary canal, in order to bring it into its new position.

It is well known that the larvae of other fixed animals may undergo a

somewhat similar change. Thus those of Ascidians and of Barnacles fix

themselves by their anterior end, and ultimately reach their adult form by

performing a kind of a somersault. The process may perhaps be explained by

supposing that some part of the anterior end or of the oral surface is

specially sensitive, and that the larva fixes itself by that portion of its

{514}body which is best fitted for ascertaining which is the proper

substance on which to fix.

BUDDING.—The formation of a new individual may take place by the outgrowth

of part of the body-wall, as in _Pedicellina_ (Fig. 243, p. 487) and in

_Bowerbankia_ (Fig. 238, p. 480). In _Pedicellina_ a young stalk is formed

by an outgrowth near one of the growing points, and the upper part of this

outgrowth becomes constricted off to form the calyx. In other cases (cf.

the growing ends of the branches in Fig. 237) a partition grows across the

body-cavity at the growing edge of the colony, and so cuts off a part

destined to become a new zooecium.

The zooecium formed in one of these ways acquires an alimentary canal by

the formation of a polypide-bud, some stages in the growth of which are

shown in Fig. 235 (p. 472). Contrary to what happens in Coelenterates and

Tunicates, in which the endoderm takes part in the budding, there is good

reason for believing that in Polyzoa the polypide-bud is developed entirely

from ectoderm and mesoderm.[583] The bud is a two-layered vesicle, attached

to the inner side of the body-wall. Its inner layer is derived from the

ectoderm, which at first projects into the body-cavity in the form of a

solid knob surrounded by mesoderm-cells. A cavity appears in the inner,

ectodermic mass, and the upper part of the vesicle so developed becomes

excessively thin, forming the tentacle-sheath, which is always developed in

the condition of retraction. The lower part becomes thicker; its inner

layer gives rise to the lining of the alimentary canal, to the nervous

system, and to the outer epithelium of the tentacles, which grow out into

the tentacle-sheath (cf. Fig. 235). The outer layer gives rise to the

mesodermic structures, such as the muscles, connective tissue, and

generative organs.

These processes are fundamentally similar, whether in the metamorphosed

larva, in a young zooecium, in an old zooecium after the formation of a

"brown body," or in the germinating statoblast of the Phylactolaemata.

{515}CHAPTER XIX

POLYZOA (_continued_)

CLASSIFICATION—GEOGRAPHICAL DISTRIBUTION—PALAEONTOLOGY—METHODS FOR THE

EXAMINATION OF SPECIFIC CHARACTERS—TERMINOLOGY—KEY FOR THE DETERMINATION OF

THE GENERA OF BRITISH MARINE POLYZOA

Our account of the Polyzoa would be manifestly incomplete without some

reference to the systematic arrangement of these animals. An outline of the

principal groups has been given on p. 475. So far, the classification is

easy, but it is otherwise when we attempt to subdivide most of the groups

any further.

Systems of classification which depend exclusively upon the external

characters of animals have been repeatedly shown to be unsatisfactory. Now

with regard to the Polyzoa, not only is it the case that the great majority

of forms are only known in their external characteristics, but current

systems of classification cannot be regarded as final, because it is not

yet certain which of the external features have most systematic value. Two

obvious points can be at once selected—namely, the character of the

zooecium and the character of the entire colony. One or two instances will

serve to show what different results are obtained by depending exclusively

on either of these characters by itself.

According to the older writers, the habit of the colony was taken as the

most important generic character; and there can indeed be no doubt that

this feature has great importance within certain limits. Any one who has

examined different species of such genera as _Flustra_, _Cellaria_,

_Bugula_, _Retepora_, etc., must feel that the form of the colony goes for

a good deal. But a consideration of other cases shows that there is great

risk in the {516}indiscriminate use of this method of arranging the

Polyzoa. The old genus _Eschara_, composed of forms with an erect

coral-like habit,[584] included species which are now placed in such

different genera as _Lepralia_, _Porella_, _Microporella_, etc. The older

works on Polyzoa include all encrusting forms of Cheilostomata, with a

completely calcareous front wall, in the genus _Lepralia_, the members of

which are now distributed in numerous widely separated genera.

As an instance of the converse arrangement—essential similarity of the

zooecia with great differences of the general habit—may be mentioned the

common _Membranipora_ (_Electra_) _pilosa_.[585] Ordinarily growing in the

form of close encrustations on seaweeds, this species may take on entirely

different habits of growth. The zooecia are now dissociated, growing in

single lines over the substratum; now forming erect tufts, composed of

single lines of zooecia or of several rows. The erect, branching habit

appears to be induced in the first instance by the character of the seaweed

on which the colony begins life. Thus colonies which encrust the thin

branches of _Corallina_ may have impressed on them something of the mode of

growth of the seaweed, so that when they extend beyond the tips of the

branches of the _Corallina_, they continue to grow in delicate branches,

which still retain more or less the same diameter as those which form their

base. An extreme variation results in the beautiful form known as _Electra

verticillata_, in which the zooecia are arranged with great regularity in

whorls, which together form erect branches.[586] But with all these

variations, the zooecia are so much alike that it is hardly possible to

regard the extreme forms as more than varieties of a single species. A

careful examination of this case would convince most observers that the

characters of the zooecium are a more trustworthy guide to classification

than those of the entire colony, a result which was first clearly stated by

Smitt, and amply confirmed by Hincks.[587]

The avicularia of the Cheilostomata afford useful help in classifying this

group; but while certain genera are always provided with avicularia, others

include some species with these organs, and other species without them.

Again, while the species {517}of some genera (e.g. _Cellepora_) possess a

great variety of forms of avicularia, the same pattern of avicularium may

characterise several widely different genera. Further, the _position_ of

the avicularium may be very different in species which are apparently

closely related. Well-developed vibracula, although constant in their

occurrence in such forms as _Scrupocellaria_ (Fig. 254) and _Caberea_ (Fig.

242), occur here and there in species of encrusting forms which are

ordinarily placed in very different families.

Now although some of these discrepancies are perhaps due to errors in

classification, whereby species which are really allied have been wrongly

placed in distinct genera, this explanation would not prove satisfactory in

all cases. Thus in _Bugula_, a genus which is specially characterised by

the high development of its avicularia, these organs are normally absent in

_B. neritina_. The fact that this species was rightly placed in the genus

has been confirmed by the discovery made by Waters[588] that avicularia

occur in specimens which are believed to be identical with that species.

[Illustration: FIG. 254.—A, Front view, and B, back view of part of a

branch of _Scrupocellaria scabra_, Van Ben., Durham Coast, × 43; _a_,

lateral avicularium; _a'_, smaller median avicularium; _ap_, membranous

aperture; _f_, fornix; _r_, rootlet; _s_, seta of vibraculum; _v.z_,

vibracular zooecium.]

1. The Cyclostomata appear to fall naturally into two main groups, (A) the

ARTICULATA, including the Crisiidae (Fig. 237), distinguished by their

erect branches, divided at intervals by chitinous joints; and (B) the

INARTICULATA, which include the remaining families, whether erect or

encrusting, agreeing in the negative character of being unjointed.

{518}2. The Cheilostomata consist of (A) the CELLULARINA, including the

flexible, erect forms, such as _Bugula_ (Fig. 233) and _Scrupocellaria_

(Fig. 254); (B) the FLUSTRINA, to which belong _Flustra_ (Fig. 232),

_Membranipora_ (Fig. 256, A, B), _Micropora_ (Fig. 256, C), and other forms

in which the front wall of the zooecium is either membranous, or depressed

and marked off by a ridge-like margin; (C) the ESCHARINA, including the

great majority of forms, in which no part of the front wall remains

membranous, the wall of the zooecium being wholly calcified.

3. The Ctenostomata comprise (A) the ALCYONELLEA or encrusting forms; and

(B) the VESICULARINA or branching forms. The zooecia in the latter

subdivision (Fig. 238) are given off from a tubular stem or stolon, which

is usually erect and branching.