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 withCaryophylliasent to him from Ilfracombe. He christened the formPhoronis 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 ofCrepina gracilis.[491]
Phoronisis 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the others, from which it is isolated by the presence of a tube in which it lives, and into which it can be completely withdrawn.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.
What little we know about the habits ofPhoronisis in the main due to the observations of Cori,[492]who studiedPh. psammophilaat 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 inCrepina 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.
fig226Fig. 226.—A piece of a matted colony ofPh. kowalevskiiCald. Slightly magnified. In most cases the tentacular head is protruding from the tube.
Fig. 226.—A piece of a matted colony ofPh. kowalevskiiCald. Slightly magnified. In most cases the tentacular head is protruding from the tube.
Fig. 226.—A piece of a matted colony ofPh. kowalevskiiCald. Slightly magnified. In most cases the tentacular head is protruding from the tube.
With regard to the habitation ofPh. 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 australisoccurs in communities of twenty to thirty, in spaces in the substance of the wall of the tube inhabited and formed by a species ofCerianthus. 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 theCerianthus. TheCerianthustubes sometimes come up empty, as we should naturally expect, the animal having dropped out; but a sufficient number ofoccupied tubes are found to show that, under ordinary circumstances, a livingCerianthusoccupies the interior of the tube and a community ofPhoronislive in its wall. This species ofPhoronisis never found anywhere else, and the species ofCerianthusis very rarely found without thePhoronis."
Ph. australisis 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 asPh. psammophila, which live in sandy places, and are termed by the Sicilian fishermen "Tubi di sabbia." The feltwork ofPh. kowalevskiiattains 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.
fig227Fig. 227.—A piece of a colony ofPh. psammophilaCori. Slightly magnified. The tubes are covered by particles of sand, small shells, etc.
Fig. 227.—A piece of a colony ofPh. psammophilaCori. Slightly magnified. The tubes are covered by particles of sand, small shells, etc.
Fig. 227.—A piece of a colony ofPh. psammophilaCori. Slightly magnified. The tubes are covered by particles of sand, small shells, etc.
The various species ofPhoronisdiffer a good deal in size; Cori gives the average length as varying from 1.5 to 7.9 mm. inPh. hippocrepiaand up to 127 mm. (6 inches) inPh. australis. Probably the very short individuals of the first-named species had not attained their adult stature.Ph. australishas 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 tentacularcrown 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. InPh. kowalevskiiandPh. psammophilathe ends are only slightly turned in, so that the crown of tentacles is truly horse-shoe shaped; but inPh. australisthey are turned in and form three coils on each side. The number of tentacles also varies, being very numerous inPh. australisandPh. 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.
fig228Fig. 228.—A specimen ofPh. buskiiM‘Int. removed from its tube and seen from behind, × about 2. (After M‘Intosh.)
Fig. 228.—A specimen ofPh. buskiiM‘Int. removed from its tube and seen from behind, × about 2. (After M‘Intosh.)
Fig. 228.—A specimen ofPh. buskiiM‘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.
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 theU-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 aU-shaped tubewhich 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.
fig229Fig. 229.—The dorsal surface ofPh. australisHas., 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.)
Fig. 229.—The dorsal surface ofPh. australisHas., 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.)
Fig. 229.—The dorsal surface ofPh. australisHas., 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 ofPhoronisis 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 theU-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"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 inPh. kowalevskiian 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. InPh. australisBenham mentions two such tubes, one on each side of the body; their precise value is obscure.
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).
fig230Fig. 230.—A schematic view of the interior of the body ofPhoronis. 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.)
Fig. 230.—A schematic view of the interior of the body ofPhoronis. 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.)
Fig. 230.—A schematic view of the interior of the body ofPhoronis. 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.)
Phoronisis 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 themultiplication 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.
fig231Fig. 231.—Three stages in the metamorphosis of the Actinotrocha intoPhoronis.A, Actinotrocha larva with the invagination (c), which will form the trunk of thePhoronislarva 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.Cis after Metschnikoff.a, Mouth;b, anus;c, invagination which ultimately forms the greater part of the body of the adult.
Fig. 231.—Three stages in the metamorphosis of the Actinotrocha intoPhoronis.A, Actinotrocha larva with the invagination (c), which will form the trunk of thePhoronislarva 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.Cis after Metschnikoff.a, Mouth;b, anus;c, invagination which ultimately forms the greater part of the body of the adult.
Fig. 231.—Three stages in the metamorphosis of the Actinotrocha intoPhoronis.A, Actinotrocha larva with the invagination (c), which will form the trunk of thePhoronislarva 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.Cis 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 inPh. 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 nerveganglion, 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 aU-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 theadult 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 tubePhoronisis 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 ofPhoronis, 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 ofPh. australisenumerates five species, including amongst themPh. 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 ofPh. 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 hippocrepiaWright.—Under this name is included the first form, described and named by Wright in 1856; alsoPh. 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 ofPh. hippocrepia. TheCrepina gracilisof 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 kowalevskiiCaldwell.—This name is given by Benham to the species from Naples described by Caldwell, and replaces the namePh. 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 australisHaswell.—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 buskiiM‘Intosh.—This species was dredged by theChallengerfrom 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 ofPh. australis, and Benham thinks that, in spite of the difference in their habitat, they may belong to the same species.(v.)Phoronis architectaAndrews.—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 psammophilaCori.—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 fromPh. psammophila, except that no sand adheres to its tube and the number of tentacles is slightly greater.In addition to the various species ofPhoronis, several distinct forms of its larva, Actinotrocha, are known, and have been named without having been traced into their corresponding adult form.
(i.)Phoronis hippocrepiaWright.—Under this name is included the first form, described and named by Wright in 1856; alsoPh. 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 ofPh. hippocrepia. TheCrepina gracilisof 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 kowalevskiiCaldwell.—This name is given by Benham to the species from Naples described by Caldwell, and replaces the namePh. 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 australisHaswell.—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 buskiiM‘Intosh.—This species was dredged by theChallengerfrom 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 ofPh. australis, and Benham thinks that, in spite of the difference in their habitat, they may belong to the same species.
(v.)Phoronis architectaAndrews.—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 psammophilaCori.—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 fromPh. psammophila, except that no sand adheres to its tube and the number of tentacles is slightly greater.
In addition to the various species ofPhoronis, several distinct forms of its larva, Actinotrocha, are known, and have been named without having been traced into their corresponding adult form.
The position ofPhoronisin the animal kingdom has formed the matter of considerable divergence of opinion amongst the naturalists who have studied it. The earlier writers regardedPhoronisas allied to the Gephyrea, and it was for a long time classed with these animals, but placed in a separate sub-Order, theGephyrea tubicola, which was opposed to theGephyrea nuda, which comprised the true Gephyrea.
Caldwell referredPhoronis, 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 genusPhoronis; (b) the Pterobranchia, including the formsCephalodiscusandRhabdopleura, 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]onPhoronisseem to indicatethat 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,Phoroniswill be removed from its present isolated and enigmatical position, and placed withCephalodiscusandRhabdopleuraamongst the Hemichordata, which will be described in Vol. VII. of this work.
BY
SIDNEY F. HARMER, M.A.
Fellow of King's College, Cambridge
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), theso-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 acolony, 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.
fig232Fig. 232.—Flustra foliaceaL., Cromer.A, Natural size,B'indicating the portion magnified inB(× 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.
Fig. 232.—Flustra foliaceaL., Cromer.A, Natural size,B'indicating the portion magnified inB(× 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.
Fig. 232.—Flustra foliaceaL., Cromer.A, Natural size,B'indicating the portion magnified inB(× 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 asuitable 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. foliaceahas 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 ofFucus, 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 aregraduallypushed 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 genusBugula. 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 furtherthat 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 livingBugulacolony, with its hundreds of swaying avicularia, and with its tentacular funnels protruding from their zooecia, and withdrawing themselves capriciously from time to time.
fig233Fig. 233.—Bugula turbinataAlder, 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 tob.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 inB.
Fig. 233.—Bugula turbinataAlder, 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 tob.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 inB.
Fig. 233.—Bugula turbinataAlder, 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 tob.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 inB.
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 acharacteristic 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).
fig234Fig. 234.—Alcyonidium albidumAlder, 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])
Fig. 234.—Alcyonidium albidumAlder, 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])
Fig. 234.—Alcyonidium albidumAlder, 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 cavitywhich 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 ofBowerbankiain 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,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 ofCrisia(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 beingprotruded (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. InFlustrathe 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.