Chapter XXIII: Introduction: Anatomy—development—sipunculoidea—priapuloidea (2)

In both sexes the larvae develop to a certain stage without showing any trace of sexual differentiation, but after this stage, the development of the male is to a certain extent arrested; in some respects, indeed, it undergoes retrogressive changes. At this time it is found clinging to the proboscis of the female, thence it makes its way to the mouth, where it undergoes its final change; and then creeping out, finds its way into the nephridium of the female, and spends the rest of its life there in a special recess cut off by a fold from the excretory part of this organ. In _Hamingia_, however, Lankester, who first described the male, did not find any in the nephridia, but found five specimens, each 1/12 inch long, within the dilated pharynx of the female.

DEVELOPMENT.—In _Bonellia_ and _Hamingia_ it seems probable that the ova are fertilised in the nephridium of the female; in the other genera they are fertilised in the water after leaving the body of the mother.

In _Thalassema_ and _Echiurus_ the growth of the embryo results in the formation of a typical Trochosphere larva, a type widely spread in the animal kingdom, being found in the Chaetopoda (Fig. 145, A), Polyzoa (p. 510), and Mollusca. The large prae-oral lobe persists in the Echiuroidea as the proboscis; the mouth is ventral in position, with usually a ring of cilia encircling the body in front of and behind it; the anus is posterior and terminal. A pair of larval excretory organs are present, and a special nervous aggregation of cells at the apex of the prae-oral lobe is usually indicated by the presence of a bunch of long cilia.

The trunk of the Trochosphere is unsegmented, and in certain groups of animals it remains so, but in Chaetopods, and in _Echiurus_ and _Thalassema_, it elongates and becomes divided {440}up into a series of somites or segments. Of these there are fifteen in _Echiurus_, and apparently eleven in _Th. mellita_; in this stage the Gephyrean larvae have again so close a resemblance to the segmenting Chaetopod larvae as to be easily mistaken for them. The segmentation is shown in the following way: (i.) the middle layer of cells or mesoblast is typically segmented, and forms septa, which separate each segment from its neighbours; (ii.) the ventral nerve-cord arises as segmentally-arranged thickenings of the epiblast, which fuse together, but retain their segmented appearance for some time; (iii.) the skin shows the segmentation of the body both by the arrangement of the pigment and by bands of cilia. The latter are replaced in the adult by rows of spines, and on the fourteenth and fifteenth segments in _Echiurus pallasii_ by the two peri-anal circles of bristles. Each bristle, like those of Chaetopods, originates from a single cell.

The anal vesicles arise quite late in the development; when they have acquired their openings into the body-cavity, they seem to take in water. In _Thalassema_, as described by Conn, this is accompanied by remarkable changes, amounting almost to a metamorphosis. The body increases in bulk fourfold, the cilia of the prae-oral ring disappear, and the animal now moves only by means of its proboscis; the pigment is absorbed, and all traces of segmentation disappear. A similar intaking of water is described by Spengel in _Bonellia_. In this genus the larva, which is coloured bright green, and has two brown eye-spots, is not such a typical Trochosphere as is that of _Echiurus_ and _Thalassema_.

Nothing is known of the development of _Hamingia_ or of _Saccosoma_.

SPECIES OF ECHIUROIDEA.—_Echiurus._ Proboscis not bifurcated at the end. Two ventral hooks and a single or double peri-anal ring of bristles. The body is to a varying extent marked {441}by rings bearing spines. Two or three (_E. chilensis_) pairs of nephridia, their external orifice often lengthened and spirally coiled. Both sexes alike.

Greef[487] mentions six species of _Echiurus_, viz. _E. pallasii_, _E. forcipatus_, _E. sitchaensis_, _E. chilensis_, _E. carabaicus_, and _E. chrysacanthophorus_; to these must be added _E. unicinctus_. It seems probable that _E. forcipatus_ of Reinhardt is identical with _E. pallasii_, although bigger, whilst _E. sitchaensis_, _E. carabaicus_, and _E. chrysacanthophorus_ are inadequately described. The distribution of the remaining three species is as follows:—

_E. pallasii_ Guérin (Fig. 223). North Sea, Atlantic, English Channel.

_E. unicinctus_ Drasche. Japan.

_E. chilensis_ Max Müller. Chili.

_Thalassema._—Proboscis rather pointed at the end, not bifurcated. No peri-anal bristles, but two ventral hook-like bristles placed anteriorly. One to three or four pairs of nephridia. The sexes resemble each other.

Greef mentions eight species of _Thalassema_ and Rietsch thirteen; three of these, however, _Th. grohmanni_, _Th. lessonii_, and _Th. pelzelnii_, were not seen by either author, and their description is taken from Diesing. There is some reason for thinking that the two first-named species are identical with _Th. neptuni_. Conn has established a new species for the specimens whose embryology he worked out at Beaufort, Virginia, and Selenka described a new species from the Challenger material.

With the exception of the three doubtful species mentioned above, the list of species of _Thalassema_ is as follows:—

_Th. neptuni_ Gaertner (Fig. 224). English Channel (Devonshire coast),
Concarneau, Mediterranean (Gulf of Marseilles), Irish coast (Dungarvan).

_Th. gigas_ Max Müller. Trieste.

_Th. vegrande_ Lampert. Philippine Islands.

_Th. baronii_ Greef. Canary Islands (Lanzarote).

_Th. formosulum_ Lampert. Shanghai and Philippine Islands.

_Th. exilii_ Fr. Müller. Brazil (Desterro).

_Th. moebii_ Greef. Mauritius.

_Th. erythrogrammon_ Max Müller. Red Sea and East Indies (Billiton).

_Th. caudex_ Lampert. Red Sea and Indian Ocean.

_Th. sorbillans_ Lampert. Philippine Islands.

_Th. mellita_ Conn. West Atlantic (Beaufort).

_Th. faex_ Selenka. North of the Faroe Islands.

{442}_Bonellia._—Proboscis very extensible and bifurcated at the end. The body and proboscis are coloured a bright green. Two ventral hook-like bristles, but no peri-anal ring. A single nephridium. The above applies to the female; the males are degenerate, and live in the nephridium or pharynx of the female.

Three (or four?) species of this genus are known.

_B. viridis_ Rolando (Fig. 220). Mediterranean, Adriatic, North Sea
(Bergen).

_B. minor_ Marion. Mediterranean (Gulf of Marseilles, Naples).

_B. suhmii_ Selenka. Off Nova Scotia. Male not known.

_B. fuliginosa_ Rolando? (Fig. 220). Mediterranean (Naples).

_Hamingia._—Proboscis not bifurcated, about as long as body. No ventral hook-like bristles. One or two nephridia, which open at the apex of one or two well-marked papillae. The above applies to the female; as in the genus _Bonellia_, the male is minute and parasitic. It has two well-marked hook-like bristles situated behind the genital pore.

This genus was first described by Koren and Danielssen as _H. arctica_. Two specimens were afterwards described by Horst as _H. glacialis_. Later Lankester described two other specimens; he was the first to find the male in the pharynx of the female. He is of the opinion that all three descriptions apply to the same species, and for this the original name _H. arctica_ must be retained.

_Hamingia arctica_ K. and D. Two hundred miles north of North Cape and in
the Hardanger Fjord.

_Saccosoma._—No proboscis. The body is flask-shaped. The mouth and anus are terminal. The ovary is anterior, and there is only one nephridium. No bristles.

Our knowledge of this remarkable Gephyrean is very incomplete, but such as it is, it is due to the careful investigations of Koren and Danielssen, who had only a single specimen at their disposition.

_Saccosoma vitreum_ K. and D. North of the Faroe Islands.

HABITS OF THE ECHIUROIDEA.—As a rule the members of this group conceal their bodies in clefts and fissures of rocks and stones, keeping up communication with the outer world by means of their proboscis. Rietsch[488] describes a specimen of _Bonellia minor_, which he placed in an aquarium, exploring with its {443}proboscis the nature of the bottom; when the animal had found a convenient crevice, it fixed its proboscis in it by means of the bifurcated end, and by its contraction drew the body up, and entered the hole, proboscis first. It then turned round, and during this operation doubtless the ventral hooks came into play; and then stretching out its proboscis, it began to explore the neighbourhood. The proboscis is evidently very sensitive, and in addition to being a locomotor organ, it is also used for the prehension of food. If cut off near the mouth, the animal does not long survive, but if a considerable portion is left the scar heals, and the lost part is probably regenerated. In captivity the animals frequently change their place of residence.

Eisig some years ago described the great extensibility of the proboscis of _B. viridis_ when confined in the tanks of the Zoological Station at Naples. When contracted the proboscis was but a few inches long, but at times it was extended till it reached the length of 1½ metre, shining through the water as a transparent green thread. The body of the _Bonellia_ was hidden under stones, but the proboscis could be seen seizing between its two ends the bodies of certain Ascidians which covered the inside of the tank, tearing them off the walls, and conveying them to the mouth along its grooved ventral surface.

The food of the Echiuroidea consists of organic matter, in the main of animal nature, but the group differs from the Sipunculoidea in not eating sand.

Rietsch describes _Thalassema neptuni_ as being more active in its movements and less sedentary than _B. minor_. The proboscis is still the chief organ of locomotion, but the trunk plays a greater part in the movements of the animal than it does in the last-named species. _Th. neptuni_ is found in cavities of stones or in the chambers worn out by the Mollusc _Gastrochaena_; when withdrawn from its house the body is found to be covered by a thick layer of tenacious viscid mucus.

{444}_Th. mellita_ was so named by Conn because it is found sheltering in the Echinid _Mellita_. "It enters the shell at the oral opening while yet very small, but once within its house it grows to its adult size, and is obliged therefore to remain during the rest of its life a prisoner." Each shell thus inhabited acquires a reddish brown horse-shoe-shaped marking, which affords a conspicuous signal that the shell contains a _Thalassema_.

_Thalassema_ is seldom found living in sand, and _Bonellia_ never, but _Echiurus_ is almost always found in U-shaped tubes or passages in the sand, which it digs out for itself by the rapid contractions of its body-wall aided by its bristles. It, like the other two genera named above, does not long remain in the same hole, but frequently changes its home. As a rule the _Echiurus_ sits near the mouth of its tube, which is often a foot or even two in depth, and sends out its proboscis in every direction; at the least sign of disturbance it withdraws into the deeper recesses. The walls of the tube are kept from falling in by a layer of mucus, which makes a smooth lining to the passage. The peri-anal bristles, which can be withdrawn or protruded at will, enable the animal to fix itself at any level in the tube.

The Echiuroidea are sometimes used by fishermen as bait. In _Echiurus pallasii_ Greef found three parasites, all of them new species. One, a Gregarine, he named _Conorhynchus gibbosus_; the others were Platyhelminthes, and were named by him _Distomum echiuri_ and _Nemertoscolex parasiticus_ respectively.

IV. ORDER EPITHETOSOMATOIDEA.

This Order includes the single Family _Epithetosomatidae_, which was established by Koren and Danielssen to contain the remarkable Gephyrean they described in 1881 under the name _Epithetosoma norvegicum_ (Fig. 225).

Unfortunately only two specimens were at their disposition, and these were badly preserved, so that many details of their structure could not be made out. The animals are of an olive-green colour, and consist of a trunk about 12 mm. long, and of a proboscis 30 mm. in length; the latter differs essentially from the proboscis of the Echiuroidea inasmuch as it is hollow, and seems to be a whip-like tubular extension of the skin. Its lumen opens into the body-cavity. Ventral to the base {445}of the proboscis is the mouth; the intestine is straight, and terminates in the anus, which is posterior. The nervous system lies between the circular and the longitudinal muscles of the body-wall, and contains a tube, the nature of which is obscure. No vascular system is known. The ovary is attached to a mesentery ventral to the anterior part of the alimentary canal, and there is a single nephridium. No anal vesicles exist.

The most remarkable feature of the genus is a series of pore-like openings, which are stated to lead from the outside into the body-cavity (Fig. 225, _a_). These are arranged four on each side, at the bottom of two slit-like depressions in the skin, which lie one on each side of the base of the proboscis, slightly dorsal to it.

These remarkable structures are without parallel amongst the Gephyrea, and, together with the peculiar character of the proboscis, justify the Norwegian naturalists in adding a new family to the group.

AFFINITIES OF THE GEPHYREA.

Before considering to what other groups of animals the Gephyrea may be allied, it is advisable to discuss the relationship of the four Orders which compose the group.

Quatrefages, in the year 1865, divided the Gephyrea into I. _Gephyrea Armata_, with which he included the Echiuroidea and _Sternaspis_,[489] and II. _Gephyrea Inermia_ or Sipunculoidea. The Gephyrea Inermia, sometimes called the Achaeta, have been extended to include the Order Priapuloidea, and opposed to the smaller sub-group the Gephyrea Armata or Chaetifera. In my opinion, however, these names now are no longer in accordance with our knowledge of the structure of the {446}animals they attempt to describe, and they should be given up. Both names had reference to the presence or absence of the two hook-like bristles described on the ventral surface of some of the Echiuroidea, but of the five genera of this family, two, _Saccosoma_ and _Hamingia_ (the latter in the female or normal form), are without these bristles, and can therefore be described neither as _Armata_ nor as _Chaetifera_. On the other hand, hook-like chitinous bristles of somewhat the same nature, though smaller in size and varying in position, are very common on the introvert of Sipunculoidea and on the body of the Priapuloidea.

Again, the association of the two last-named Orders in one sub-group is, to my mind, an error. The Priapuloidea have little in common with the Sipunculoidea; almost the only real point of resemblance is the power of protruding the anterior part of the alimentary canal, and withdrawing it by the aid of retractor muscles. But in the Priapuloidea this power exists to a very small extent, and it is a power shared by very many animals besides the Gephyrea. The terminal anus of the former is a feature shared by the Echiuroidea and by _Epithetosoma_, but these have little else in common with the Priapuloidea. On the other hand, the entire absence of any head appendages, such as the proboscis of the Echiuroidea and the tentacles or tentacular membrane of the Sipunculoidea, the absence of a vascular system, of nephridia or anal vesicles, taken together with the straight intestine which occurs elsewhere only in _Epithetosoma_, the persistent connexion of the nervous system with the epidermis, the unique character of their excretory system and of the reproductive organs, are all features in which the Priapuloidea differ from the more normal members of the other three Orders. These constitute a list of peculiarities which are at least as important, and probably even more important, than those which characterise the Sipunculoidea and the Echiuroidea. Thus the Priapuloidea should, I think, be regarded as a distinct Order, which occupies a very isolated position in the group.

Until we know something about the development of _Halicryptus_ and of _Priapulus_, it will be difficult to say whether the Order is more nearly allied to one or the other of the two great Orders of Gephyrea, whether it is very primitive or very specialised. The connexion of the entire nervous system with the epidermis and the absence of a vascular system are both {447}rather primitive features, and so is the Platyhelminthine character of the excretory organs. With regard to the vascular system, however, it should be pointed out that it arises very late in the larva of those Gephyrea whose development is known, and that it does not seem to correspond with the vascular system of other animals; it has no fine vessels or capillaries connected with it, and apparently does not act so much as the channel of the circulatory medium, but more as a mechanism for the expansion of the head appendages, the tentacles in the Sipunculoidea and the proboscis in the Echiuroidea; moreover, it is absent in some genera of the former, such as _Onchnesoma_, _Tylosoma_, and _Petalostoma_, where there are no tentacles.

The conclusion of the whole matter seems to be that the Priapuloidea are an isolated Order retaining many primitive features, and having no closer affinities to the Sipunculoidea than to the Echiuroidea.

Hatschek came to the conclusion, from his work on the development of _Echiurus_, that the Echiuroidea are true "Annelids," and from the presence and mode of formation of the bristles, that they are related to the Chaetopods. In this view he is confirmed by Conn, who worked out the development of _Thalassema_. This relationship is further confirmed by the discovery of Sluiter's that _Sternaspis_, the genus of Chaetopods which in other respects most nearly resembles the Gephyrea, has in one of its species (_S. spinosa_) a well-marked bifid proboscis, which, like that of the Echiuroidea, is thrown off at the least disturbance. Thus it seems fairly well established that the Echiuroidea are closely connected with the Chaetopoda, for although the only traces of segmentation they retain in the adult are the serially-repeated nephridia of _Thalassema_ and _Echiurus pallasii_, and the two rows of peri-anal bristles in the latter, and possibly the circular nerves given off from the ventral cord, yet the larva is fully segmented, and in other respects is almost typically Chaetopodan.

The relationship of the Sipunculoidea to the Echiuroidea is a more doubtful point. Hatschek is inclined to separate them, and in this he is again supported by Conn. Embryology unfortunately does not help us much. The early stages and larvae of _Sipunculus nudus_ and of _Phascolosoma elongatum_ have been investigated by Hatschek and by Selenka respectively. In neither genus is there any trace of segmentation or of Annelid {448}features, with the possible exception of the bristles on the larval _Phascolosoma_. On the other hand, it must be remembered that the development of _Sipunculus_ is remarkably abbreviated, and that such stages may have dropped out, the larvae hardly differing more from the Trochosphere of _Echiurus_ and _Thalassema_ than does that of _Bonellia_, an undoubted Echiurid. Still the facts that there is never a head-kidney present, that there is no trace of segmentation, and that at no stage is the anus terminal, must have a certain weight.

If we leave out of account the larval history, which, although pointing to a difference in the nature of the two families, is by no means decisive, and consider the adult structures, we find very considerable evidences of affinity. Taking firstly the main points of difference, we find these to be (i.) the nature of the cephalic appendages, either a proboscis or some modification of tentacles; (ii.) the position of the anus; (iii.) the presence of anal vesicles; (iv.) the number of the nephridia, never more than one pair in Sipunculids; and (v.) the difference in origin of the chaetae. Of these most undoubtedly the first is the most important. The Echiuroidea have retained the prae-oral lobe of the larva in the form of a solid outgrowth of the body, which outgrowth has carried with it the nerve-ring and vascular ring which surround the mouth. This has been lost in the Sipunculoidea, but is, I think, represented by a modified patch of epidermis which lies dorsal to the mouth and just above the brain. A solid extension of the skin in this region, which involved the nervous and vascular systems, would bring about the same relation of parts as is found in the Echiuroidea. The tentacular membrane or tentacles of the Sipunculoidea have such a variety of form and arrangement, whilst all subserving the same end, that I am inclined to believe that they have originated within the limits of the family.

The position of the anus in the Sipunculoidea is one common to very many animals which live embedded in sand or in tubular holes; it is probably not primitive, as in the larva of _Sipunculus_ it is near the posterior end, and becomes more dorsal as the larva elongates.

The anal vesicles of the Echiuroidea probably have no representative in the Sipunculoidea. In appearance and position they are very like the little tufts which are found on the rectum of {449}_Sipunculus_, but since these open neither into the body-cavity nor into the alimentary canal, it is hardly fair to compare them.

The resemblances between the Orders seem to me, on the whole, to outweigh the differences. The general structure of the skin, the coiled alimentary canal, with its ciliated groove, supported by strands of muscles, the vascular system which gives off no capillaries, the structure of the brown tubes, the existence of chitinous hooks or bristles, the nervous system with its single unsegmented ventral cord, the formation of the generative organs, all point to a sufficiently close resemblance to justify us in classing the two Orders together. In addition to these there are considerable histological resemblances which cannot be discussed here, but which have a certain weight.

To sum up, it seems probable that the Echiuroidea are derived from the Chaetopoda, and that their nearest ally in this group is _Sternaspis_; and that the Sipunculoidea are allied to the Echiuroidea, but have further departed from the Annelid stock, and have lost even those traces of affinity with the parent group which have been preserved in the development of _Echiurus_ and _Thalassema_.

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.

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.

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.

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.

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).

_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.

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