GEPHYREA AND PHORONIS

fig210Fig. 210.—Pontobdella muricata.× 1. (After Bourne.)

Fig. 210.—Pontobdella muricata.× 1. (After Bourne.)

The members of this family all inhabit fresh water; they have the habit of depositing the eggs separately, which are then fixed to the ventral surface of the body, and the young when hatched arestill protected by the parent, returning to its body for shelter. The type genusGlossiphonia( =Clepsine) is common in Europe, and has many species. The Mexican and AmazonianHaementeriacontains a number of species, of which the MexicanH. officinalisis used in medicine; but according to Miguel Jimenez, its use is apt to be attended with unpleasant symptoms. Drowsiness, a buzzing in the ears, and the development of a painful rash are some of the effects produced by its bite. It is disputed whether the animal's saliva or foreign matter introduced by it into the wound are the cause of the symptoms.

Mesobdellaof Blanchard[467]is said to be intermediate between this family and the next. Each segment has three annuli, as in this family, but the leech has three jaws, as inHirudo.

Sub-Order 2. Gnathobdellae.—Hirudinea without a proboscis, generally with jaws; the blood is red; the eggs are invariably deposited in cocoons.

Fam. 1.Gnathobdellidae.—Pharynx with three denticulate jaws.

This family as well as the next is terrestrial or fresh-water in habit. It contains a number of generic types, including the medicinal leech,Hirudo medicinalis, and the horseleech,Haemopis(Aulastomum)gulo. The former can be distinguished from the latter by its power of contracting itself into an oval olive-shaped form, which power is not possessed by the horseleech; the latter has, moreover, only two caeca, while the common leech has ten pairs of these appendages of the intestine. The genusLimnatisis called after the Greek wordλιμνῆτις, which Theocritus applied to the leech. It is found in the Nile, and caused serious inconvenience to the army of Napoleon. His soldiers in drinking at pools sucked up the small leeches not thicker than a horse's hair, whose presence in the hinder part of the mouth cavity produced divers objectionable results, such as spitting of blood and hindered respiration.

Fam. 2.Herpobdellidae.—Pharynx without denticulate jaws, with three unarmed chitinous plates.

A characteristic genus of this family isTrocheta, which is so common at the Zoological Society's Gardens and in the Regent's Park, and which has been met with in other places near London; it is in this country an introduced species, but is found in manyparts of the continent. It is a land-leech, and lives upon earthworms.

The genusHaemadipsa, which M. Blanchard places in a special sub-family, contains a number of species which are for the most part land-leeches. Land-leeches occur in many parts of the world, but chiefly in the tropics—in India, Ceylon, Java, South America, etc. They lie in wait for their prey, upon the ground as a rule; but they may ascend herbs and shrubs to gain a better outlook when they are aware of an approaching footstep. A vivid account of the ferocity of these tiny Annelids in Ceylon can be read in Sir J. E. Tennent'sNatural History of Ceylon. They have been said to be so pugnacious and so poisonous that persons surprised in their sleep by the pests have succumbed to their united efforts. A whole battalion of English soldiers decamped on one occasion from a wood which was overflowing with land-leeches. The familiar misquotation "lethalis hirudo" might well be applied to this species. Professor Whitman has written much upon the habits of the land-leech of Japan (Haemadipsa japonica), which bites so softly that its presence cannot be detected except for the stream of blood which trickles from the wound. While it is feeding it emits from the pores of the nephridia a clear fluid, which, as it appears, is used to keep the skin moist; when unduly dried the same phenomenon occurs. It is curious that in this and other leeches the nephridia should play a part which in the earthworm is played by the dorsal pores; in both animals the glands of the skin are also concerned with the same duty.

The purely aquatic leeches swim by undulations, and also crawl by the help of the two suckers, like a "Geometer" caterpillar. But when a land-leech is dropped into the water it at once sinks to the bottom and crawls out; it does not swim, but can survive immersion for a long period. In this it resembles the earthworms, which can also survive a prolonged immersion, and even in the case of some are indifferent to the medium, land or water, in which they live; the land-leech, however, is entirely dependent upon damp surroundings; a dry air is fatal to it. The land-leech of Japan leaves a slimy trail behind it as it crawls, in this respect recalling the land PlanarianBipalium kewense.

ARTHUR E. SHIPLEY, M.A.

Fellow and Tutor of Christ's College, Cambridge

GEPHYREA

INTRODUCTION—ANATOMY—DEVELOPMENT—SIPUNCULOIDEA—PRIAPULOIDEA—ECHIUROIDEA—EPITHETOSOMATOIDEA—AFFINITIES OF THE GROUP.

The animals included in the above-named group were formerly associated with the Echinodermata. Delle Chiaje[468]states that Bohadsch of Prague in 1757 was the first to give an accurate description ofSipunculusunder the name ofSyrinx, but Linnaeus, who noted that in captivity the animal always kept its anus directed upwards, re-named itSipunculus. Lamarck[469]placed the Gephyrea near the Holothurians; and Cuvier[470]also assigned them a position amongst the Echinoderms. He mentionsBonellia,Thalassema,Echiurus,Sternaspis, and three species ofSipunculus, one of which,S. edulis, "sert de nourriture aux Chinois qui habitent Java, et qui vont la chercher dans le sable au moyen de petits bambous préparés."

The name Gephyrea[471]was first used by Quatrefages, who regarded these animals as bridging the gulf between the Worms and the Echinoderms. He included in this group the genusSternaspis(videp.335), now more usually classed with the Chaetopoda.

The Gephyrea are exclusively marine. They are subcylindrical animals, which can either retract the anterior end of their body—the introvert—carrying the mouth into theinterior; or are provided with a long flexible but non-retractile proboscis. The latter is easily cast off. They usually bear spines or hooks of a hard chitinous character, secreted by the epidermis or outermost layer of cells. The mouth is at the base of the proboscis or at the end of the protractile part, the anus is at the other end of the body or on the dorsal surface. The nervous system consists of a ring round the mouth and of a ventral nerve-cord. A vascular system is present as a rule. Nephridia are found which act as excretory organs, and in most cases also as ducts for the generative cells. The Gephyrea are bisexual, and the male is sometimes degenerate.

The group may be divided into four Orders:—(i.) Sipunculoidea; (ii.) Priapuloidea; (iii.) Echiuroidea; (iv.) Epithetosomatoidea; of these the first is by far the largest, both in number of genera and of species.

The Anatomy of Sipunculus nudus.

External Characters.—The body ofS. nuduswhen fully extended may attain a length of a foot, or even a little more; in this condition it is seen to consist of two portions, the anterior of which is, however, retracted into the other when the animal is disturbed. The retractile portion is sometimes termed the proboscis, but as its nature is entirely different from that of the proboscis of the Echiuroidea, it is better to refer to it as theintrovert. Special retractor muscles are attached on the one hand to the body-wall about half-way down the body, and on the other hand are fused into a muscular sheath which surrounds the gullet, just behind the mouth. When these muscles contract, they withdraw the introvert into the rest of the body ortrunkin much the same way as the finger of a glove may be drawn into the hand, by a thread fastened to the inside of its apex. The introvert is protruded by the contraction of the circular muscles of the body-wall. These exert a pressure on the fluid which fills the body-cavity, and by this means the sides of the introvert are forced forward until finally the head is exposed.

The introvert occupies about one-sixth or one-fifth of the total body length. It is somewhat narrower than the trunk, and is covered by a number of small flattened papillae, some of which lie with their free ends directed backward, overlappingone another like tiles on a roof. In some other genera, asPhymosoma, the introvert bears rows of horny hooks, which are apt to fall off as the animal grows old.

The trunk has from thirty to thirty-two longitudinal furrows, the elevations between which correspond with a similar number of muscles lying in the skin. This longitudinal marking is crossed at right angles by a circular marking of similar origin, the elevations of which correspond with the circular muscles in the skin. These two sets of markings thus divide the skin of the trunk into a number of small square areas, very regularly arranged (Fig. 212).

The outline of the trunk is more or less uniform, but it is capable of considerable change according to the state of contraction of its muscles. The circular muscles, for instance, may be contracted at one level, thus causing a constriction at this spot. The colour ofS. nudusis a somewhat glistening greyish-white.

fig211Fig. 211.—Right half of the anterior end ofSipunculus nudusL., seen from the inner side and magnified.a, Funnel-shaped grooved tentacular crown leading to the mouth;b, oesophagus;c, strands breaking up the cavity of the tentacular crown into vascular spaces;c', heart;d, brain;e, ventral, ande', dorsal retractor muscles;f, ventral nerve-cord;G, vascular spaces in tentacular crown.

Fig. 211.—Right half of the anterior end ofSipunculus nudusL., seen from the inner side and magnified.a, Funnel-shaped grooved tentacular crown leading to the mouth;b, oesophagus;c, strands breaking up the cavity of the tentacular crown into vascular spaces;c', heart;d, brain;e, ventral, ande', dorsal retractor muscles;f, ventral nerve-cord;G, vascular spaces in tentacular crown.

The anterior end of the fully-expandedSipunculusmay be termed the head; here the skin is produced into a frayed fringe which stands up in the shape of a funnel round the mouth. This fringe is grooved on its internal surface with numerous little gutters, all of them lined with cilia, which by their constant motion keep up a current which sweeps food into the mouth.The fringe may be in the form of a simple ring round the mouth, or the ring may be folded in at the dorsal side so as to take the form of a double horse-shoe (Figs. 211 and 212).

Body-wall.—The glistening appearance ofSipunculusis due to the cuticle, a chitinoid layer which is secreted by the external layer of cells, theepidermis. Beneath this lies a layer of connective tissue, which is not always present in other Gephyrea; within this lies a layer of circular muscles arranged in bundles, then comes a very thin sheath of oblique muscular fibres, then a thicker layer of longitudinal muscles, and finally a layer of peritoneal epithelial cells, which inSipunculusare for the most part ciliated.

Scattered over the surface of the body, and opening by narrow tubes which pierce the cuticle, are a number of glandular bodies which may be either bi- or multi-cellular. The glandular cells are apparently enlarged and modified epidermal cells; they are arranged in a cup-shaped manner, with their apices directed towards the orifice. They are crowded with granules, which are presumably poured out over the cuticle, but the exact function of the secretion is entirely unknown. They have a well-developed nerve supply.

Digestive System.—The mouth lies in the centre of the fringe, and is not provided with any kind of jaw or biting armature; it leads directly into the thin-walled alimentary canal, the first part of which is ciliated. The alimentary canal is not marked out into definite regions, but passes as a thin-walled semi-transparent tube to the posterior end of the body, and then turns forward again and opens to the exterior by an anus situated about an inch below the junction of the introvert with the trunk, on the median dorsal line. The descending and ascending limbs of the alimentary canal are coiled together in a spiral, which may be more or less close in different individuals. The whole is supported by numerous fine muscular strands, which pass from the walls of the intestine to the skin, and by a spindle-muscle, which runs from the extreme posterior end of the trunk up the axis of the spiral and terminates in the skin close to the anus.

No glands open into the alimentary canal at any point of its course, but near the anus a simple diverticulum, or pocket, of unknown function arises. The size of this outgrowth differsenormously in different individuals. The alimentary canal near the anus also bears two tuft-like organs, which, however, do not open into the intestine, but probably have some function in connexion with the fluid in the body-cavity.

Along the whole course of the alimentary canal there runs a ciliated groove, into which the food does not pass, but the cilia of which probably keep in motion a current of water whose function may be respiratory.

fig212Fig. 212.—Sipunculus nudusL., with introvert and head fully extended, laid open by an incision along the right side to show the internal organs. × 2.a, Mouth;b, ventral nerve-cord;c, heart;d, oesophagus;e, intestine;f, position of anus;g, tuft-like organs;h, right nephridium;i, retractor muscles;j, diverticulum on rectum. The spindle-muscle is seen overlying the rectum.

Fig. 212.—Sipunculus nudusL., with introvert and head fully extended, laid open by an incision along the right side to show the internal organs. × 2.a, Mouth;b, ventral nerve-cord;c, heart;d, oesophagus;e, intestine;f, position of anus;g, tuft-like organs;h, right nephridium;i, retractor muscles;j, diverticulum on rectum. The spindle-muscle is seen overlying the rectum.

Vascular System.—On the dorsal surface of the anterior end of the alimentary canal lies a contractile vessel, usually termed the heart. It is a tube about an inch long, ending blindly behind, but opening in front into a ring-shaped space surrounding the mouth and partially enveloping the brain. From this ring-like vessel numerous branches are given off which pass into the fringe round the mouth, and probably the chief function of the heart is by its contraction to force fluid into this fringe, and so to extend it. The heart contains a corpusculated fluid.A similar but shorter tube is found on the ventral surface of the anterior end of the alimentary canal in the species in question; it also opens into the ring which surrounds the mouth.

Respiratory System.—There are no special respiratory organs, and it has long been a matter of dispute where the respiration of Gephyrea is carried on. The oxygenation of the blood probably takes place to some extent through the walls of the oral fringe, but the blood which receives its oxygen at this spot is limited in its distribution, and could only supply the brain and head. It seems probable that the remaining organs are supplied with oxygen by the fluid of the body-cavity, which bathes them on all sides. This might obtain its oxygen from the blood in the heart, or more probably, through the thin walls of the intestine, from the stream of water which is maintained by the ciliated groove described above. Quite recently a form—S. mundanus, var.branchiata—has been described[472]with thin-walled papillae covering parts of the skin. These papillae are full of corpuscles, and are regarded by their discoverer as branchiae.

Body-Cavity.—The pinkish fluid of the body-cavity contains numerous corpuscles, the products of the reproductive organs (either ova or spermatozoa), and some curious unicellular bodies known as "urns." The latter are shaped like a bowl with a ciliated rim, and are formed from the budding of certain cells on the walls of the dorsal blood-vessel.[473]Their function is unknown, but they resemble certain multicellular bodies found in the body-cavity ofPhascolosoma. The generative cells found in the body-cavity are further considered below. The true corpuscles are either biconcave round corpuscles coloured with a chemical substance, the haemerythrin of Krukenberg, which apparently plays the same rôle as haemoglobin in other animals; or amoeboid corpuscles, which, though rare inSipunculus, are very numerous inPhascolosoma.

Nervous System.—The nervous system ofSipunculusconsists of a brain or cerebral ganglion, a circumoesophageal ring surrounding the gullet, and a ventral nerve-cord. The brain is a small bi-lobed nervous mass situated on the dorsal surface of the oesophagus, in the angle between the right and left dorsal retractor muscles close to their point of insertion. Numerousnerves arise from it, and pass to the fringe surrounding the mouth and to neighbouring parts. At the sides, the brain is continued into two stout nerve-cords which encircle the oesophagus, and meeting, fuse together in the median ventral line to form the ventral nerve-cord (Fig. 211). The latter is of the same diameter throughout, and shows no signs of segmentation; it is oval in section, and consists of small ganglion cells heaped up on the ventral surface,i.e.next the skin, and of numerous fibres situated dorsally. The cord gives off many nerves, which usually arise in pairs. These pass into the skin, and forming rings, run round the body, and give off finer nerves as they go.

The nerve-cord is supported by numerous strands of muscle which pass to it from the skin. These are especially long in the region where the introvert joins the trunk, and thus allow free play to the nerve-cord when the former is being protruded or retracted.

Sipunculusis not well provided with sense-organs, but in an animal which lives buried in sand we should not expect to find these very highly developed. On the introvert there are certain patches of epithelium bearing long stout cilia, which have been regarded as tactile in function, and there is a tubular infolding reaching the brain, which almost certainly has some sensory function. Ward[474]has termed this "the cerebral organ." It consists of a duct lined with ciliated cells, which opens to the exterior in the middle dorsal line outside the tentacular fringe. The duct leads down to the brain, and expands at its lower end into a saucer-shaped space, covering that portion of the brain where its substance is continuous with the external epithelium. InPhymosomathis cavity is produced into two finger-shaped processes, which are sunk into the brain and are lined by cells crowded with a dense black pigment.[475]They are probably rudimentary eyes, perhaps distinguishing only between darkness and light. The pits appear to be absent inSipunculus nudus, but Andrews states they are found, although without pigment, inS. gouldii.[476]

Excretory System.—The excretory organs or "brown tubes" are typicalnephridia, that is to say, they consist of tubeswith glandular walls which open on the one side to the exterior, and on the other by means of a ciliated funnel-shaped opening into the body-cavity. In Gephyrea one wall of the tube is produced into a long diverticulum or sac which hangs down into the body-cavity, and is usually supported by muscle-fibres running to the body-wall. The lower end of the sac is broken up into a number of crypts or pits, lined by large glandular cells crowded with brown pigment. The pigment-granules are secreted into the cavity of the sac, and leave the body through the external opening; they probably consist of the nitrogenous excreta of the animal. The upper end of the sac, into which both the external and internal orifices open, is usually enlarged, and its walls are very muscular. As in so many other animals, the nephridia serve as ducts through which the reproductive cells leave the body of the parent.

Reproductive System.—The Gephyrea are bisexual. InSipunculusthe testes and ovaries are found in the same position in the two sexes, and are indistinguishable without microscopic investigation. They each consist of small ridges situated at the lower end of the ventral retractor muscles, just where the latter take their origin from the longitudinal muscles of the skin. At this level the cells which line the body-cavity on the inside of the skin are heaped up, and become modified in the one case into ova or eggs, and in the other into the mother-cells of the spermatozoa. This method of forming the reproductive organs from modified cells lining the body-cavity is very common in the higher animals; but it is seen in its simplest and least modified form in the Sipunculidae.

The eggs break away from the ovary in a very undeveloped condition, but whilst floating about in the body-cavity they increase in size and secrete a thick membrane around them. They have a well-marked nucleus, and are oval in outline.

The mother-cells of the spermatozoa also break away in an immature condition, and complete their development in the nutritive fluid of the body-cavity. They divide into a number of spermatozoa, usually eight or sixteen, which remain in contact. They each develop a tail, which projects outwards, and aids the cluster in swimming along. These clusters of spermatozoa are about the same size as the ova of the female, and, like them, make their way into the "brown tubes." The exact way inwhich this is accomplished is not very clear, but the cilia on the funnel-shaped internal opening of the tube seem to have some power of selecting the generative cells when they come within their reach, and of passing them on, whilst they reject the much smaller corpuscles of the perivisceral fluid, which are never found in the nephridia.[477]Once inside the internal opening, the clusters break up and the spermatozoa escape singly into the sea. Here they meet with and fertilise the eggs which have escaped from the body of the female.

fig213Fig. 213.—Larva ofSipunculus nudusL. × 150. (After Hatschek.)a, Mouth;b, anus;c, excretory organ;d, glandular appendage of oesophagus;e, wall of stomach over which the retractor muscle runs;f, invaginated sense-organ at aboral pole.

Fig. 213.—Larva ofSipunculus nudusL. × 150. (After Hatschek.)a, Mouth;b, anus;c, excretory organ;d, glandular appendage of oesophagus;e, wall of stomach over which the retractor muscle runs;f, invaginated sense-organ at aboral pole.

Development.—Hatschek,[478]who investigated the development ofSipunculus nudusat Pantano, an inlet of the sea near Messina, states that the spawning takes place during the night, and ceases about July 10. The rate of development depends upon the temperature, but the larvae usually free themselves from the egg-membrane during the third day. When hatched the embryos lengthen out a good deal, and take the form represented in Fig. 213. The larva swims actively by means of a ring of stout cilia, which encircle the body just behind the mouth. Other shorter cilia are found on the head, continuing into the lining of the mouth, and a little bunch of them is situated at the extreme posterior end. The alimentary canal is already formed, and is twisted, so that the anus lies dorsally, but not so far forward as it does in the adult. A glandular structure opens into the mouth, and another body of unknown function is connected with the oesophagus; both these disappear during larval life. A pair of excretory tubules, theforerunners of the brown tubes, are found, and the chief muscle tracts are already established. The nervous system is still in close connexion with the skin, from the outer part of which it is derived; the cerebral thickening bears two eye-spots.

The fluid of the body-cavity contains corpuscles, which are kept in active circulation by the constant contractions of the body-wall, and by numerous tufts of cilia which are borne on the inner surface of the skin. The dorsal blood-vessel is one of the latest organs to arise.

The larva swims actively about for a month, during which time it increases greatly in size; it then undergoes a somewhat sudden metamorphosis. The ciliated ring and the structures related to the oesophagus begin to disappear, the distinction between the head and the rest of the body is obliterated, and the head becomes relatively small. The mouth changes its position, and becomes terminal instead of being somewhat ventral, and the tentacular membrane begins to appear. At the same time the larva relinquishes its free-swimming life, and sinks to the bottom; it begins creeping amongst the sand by protruding and retracting the anterior part of its body, and takes on all the characters and habits of the adult.

I. Order Sipunculoidea.

Besides the genusSipunculus, the Order Sipunculoidea includes ten other genera. A key to these, taken for the most part from Selenka's admirable monograph, is given on page 424.

Phascolosomacontains, in comparison withSipunculus, only small species, and it is easily distinguished by the fact that the longitudinal muscles are fused into a continuous sheath. As a rule the skin is smooth. A few species bear hooks, which are generally scattered irregularly and not arranged in transverse rows, as inPhymosoma(Fig. 214) and most of the other genera.

The fold which inS. nudussurrounds the mouth may be in the same species bent in so as to take the form of a double horse-shoe, the opening of which is always dorsal, just above the brain; in this case the mouth is crescentiform. In other genera the fold is broken up into discrete tentacles, and these are variously arranged; inDendrostomathey are grouped together in four or six bundles round the mouth, but the more usualarrangement is the horse-shoe-like row of tentacles which overhang the crescentiform mouth, as inPhymosomaand some species ofAspidosiphon.

The ventral side of each tentacle is grooved and ciliated, and the grooves are continued into the ciliated mouth. Their dorsal surface is pigmented, and in the hollow of the horse-shoe lies a deeply pigmented epithelium covering the brain.

A blood-vessel courses up each tentacle, and usually two channels return the blood to the vascular ring which surrounds the mouth. In those forms which possess tentacles on the dorsal side of the mouth only, the ventral part of the vascular ring lies in the lower lip, which is tumid and swollen. The brain supplies a nerve to each tentacle.

When the introvert is retracted the tentacular ring is withdrawn and to some extent collapsed; in this condition it would be almost touching the rough external surface of the introvert. In some species ofPhymosomathe delicate appendages of the head are guarded from the hooks on the introvert by a thin membrane or collar,[479]which completely ensheaths the retracted head.

fig214Fig. 214.—A,Phymosoma granulatumF. S. Leuck. × 2.B, Head of the same. × 4.a, Pigmented pit leading to brain. The crescentiform mouth on the lower side of the figure is overhung by the tentacles.

Fig. 214.—A,Phymosoma granulatumF. S. Leuck. × 2.B, Head of the same. × 4.a, Pigmented pit leading to brain. The crescentiform mouth on the lower side of the figure is overhung by the tentacles.

When the introvert is fully extended the dorsal blood-vessel contracts and sends its blood forward into the vascular ring, and thence into the tentacles or tentacular fold, which are thus erected. In several species ofSipunculus, asS. nudus,S. norvegicus,S. robustus,S. tesselatus, there is a ventral blind tube as well as a dorsal, into which the blood is withdrawn when the head is retracted. In many other species in various genera, such asPhymosomaweldoniiandPh. asser,Dendrostoma signifer,S. vastus, the lumen of the dorsal vessel is increased by numerous hollow blind processes which it bears, hanging freely into the body-cavity. Three very small genera of Sipunculids—Onchnesoma,Petalostoma, andTylosoma—are devoid of all trace of vascular system and of tentacles; the mouth opens in the centre of the anterior end of the introvert. InOnchnesomathe dorsal part of the lip is somewhat produced, so that the head has somewhat the shape of a Doge's cap, and inPetalostomathere are two leaf-like processes of the body-wall which guard the mouth.

The extent to which the intestine is coiled varies very much even in the same species; the axis of the coil is often supported by a spindle-muscle, but this is sometimes absent. The caecum, which opens into the rectum ofS. nudus, is again a very variable structure, and when it is present varies remarkably in size.

The food of Sipunculids seems to consist almost entirely of sand, and their only nourishment must be such small microscopic organisms or particles of animal and vegetable débris as are to be found mixed with the sand. The alimentary canal is, as a rule, quite full of sand, and yet in spite of the tenuity of its walls they never seem to be ruptured. If the contents of the digestive tube be washed out with a pipette, it will be found that it requires considerable force to dislodge many of the sand-particles lying next the wall. These are more or less embedded in crypts or pockets of the wall, and as the sand passes along the intestine they probably serve as more or less fixed hard points, against which the sharp edges of the sand particles are worn off. Amongst the sand are usually to be found pieces of shell, sometimes with a diameter equal to that of the alimentary canal; these are usually rounded, but their angles may have been removed by attrition before they entered the mouth of the Sipunculid.

InS. tesselatusthe sand is to some extent held together by a mucous deposit; in those cases where there is no sand in the intestine, there is always a coagulum of mucus, and the walls are contracted and thick; when full of sand the walls are tensely stretched and very thin. This thinness of the wall of the alimentary canal seems ill-adapted to a diet of sand, nevertheless it is also met with in other great sand-eating groups of animals, such as the Echinids and the Holothurians.

The enormous amount of sand and mud which passes through the bodies of the Sipunculids shows that they must take a considerable part in modifying the mineral substances which form the bottom of the sea. Just as earthworms, as shown by Darwin, play a considerable rôle in the formation of soil, so must these animals, in conjunction with Echinids and Holothurians, effect considerable modifications in the sand and mud which pass through their bodies. Mr. J. Y. Buchanan[480]is "led to believe that the principal agent in the comminution of the mineral matter found at the bottom of both deep and shallow seas and oceans, is the ground fauna of the sea, which depends for its subsistence on the organic matter which it can extract from the mud." The minerals at the bottom of the sea are exposed to a reducing process in passing through the bodies of the animals which eat them, and subsequently to an oxidising process due to the oxygen dissolved in the sea-water acting on the minerals extruded from the animals' bodies.

The rate at which the sand passes through the body ofSipunculusis unfortunately unknown, but that at any one moment a considerable quantity is contained in the intestine is shown by the fact that the average weight of five specimens ofS. nudusfrom Naples, taken at random, was 19.08 grms., whilst the average weight of sand washed out of their alimentary canal was 10.03 grms. The sand contained in five other specimens of the same species measured respectively 6 c.c., 7 c.c., 6.5 c.c., 7.5 c.c., and 7.5 c.c., giving an average of 6.9 c.c. for each individual.

OnchnesomaandTylosomahave only one retractor muscle;AspidosiphonandPhascolionhave, as a rule, two;PhymosomaandSipunculushave four, and perhaps this is the more usual number.

Phascolion,Tylosoma, andOnchnesomahave but one "brown tube"; inPhascolionthis is the right, inOnchnesomait is sometimes the right and sometimes the left that persists. Most other genera retain two, but there are many exceptions; for instance,Phascolosoma squamatumhas but one, and so hasAspidosiphon tortus, and in both cases it is that of the left side. No Sipunculid has more than two. It has been pointed out by Selenka that those species which have but one browntube are, as a rule, inhabitants of tubes or shells, and do not move actively about in the sand.

The eggs of all members of the family, with the exception of the genusPhymosoma, are spherical, but those of the last-named genus are elliptical. They are always surrounded by a thick membrane, the "zona radiata," pierced by numerous pores.

Aspidosiphon(Fig. 215) is easily recognised by the presence of two symmetrically-arranged cuticular shields, one at each end of the trunk. These are formed by the fusion of minute cuticular plates, such as exist in the skin of most Sipunculids. The posterior shield is radially symmetrical, but the anterior is somewhat like the shell of a Pecten, and symmetrical only about one plane. The introvert is protruded from the acute angle of the anterior shield, and when extended lies almost at right angles to the trunk, instead of being, as is usually the case, in the same straight line with it. In many specimens, and these seem as a rule to be the older ones, a deposit of calcium carbonate takes place over these shields, covering over and concealing their external markings.

Cloeosiphon(Echinosiphon) has a calcareous ring, consisting of four or five rows of lozenge-shaped calcareous bodies forming a close mosaic, arranged round the base of the introvert, which when extended is in the same straight line as the trunk. Each piece bears a brown spot, which is said to be the pore of a gland (Fig. 217).GolfingiaLankester, has a cylindrical horny thickening at the anterior end of the trunk and another at the posterior.

fig215Fig. 215.—Aspidosiphon truncatusKef. × 2.a, Introvert partially extended, but not sufficiently to show the head.

Fig. 215.—Aspidosiphon truncatusKef. × 2.a, Introvert partially extended, but not sufficiently to show the head.

Key to the Genera of Sipunculoidea.[481]

I. The longitudinal muscles in the body-wall divided into 17-41 distinct bundles. Four retractor muscles.A. Body covered with papillae. Numerous filiform tentacles which seldom (or never?) surround the mouth, but stand above and dorsal to it in a horse-shoe, with the opening dorsal. No rectalcaecum. Hooks usually present. Four retractors (inPh. Rupelliionly two?). Heart almost always without caeca. Eye-spots always present. Eggs oval, flat, reddish. Almost entirely small tropical species1.PhymosomaB. Body devoid of papillae. Tentacular membrane surrounds the mouth in a circlet. Rectum with one or more caeca (exceptS. edulis?). Hooks absent except inS. australis. Eggs spherical. The tentacular membrane contains a vascular network. A ventral contractile vessel usually present in addition to the heart. Mostly large forms. Found in all seas2.SipunculusII. The longitudinal muscles in the body-wall form a continuous sheath, and are not split up into bundles.A. Two brown tubes. Numerous tentacles form a wreath round the mouth. Alimentary canal forms a complete spiral, free behind except inPh. Hanseni. Spindle-muscle usually present. One or more ligaments present, but only on the anterior convolutions of the intestine. Adhesive papillae always absent. Hooks very frequently absent. Eggs spherical. Found in all seas.3.PhascolosomaB. Two free brown tubes. Only four or six plumed tentacles. A complete intestinal spiral, not attached behind. Spindle-muscle always present. One or more ligaments present, but only on the anterior convolutions of the intestine. Hooks are present, but sometimes fall off early in life. Heart usually bears caeca. Found only in the tropics.4.DendrostomaC. Only one brown tube, that of the right side, present; it is attached to the body-wall throughout its entire length. Numerous tentacles form a circle round the mouth. The alimentary canal forms no spiral, or an incomplete one. No spindle-muscle, but the intestine is attached to the body-wall throughout its length by numerous ligaments. Adhesive papillae often present. Not more than two retractors. Spherical eggs. Inhabits Mollusc shells or tubes. Found in all seas5.PhascolionIII. At both ends of the trunk a distinct horny shield, or tube-like cornification, or a calcareous ring at the anterior end of the trunk. Hooks sometimes present. Longitudinal muscles continuous or split up into bundles.A. A shield at both ends of the trunk. Introvert excentric, arising from the ventral side of the anterior shield. Tentacles small and few in number, arranged in a horse-shoe above the mouth. A spindle-muscle, which arises from the posterior end of the body, traverses the intestinal coil. Two retractors only, these are the ventral; they are frequently fused together from their point of origin.6.AspidosiphonB. A calcareous ring surrounds the anterior end of the trunk, from the middle of which the introvert is extruded. Longitudinal muscles continuous. Hooks bifid. Tropical.7.CloeosiphonC. A corneous ring, from which the introvert issues, surrounds the anterior end of the trunk, and the posterior end of the trunk isproduced into a corneous spike. Six pinnate tentacles encircle the mouth. Four retractors. Hooks present on the introvert. Longitudinal muscles continuous. Intestine not coiled throughout in a spiral nor fastened posteriorly. Spindle muscle present.8.GolfingiaIV. No tentacles, but two leaf-like extensions of the body-wall guard the mouth. Four retractors. Few intestinal loops, quite free. No vascular system.9.PetalostomaV. No tentacles, no vascular system. One retractor, and one segmental organ.A. Introvert long. Body small, pear-shaped.10.OnchnesomaB. No introvert (?). Body cylindrical, thickly covered with papillae, which are larger and more crowded at both ends of the trunk.11.Tylosoma

I. The longitudinal muscles in the body-wall divided into 17-41 distinct bundles. Four retractor muscles.

A. Body covered with papillae. Numerous filiform tentacles which seldom (or never?) surround the mouth, but stand above and dorsal to it in a horse-shoe, with the opening dorsal. No rectalcaecum. Hooks usually present. Four retractors (inPh. Rupelliionly two?). Heart almost always without caeca. Eye-spots always present. Eggs oval, flat, reddish. Almost entirely small tropical species1.PhymosomaB. Body devoid of papillae. Tentacular membrane surrounds the mouth in a circlet. Rectum with one or more caeca (exceptS. edulis?). Hooks absent except inS. australis. Eggs spherical. The tentacular membrane contains a vascular network. A ventral contractile vessel usually present in addition to the heart. Mostly large forms. Found in all seas2.Sipunculus

1.Phymosoma

B. Body devoid of papillae. Tentacular membrane surrounds the mouth in a circlet. Rectum with one or more caeca (exceptS. edulis?). Hooks absent except inS. australis. Eggs spherical. The tentacular membrane contains a vascular network. A ventral contractile vessel usually present in addition to the heart. Mostly large forms. Found in all seas

2.Sipunculus

II. The longitudinal muscles in the body-wall form a continuous sheath, and are not split up into bundles.

A. Two brown tubes. Numerous tentacles form a wreath round the mouth. Alimentary canal forms a complete spiral, free behind except inPh. Hanseni. Spindle-muscle usually present. One or more ligaments present, but only on the anterior convolutions of the intestine. Adhesive papillae always absent. Hooks very frequently absent. Eggs spherical. Found in all seas.3.PhascolosomaB. Two free brown tubes. Only four or six plumed tentacles. A complete intestinal spiral, not attached behind. Spindle-muscle always present. One or more ligaments present, but only on the anterior convolutions of the intestine. Hooks are present, but sometimes fall off early in life. Heart usually bears caeca. Found only in the tropics.4.DendrostomaC. Only one brown tube, that of the right side, present; it is attached to the body-wall throughout its entire length. Numerous tentacles form a circle round the mouth. The alimentary canal forms no spiral, or an incomplete one. No spindle-muscle, but the intestine is attached to the body-wall throughout its length by numerous ligaments. Adhesive papillae often present. Not more than two retractors. Spherical eggs. Inhabits Mollusc shells or tubes. Found in all seas5.Phascolion

3.Phascolosoma

B. Two free brown tubes. Only four or six plumed tentacles. A complete intestinal spiral, not attached behind. Spindle-muscle always present. One or more ligaments present, but only on the anterior convolutions of the intestine. Hooks are present, but sometimes fall off early in life. Heart usually bears caeca. Found only in the tropics.

4.Dendrostoma

C. Only one brown tube, that of the right side, present; it is attached to the body-wall throughout its entire length. Numerous tentacles form a circle round the mouth. The alimentary canal forms no spiral, or an incomplete one. No spindle-muscle, but the intestine is attached to the body-wall throughout its length by numerous ligaments. Adhesive papillae often present. Not more than two retractors. Spherical eggs. Inhabits Mollusc shells or tubes. Found in all seas

5.Phascolion

III. At both ends of the trunk a distinct horny shield, or tube-like cornification, or a calcareous ring at the anterior end of the trunk. Hooks sometimes present. Longitudinal muscles continuous or split up into bundles.

A. A shield at both ends of the trunk. Introvert excentric, arising from the ventral side of the anterior shield. Tentacles small and few in number, arranged in a horse-shoe above the mouth. A spindle-muscle, which arises from the posterior end of the body, traverses the intestinal coil. Two retractors only, these are the ventral; they are frequently fused together from their point of origin.6.AspidosiphonB. A calcareous ring surrounds the anterior end of the trunk, from the middle of which the introvert is extruded. Longitudinal muscles continuous. Hooks bifid. Tropical.7.CloeosiphonC. A corneous ring, from which the introvert issues, surrounds the anterior end of the trunk, and the posterior end of the trunk isproduced into a corneous spike. Six pinnate tentacles encircle the mouth. Four retractors. Hooks present on the introvert. Longitudinal muscles continuous. Intestine not coiled throughout in a spiral nor fastened posteriorly. Spindle muscle present.8.Golfingia

6.Aspidosiphon

B. A calcareous ring surrounds the anterior end of the trunk, from the middle of which the introvert is extruded. Longitudinal muscles continuous. Hooks bifid. Tropical.

7.Cloeosiphon

C. A corneous ring, from which the introvert issues, surrounds the anterior end of the trunk, and the posterior end of the trunk isproduced into a corneous spike. Six pinnate tentacles encircle the mouth. Four retractors. Hooks present on the introvert. Longitudinal muscles continuous. Intestine not coiled throughout in a spiral nor fastened posteriorly. Spindle muscle present.

8.Golfingia

IV. No tentacles, but two leaf-like extensions of the body-wall guard the mouth. Four retractors. Few intestinal loops, quite free. No vascular system.

9.Petalostoma

V. No tentacles, no vascular system. One retractor, and one segmental organ.

A. Introvert long. Body small, pear-shaped.10.OnchnesomaB. No introvert (?). Body cylindrical, thickly covered with papillae, which are larger and more crowded at both ends of the trunk.11.Tylosoma

A. Introvert long. Body small, pear-shaped.

10.Onchnesoma

B. No introvert (?). Body cylindrical, thickly covered with papillae, which are larger and more crowded at both ends of the trunk.

11.Tylosoma

Species of Sipunculoidea.—The genusPhymosoma(Fig. 214) contains more species than any other genus of Sipunculoidea, and they are all of fair size. Twenty-seven species are known, of which seventeen occur in the Malay Archipelago, thirteen being found there alone.Phymosomaaffects shallow water, the deepest specimens being taken at a depth of about 50 fathoms; this may be due to the fact that they flourish only in comparatively warm water. With very few exceptions, they are found only in tropical seas, very often living in tubular excavations made in soft coral rock.

The genusSipunculuscontains sixteen species. They are the largest and the most conspicuous members of the group. They have a very wide distribution, some species, asS. nudus(Fig. 212) andS. australis, being almost cosmopolitan. They are most common in temperate and tropical seas, butS. norvegicusandS. priapuloidesare found far north, but always at considerable depths, 100 to 200 fathoms.

The following account of the habits ofSipunculus gouldiiis taken from Mr. Andrews'[482]paper on that species:—

"ThisSipunculusis very abundant in certain small areas of compact, fine sand darkened by organic matter and not laid bare at ordinary low tide. In such places, only a few square metres in extent, they pierce the sand in all directions to a depth of more than half a metre, making burrows with persistent lumen running from the surface downward and then laterally, but with no regularity in direction.

"Kept in aquaria, the dependence of the animal upon thenature of the sand and its method of locomotion may be readily observed. A vigorous individual buries itself in a few moments in the following manner: Running out the introvert to nearly its full extent, and applying it to the surface of the sand till some spot of less resistance is found, the animal still further expands the introvert so that it penetrates the sand, provided this is not too dense and firm, for then the body is merely shoved backward. When the introvert is inserted, the contraction of the longitudinal muscles of the body-wall brings the whole body forward somewhat, in case the introvert is fixed in the sand. In case soft ooze was present, this fixation did not take place, and the introvert was merely pulled out again, but when the sand was of the right consistency the introvert was fixed by becoming much swollen at the tip, and then constricted just posterior to this swollen area. This bulb-like area exerts lateral pressure on the sand, as could be seen by movements of the grains. The swelling of the anterior end of the introvert is brought about by the body-wall contracting elsewhere, and forcing in liquid to distend that end. Owing to the curved form assumed by the body in the normal contracted state when first removed from its burrow, the entrance of the introvert may often be nearly vertical, and hence the entire body is soon raised nearly upright in the water above the sand. If the body has thus been warped forward sufficiently to become somewhat fixed in the sand, the introvert is rolled in and again thrust forward from this new point of resistance, and so on till the animal is entirely buried. This locomotion increases in speed as the creature becomes more completely surrounded by sand, and is the only means of moving from place to place.

"On a smooth surface, or on one not presenting the right degree of resistance, theSipunculusdoes not change its position, but remains till death finally occurs, rolling its introvert in and out and contracting its body-wall to no purpose.

"The essential factors in the mechanism bringing about this hydrostatic locomotion are an elongated contractile sac filled with liquid, and some means of definitely co-ordinating the contractions of the sac.

"In natural environment the animals are found with sometimes one, sometimes the other end nearer the surface of the sand: in the aquaria the same was observed, but when thewater became stagnant and impure the anterior end with expanded branchiae was often protruded somewhat above the surface of the sand."

The genusPhascolosomacontains at least twenty-five species, for the most part small.Ph. margaritaceum, however, measures[483]10 cm. in length, andPh. flagriferum, 13 cm. The latter is produced at the hinder end of its trunk into a long whip-like process, which recalls the horny spike ofGolfingia. Most species live free, but a few inhabit the shells of dead Gasteropods or ofDentalium, or the abandoned tubes of worms. They occur in practically all seas.

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