Chapter X: Introduction: Nature of Cestodes—occurrence of Cestodes—the Tape-Worms of (2)

_Cephalothrix bioculata_ Oerst., _C. linearis_ Rathke.

_Valencinia lineformis_ M‘Int.

A most important monograph by Bürger[143] on Nemertines has just been published, but unfortunately it appeared too late to be adequately considered here. He gives an elaborate account, illustrated by admirable figures, of the present state of our knowledge of this group, and his work will be indispensable to future students of the subject. The older systems of classification are criticised, and the following scheme is adopted in their place:—

ORDER I. PROTONEMERTINI (= part of the Palaeonemertea, e.g. _Carinella_).—The brain and lateral nerve-cords lie outside the muscle layers in the epithelium or below the basement membrane. The body-wall consists of the following layers: epidermis, basement membrane, circular muscles, and longitudinal muscles. The mouth lies behind the brain. The proboscis is unarmed.

ORDER II. MESONEMERTINI (= part of the Palaeonemertea, e.g. _Cephalothrix_).—The characters of this Order are similar to those of the Protonemertini except that the brain and lateral nerve-cords lie in the muscle layers.

ORDER III. METANEMERTINI (= Hoplonemertea).—The brain and lateral nerve-cords lie in the parenchyma of the body internal to the muscle layers. The layers of the body-wall are {113}similar to those of the Protonemertini. The mouth lies in front of the brain. The proboscis is armed. At the junction of the fore- and mid-gut a diverticulum is given off which projects forwards beneath the fore-gut and ends blindly in front.

ORDER IV. HETERONEMERTINI (= Schizonemertea, and the genera _Eupolia_ and _Valencinia_, placed provisionally by Hubrecht in the Palaeonemertea).—The body-wall consists of the following layers: epidermis, thick cutis, and an outer and an inner longitudinal muscle layer separated from one another by a circular muscle layer. The brain and lateral nerve-cords lie between the outer longitudinal and the circular muscle layers. The mouth lies behind the brain. The proboscis is unarmed.

DEVELOPMENT OF THE NEMERTINEA.—The development of the Palaeonemertea is at present not known: in the Schizonemertea a larval stage occurs; while in the Hoplonemertea the egg develops directly without undergoing any metamorphosis.

There are two forms of larva characteristic of the Schizonemertea, known respectively as Pilidium and the Type of Desor. The Pilidium is hatched early and leads a free-swimming existence, whereas the Type of Desor, though in many respects resembling it, never passes through the free-swimming phase.

The Pilidium (Fig. 60) is a helmet-shaped larva bearing a tuft or spike dorsally, and prolonged downwards laterally into a pair of lobes. The whole larva is covered with cilia, there being a specially strong band round its ventral surface. The dorsal spike is composed of a bunch of strongly developed cilia or of a long flagellum. The alimentary canal consists of a sac constricted into {114}oesophageal and gastric regions (Fig. 60, _oes_ and _st_). In this condition the larva swims about freely in the water. The helmet-shaped Pilidium-skin forms no part of the future Nemertine, the skin of which is developed as ingrowths from it; these meet one another and unite to form a complete covering round the alimentary canal; the larval skin is then cast off, and by a series of gradual steps the embryo develops into the adult.

HABITS.—Nemertines are often found under stones between high- and low-water marks, lying on sandy or muddy bottoms. They are usually in the form of coiled masses, and are generally in a state of quiescence. Hence it is probable that their period of activity is during high-water, and that when left by the receding tide they subside into a resting condition.

The large kinds, such as _Lineus marinus_, seem to be always found living alone, but some of the smaller kinds, notably _Tetrastemma dorsale_ and _Prosorhochmus claparedii_, have gregarious habits and live in masses, the coils of the different individuals being inextricably mixed.

Some species, such as _Micrura purpurea_, _Amphiporus pulcher_, and _Cerebratulus angulatus_, frequent empty bivalve shells, while Nemertines are often found in empty limpet shells adhering to rocks in tidal pools. Other smaller forms resort to no such definite protection, but live among seaweeds; some of these remain naked, while others secrete for themselves tubes of a membranous or gelatinous consistency. _Borlasia elizabethae_ lives in a burrow of clay.

Nemertines are commonly dredged from a depth of six or eight fathoms. They may sometimes be found floating on the surface of the water, and some possess the power of swimming rapidly, propelling themselves by a lateral motion of the tail, the sides of which are in such cases prolonged into a thin fin-like edge. This mode of progression is usually adopted by those which frequent deep water. A pelagic Nemertine (_Pelagonemertes_) was discovered by Moseley near the southern verge of the South Australian current, being found in a trawl with deep-sea forms from a depth of 1800 fathoms. This animal was leaf-like in shape, bluntly pointed behind and rather square in front.

The power possessed by Nemertines of secreting mucus is very great, their course being often traceable by the tracks which they leave behind them. Many of them glide along with great rapidity, a mode of progression which is probably due to the {115}cilia covering the whole outer skin, and to the extreme contractility of the muscles of the body-wall. In some locomotion is effected by the proboscis; this is protruded and attaches itself by means of its spines to some foreign body, after which the body is drawn up after it. This has been specially observed in a land form, _Tetrastemma agricola_, discovered by Willemoes-Suhm in the Bermudas. On solid bodies the movement is a kind of crawling action, the head and mouth acting as suckers in much the same way as in many Leeches.

Most Nemertines can be very readily kept in confinement. The chief apparent effect of such a life is a loss of colour, the animal gradually becoming pallid in hue. Owing also to the absence of proper food they diminish very much in size, though even when all food is kept away an animal will sometimes continue to live as long as eighteen months.

FOOD.—Nemertines are carnivorous in their habits and are very voracious, devouring any prey which comes in their way, whether it be living or dead. No animal food seems to come amiss to them, and they will devour creatures of considerable size. When in contact with its prey, the Nemertine dilates its mouth to a large extent, and the anterior end of the oesophagus is thrust out and engulfs the animal. Chaetopods form a favourite food material, the whole animal being swallowed quite regardless of the hard chitinous bristles and spines with which it is beset. The soft parts are gradually digested, the bristles and other indigestible portions being extruded by the anus. The larger spines often pass out by perforating passages through the wall of the intestine and through the body-wall. The aperture thus formed appears speedily to heal after the foreign body has been extruded.

The carnivorous habits of Nemertines even extend to cannibalism, and when kept in confinement they frequently devour one another. For this reason it is unsafe to keep large and small kinds together, as the small ones speedily disappear, being used as food material by the large. If one be divided into several pieces, the pieces are very rapidly demolished by other individuals.

REGENERATION.[144]—This power is, no doubt, of great service to these animals, since injury, or even violent local irritation, often causes complete rupture at the point affected. It seems that the {116}chief power of regeneration is situated in the head, as, if a very short piece be broken off the anterior end of the body, it very rapidly reproduces itself into a new individual. The hind end of the original body often lives for a considerable time, but it does not in most cases appear to possess the power of reproducing a head, and after existing for a time it dies. For a while, however, it so far retains its vital powers that the generative products continue to grow, and actually attain to perfection. Severe wounds also heal very quickly and completely, and all local injuries are speedily repaired.

Owing to the force with which it is shot out, the proboscis is often completely severed from the body, and in such a case the animal grows a new one in an extremely short space of time. The proboscis thus broken off retains its power of movement and contractility for a considerable time, and has been more than once mistaken for a worm. This great vital power is probably due to the great development of nervous tissue, the proboscis being usually richly supplied with nerve plexuses.

One large form, _Lineus sanguineus_, seems to possess great recuperative powers. It shows a marked tendency to break up into pieces, when not only the head end, but also the other portions develop into perfect animals, each one growing a head and all the organs belonging to it. Thus in this case an animal may multiply by a simple process of transverse fission, and form numerous complete individuals.

BREEDING.—The breeding season only appears to cease in the extreme of winter. Different genera and species seem to mature their generative products at different times.

In the armed Nemertines the eggs are deposited separately, and are not connected together except by such accidental mucus as the animal deposits normally; but in the unarmed a special mucous secretion forms a thick investment for the eggs.

M‘Intosh[145] has observed the process of the deposition of the male and female products in _Nemertes gracilis_. He put into a glass vessel a male and female of this species in which the products were apparently ripe. Soon spermatozoa began to issue in wreath-like jets from the body of the male, at first from the middle region of the body, and afterwards anteriorly and posteriorly, until the animal was enveloped in a dense cloud of {117}spermatozoa. The whole process only lasted a few minutes. When all the spermatozoa had apparently been given out, the female was seen to protrude her head from the sand; she then passed to the side of the vessel and deposited a group of eggs about three inches distant from the spermatozoa.

With only a few exceptions Nemertines are oviparous. _Prosorhochmus claparedii_, _Tetrastemma obscurum_, and _Monopora vivipara_ have been observed to contain embryos at certain times of the year. In other forms the eggs are laid when ripe, and development takes place subsequently to their deposition.

GEOGRAPHICAL DISTRIBUTION.—Nemertines have been found in all seas from the arctic to the equatorial regions. Many forms are found in the British Isles both between tide-marks and also at greater depths around our coasts. Some genera seem to be confined to warm climates and others to cold; while others appear to be indifferent to climate, and to subsist equally well under very various degrees of temperature. So far as is known, the land forms are all indigenous to warm countries.

LAND FORMS.—Land forms, which occur on or in moist earth under stones or decaying vegetable matter, have been discovered and described by Semper,[146] Willemoes-Suhm,[146] and von Graff.[146]

The species found by Semper, and called by him _Geonemertes palaensis_, lives under damp leaves and the roots of trees on Pelew Island in the North Pacific. It is about 2 inches long, of a reddish-white colour, with narrow, brownish-black, longitudinal stripes on its dorsal surface. It possesses six eyes and very small cephalic slits and cerebral organs. The proboscis is armed, and opens by the mouth instead of by a special pore.

The same peculiarity as to the opening of the proboscis is found in _Geonemertes chalicophora_, discovered by von Graff in pots of _Corypha australis_ in the palm-house at Frankfurt-on-Main. He found specimens on and beneath the surface of the earth. As it was only found in pots in which this Australian plant was growing, von Graff thought it almost certain that it was a native of Australia. Those found below the surface of the earth were surrounded by a transparent tube in which particles of earth were embedded. The animal is small, only about two-fifths of an inch in length. The colour is milk-white, with a small quantity of red pigment anteriorly: there are four eyes, and the cephalic slits are absent.

{118}The species which was discovered by Willemoes-Suhm, and named by him _Tetrastemma agricola_, lives under stones in damp earth in the Bermudas. It differs from the other two in that the proboscis opens by a special terminal aperture. It measures nearly an inch and a half in length, and, like _G. chalicophora_, is milk-white in colour. It resembles it also in possessing four eyes, and in the absence of cerebral organs and cephalic slits.

FRESH-WATER FORMS.—In most cases the descriptions of fresh-water forms are so vague and incomplete that it is difficult to determine whether or not they are different species.

They are probably more numerous than is at present known, and are certainly scattered widely over the face of the earth, since they have been found in Nicaragua, at Tashkend in Turkestan, and at Philadelphia and Monroe in the United States.

A form of which we have a full description is _Tetrastemma aquarum dulcium_, found by Silliman[147] at Monroe, under stones in brooks in company with Planarians. It is a small worm of a red or pink colour, about half an inch in length, and it possesses usually three pairs of eyes. The proboscis is armed, and opens by a separate aperture. The excretory system consists of a vessel on each side of the body, each opening externally by a pore, and internally dividing into numerous branches which end in ciliated expansions. An individual of the same species was found by Beddard in one of the tanks in the Botanical Gardens in Regent's Park, but as the tank is one in which tropical plants are grown, it had almost certainly been introduced among the roots of the plants, and cannot be considered as a British species.

A fresh-water Nemertine belonging to the genus _Tetrastemma_ was, however, found by Benham[148] on the roots of some water plants in the Cherwell at Oxford. The specimen was of a bright orange colour and measured half an inch in length.

Du Plessis[149] found another fresh-water form on the lower surface of stones in shallow pools on the shores of the Lake of Geneva, and named it _Tetrastemma lacustre_. It is a small animal, the largest specimens being rather over an inch in length.

Another European genus was found in 1893 by F. E. Schulze in Berlin. It has been fully described by T. H. Montgomery,[150] who has given it the name of _Stichostemma eilhardii_.

{119}PARASITIC FORMS.—The genus _Malacobdella_ was found by von Kennel[151] in large numbers living on _Cyprina islandica_, a Lamellibranch Mollusc, in the harbour at Kiel; and it has also been described by Riches[152] as a British form. It is attached to its host by means of a large round sucker situated at the posterior end of the ventral surface, while the rest of the body waves about freely in the mantle-cavity. It is perhaps hardly correct to describe this animal as parasitic, since it does not appear to obtain its nutriment at the expense of the host by preying on its juices. The advantage of its position is, however, obvious, since a perpetual current of water is kept up in the mantle-cavity of the Mollusc, and from the stream the Nemertine is able to pick out and take for itself any food material which it considers suitable. At the same time it is not subjected to the influence of the winds and waves, as the shell of the mollusc acts as a barrier to prevent the entrance of disturbing elements.

_Malacobdella_ is short and broad, somewhat flattened dorso-ventrally. The anterior end is bluntly rounded. The mouth opens into a wide pharynx, which is constricted behind and then passes into the intestine; this after a few coils opens by the anus situated dorsally immediately above the sucker. The proboscis opens into the pharynx.

PALAEONTOLOGY.—Nemertines are unknown in a fossil state; this is probably owing to the softness of their bodies, which would render their preservation extremely improbable.

AFFINITIES.—Until recently the Nemertines were regarded as a sub-order of the Turbellaria. They were afterwards separated from the Turbellaria and placed as a distinct class of the phylum Platyhelminthes.

Some zoologists have considered them to be so different in many respects from the other classes of the Platyhelminthes as to justify their being altogether separated from that phylum, and treated as a distinct group.

{120}If, however, the recent work of Bürger on the excretory system is to be relied upon, the existence of flame cells would be a strong reason for classing them among the Platyhelminthes.

Hubrecht[153] has instituted an interesting comparison between Nemertines and Vertebrates. He compares the median dorsal nerve of Nemertines to the spinal cord of Vertebrates; the lateral nerve-cords to the nerve of the Vertebrate lateral line; and the lateral swellings which constitute the brain in Nemertines to the lateral ganglia of the cephalic region in Vertebrates. This view is strengthened by the existence of transverse nerves connecting the lateral and dorsal nerves of Nemertines, since these may be compared with the spinal nerves of Vertebrates. He suggests that both Nemertines and Vertebrates may have arisen from a vermiform animal possessing a nervous layer in the form of a plexus of nerve-fibres, the nerve tissue having become concentrated along three lines to form a median dorsal and two lateral nerve trunks; the former being specially developed in the Vertebrata and the latter in the Nemertines. Hubrecht further suggests that the notochord of Vertebrates may be a survival of the proboscis sheath of Nemertines, while the proboscis of the latter may be represented by the invagination to form the pituitary body in Vertebrates.

Certain authors[154] have suggested that indications exist of a relationship between Nemertines and _Balanoglossus_.

The features which are supposed to indicate this are the elongated vermiform shape showing no external signs of segmentation; the ciliated smooth skin and the possession of unicellular mucous glands; and the protrusible proboscis, which may be comparable to the non-retractile proboscis of _Balanoglossus_, a comparison which is strengthened by the fact that in some Nemertines a sheath of nerve-fibres exists in the wall of the proboscis corresponding to the nerve plexus in the proboscis of _Balanoglossus_. In both cases an ectodermic nerve plexus exists with local thickenings along definite lines, although these lines are not the same in the two cases. Both possess a straight alimentary canal, ending in a terminal anus and thrown out into paired lateral caeca, between which are the paired metamerically-arranged generative sacs.

NEMATHELMINTHES & CHAETOGNATHA

BY

ARTHUR E. SHIPLEY, M.A.

Fellow and Tutor of Christ's College, Cambridge.

{123}CHAPTER VI

NEMATHELMINTHES