Chapter IX: Introduction: Nature of Cestodes—occurrence of Cestodes—the Tape-Worms of (1)

MAN AND DOMESTIC ANIMALS—TABLE OF THE LIFE-HISTORIES OF THE PRINCIPAL CESTODES OF MAN AND DOMESTIC ANIMALS—STRUCTURE AND DEVELOPMENT OF CESTODES—TABLE FOR THE DISCRIMINATION OF THE MORE USUAL CESTODES OF MAN AND DOMESTIC ANIMALS—CLASSIFICATION.

The Cestodes or Tape-worms are exclusively endoparasitic Platyhelminthes living, in the adult condition, in the alimentary canal of Vertebrates, with the exception of _Archigetes_ (Fig. 37), which may become mature in the body-cavity of _Tubifex_. In relation with this wholly parasitic existence, the Cestodes exhibit certain characteristic modifications in structure and mode of development, such as the formation, by the segmentation of the "neck," of a (usually) long chain of "proglottides" or joints, which form the "body" of the Cestode; and the entire absence of an alimentary tract, both in the larva and adult. As an adaptation to the fixed mode of life, the anterior end (head, scolex) is modified to form an adhering organ. Various adaptive forms of larvae are known. These live in the internal organs of one or more intermediate hosts, and are transferred to the final host passively during a meal. Lastly, there is the curious metamorphosis by which the adult is formed from a portion (scolex) of the larva.[94]

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_Taenia solium_, from man (Fig. 39, B), or _Echinobothrium_ (Fig. 36), from an Elasmobranch fish, is fixed to the mucous lining of the intestine of its host by means of a radially-constructed apparatus of four suckers and a circlet of hooks (Fig. 39), which are borne by the "head" or "scolex," being that part of the worm which is directly derived from part of the larva, and which contains the central, commissural portion of the nervous system. Firm adhesion to the host's intestine is necessary, in order to avoid the loosening action of the peristaltic movements of the intestine as the food passes along. The heads of different Cestodes exhibit a marvellous variety of suckers and hooks, from a mere muscular depression in _Schistocephalus_, to the compound proboscides of _Tetrarhynchus_[96] which is found in Elasmobranchs. The jointed body, often of enormous length (up to 20 yards in _Bothriocephalus latus_), is usually separated from the head by a slender neck, from which the proglottides are segmented off from behind forwards, and become more and more individualised as they recede farther away from the neck by the intercalation of younger joints. Thus in Fig. 36 the mature, distal proglottis has passed through all the stages represented by the other segments.

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The longitudinal muscles, the nerves, and excretory vessels which supply the proglottides are continuous throughout and with those of the head. Each joint contains at first male genitalia comparable with those of a Trematode; then the female organs develop, and finally self-fertilisation follows. The Cestodes feed through their skin, probably by the aid of fine protoplasmic processes, which penetrate the tough investing membrane and absorb the already digested food which bathes them. When a proglottis of _Calliobothrium_ is approaching maturity it separates from the parent, the broken ends of muscles, nerves, and excretory vessels speedily heal, and it is now capable of continued growth and of fairly active movement if it remains in the intestine of the host. According to van Beneden, it may even attain a size equal to, or exceeding, that of the whole parent or "strobila."[97] These considerations led Leuckart, von Siebold, P. J. van Beneden, and others, to Steenstrup's conclusion that a jointed tape-worm is really a colony composed of two generations—the head and neck derived from the larva, and the proglottides produced by the segmentation of the neck.[98] This view of the colonial nature of jointed Cestodes was generally adopted from 1851 to 1880. During the last fifteen years, however, the varied interpretations of the facts of the ontogeny of this group have led some authors to adopt the monozootic view (that a Cestode is one individual), others are still of the older opinion, and Hatschek (_Lehrbuch_, p. 349) and Lang take up intermediate positions. Lang considers that the formation of the joints of a tape-worm from a small fixed "scolex," is not only largely comparable with the strobilation of a _scyphistoma_ and the consequent formation of a pile of medusae, as in the life-history of _Aurelia_, but {77}that both processes have arisen from the power of regenerating the necessary organs in each of the new segments. The result in both cases is the rapid formation of a number of joints, which gradually separate from the parent, to carry the eggs and young to new stations. Just as some Coelenterata (_Lucernaria_) may be regarded as not having advanced much beyond a scyphistoma stage, so there are unisegmental Cestodes (e.g. _Archigetes_, Fig. 37) which have remained as a slightly altered but sexual scolex, directly comparable with a Trematode, and, as all authors are agreed, representing one generation only. Such monozootic forms are now classed as a special family, the Cestodaria or Monozoa, of which _Caryophylleus mutabilis_, from the intestine of various Cyprinoid fish, is the most abundant representative, while _Amphiptyches_ (_Gyrocotyle_) _urna_, from _Chimaera monstrosa_ of the northern hemisphere, is paralleled by _A. rugosa_, found in _Callorhynchus antarcticus_ of the southern seas.

OCCURRENCE OF CESTODES.—The distribution of Cestodes and their larvae is analogous to that of the digenetic Trematodes, although the absence of an alimentary canal limits the habitat of the mature worms to certain sites, such as the blood-vessels, the lymphatic and coelomic spaces, and the digestive system, where their body may be bathed by a nutritive fluid. Almost all groups of Vertebrates are attacked by Cestodes. Those of fishes, and particularly of Elasmobranchs, are distinguished by certain structural and developmental features; those of birds by {78}others; those of mammals, by a third set of characters. The young stages of the Cestodes of Sharks and Rays occur encysted in the body-cavity, or in the pyloric appendages, of Teleosteans, which probably swallow them along with those invertebrate animals upon which they prey. The larvae of the Cestodes of carnivorous mammals or piscivorous birds, live respectively in herbivores and fishes, but how the latter are infected we know in very few instances. Cestode larvae are known to occur in many Invertebrates, and occasionally are taken free swimming in the sea, presumably crossing from one host to the next. Ctenophores, Siphonophores, Copepods, Ostracods, Decapods, various Molluscs especially Cephalopods, Earthworms, and other Annelids, are the intermediate hosts of these larvae (see Fig. 38), the fate of which, however, has been determined in but few cases.

OCCURRENCE OF CESTODES IN MAN.[99]—Tape-worms, either in the adult or larval stages (bladder-worms), have, from ancient times, been known to occur in man, and in the animals that serve him as food. Until comparatively recent times, however, the true nature of these parasites, and particularly of "hydatids" (cystic larvae), was unrecognised. Up to the seventeenth century the larvae were regarded as abscesses or diseased growths of the affected organs, and it was only at the close of that century that their animal nature was even suggested. Even at the beginning of the nineteenth century, three modes of origin of Cestodes—by "generatio aequivoca" from the tissues of the body, or by the union of previously distinct proglottides, or again by metamorphosis of free-living worms drunk with water by cattle or birds (as Linnaeus suggested)—were still variously held, at a time when Malpighi, Pallas, and Goeze had recognised the true connexion between the cystic and segmented states of _Taenia crassicollis_ (the cat tape-worm), and when Goeze had seen the eggs of _Taeniae_, and Abildgaard[100] had even conducted the first helminthological experiments (conversion of the larval _Schistocephalus_, Fig. 40, into the adult form).

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Generally speaking, "a tape-worm" in Western Europe will prove to be _Taenia saginata_ Goeze (the beef tape-worm, Fig. 39, A), exceedingly prevalent also in the East, and indeed cosmopolitan, occurring wherever the infected flesh of the ox is eaten in a raw or half-cooked state. Its attacks are fortunately not usually severe. _Taenia solium_ Rud. (the pork tape-worm) is found wherever the pig is kept as a domestic animal, and has consequently a world-wide distribution. Its size (6-9 feet long) and powers of adhesion would alone render _T. solium_ a formidable parasite. But the danger of its presence in the body of man, or in the flesh of pigs, lies in the fact that the larva or bladder-worm (known as _Cysticercus cellulosae_) can live in the most varied organs. Thus if by accident a mature proglottis be eaten, the embryos escape, bore their way into the wall of the stomach, and entering the portal vein, may reach in time the muscles, the brain, the eye, or even the heart itself, and attain the cystic condition. Even more disastrous may be the result, should some ripe joints of a mature worm work their way from the intestine back towards the stomach. Should this happen (and though it has not been directly proved, the possibility is to be reckoned with), the result would be the release of vast numbers of embryos capable of inflicting fatal injury on the host. An abnormal _Cysticercus_ of this species is probably the _Taenia_ {80}(_Cysticercus_) _acanthotrias_ Weinl. (see, however, Leuckart, _loc. cit._ p. 711).

_Taenia_ (_Hymenolepis_) _nana_ v. Sieb.[101] is found in man in Egypt, Italy, England, Servia, Argentine Republic, and the United States. Though small (¾-1 inch long), its numbers usually excite digestive and nervous disorders of considerable severity, more serious, indeed, than those caused by the commoner tape-worms. _H. diminuta_ Rud. (_flavopunctata_ Weinl.), normally found in Rodents, has been rarely recorded in man. _Taenia_ (_Dipylidium_) _caninum_ L. (= _T. cucumerina_ Bloch = _T. elliptica_ Batsch), the commonest parasite of pet cats and dogs, and _T._ (_Davainea_) _madagascariensis_ Davaine, have occasionally been recorded from infants and young children. But the attacks of these species are insignificant in comparison with those of the cystic stage (_Echinococcus polymorphus_) of a tape-worm (_T. echinococcus_ v. Sieb.) which lives when mature in the dog.

_Echinococcus_ is most frequent in Iceland, where it affects 2 to 3 per cent of the population, and a still larger proportion of sheep; while in Copenhagen, Northern Germany, some districts of Switzerland, and Victoria it is not uncommon, but is frequently found during _post-mortem_ examinations when no definite symptoms of its presence had been previously noticed. _Echinococcus_[102] varies greatly in size, form, and mode of growth, but is distinguished in the formation not of one scolex only, as in the _Cysticercus_, but in the production of a number of vesicles, usually from the inner wall. Within these, large numbers of scolices may be developed. The whole organism continues to swell by the formation of a watery liquid within it, and if its growth be rapid the fluid tension may cause the rupture of the enclosing connective-tissue capsule formed around the parasite, at the expense of the host, and the protrusion of the daughter vesicles. It is the consequent injury to the surrounding organs of the host, at this critical stage, often only reached after the lapse of several years, that occasions serious or even fatal results. Zoologically, _Taenia echinococcus_ and _T. coenurus_ are interesting, since they exhibit an {81}indubitable alternation of asexual generations in the larval state, with a sexual adult stage.

_Bothriocephalus latus_ Brems., the broad tape-worm, which attains a length of 20-30 feet, or even more, occurs in man endemically in the eastern Baltic provinces, certain parts of Switzerland, generally throughout Russia (especially near Kasan), in North America, and commonly in Japan,—that is, in districts where the population partake largely of pike or other fish in a raw or partially-cooked state. Elsewhere it occurs sporadically, and in Munich, where it was unknown before 1880, its presence has been traced to emigrants from infected districts, who settled on the shores of the Starenberger Lake, from which Munich was supplied with fish. How the pike, the usual but not invariable intermediate host, becomes infested (and its musculature is frequently riddled with the larvae) we do not accurately know, but some Invertebrate, the prey of the pike, is probably the first host into which the free-swimming ciliated larva (Fig. 42) finds its way. In Greenland, _B. cordatus_ is very common in the dog, and probably also in man, though few cases have been recorded. _B. mansoni_ Cobb. (= _B. liguloides_ Leuck.) was, till recently, known only in the larval state from China and Japan. Iijima, however, has found older specimens in the latter country. _B. cristatus_ Dav. is a species founded somewhat doubtfully on two fragments found, one in a child, the other in a man, in France.

OCCURRENCE OF CESTODES IN DOMESTIC ANIMALS.[103]—Among domestic animals, the dog is, undoubtedly, the most frequently attacked by Taeniae. Six species of _Taenia_ (_T. serrata_, _marginata_, _coenurus_, _echinococcus_, _krabbei_, and possibly _T. serialis_), _Dipylidium caninum_ (the commonest form), _Mesocestoides lineatus_, and three or four species of _Bothriocephalus_ have been found in the dog. The table of life-histories (p. 83) shows that sheep, rabbits and other Rodents serve as the intermediate hosts, in which the cystic stages of the species of _Taenia_ are found. Hence the prevalence of _T. serrata_ in a given locality is connected with the abundance there of the rabbit and hare, in which the larva (_Cysticercus pisiformis_) occurs. _Bothriocephalus cordatus_ develops from the young stage present in the fish which the Icelanders give to their dogs. In Iceland and certain parts of {82}Australia _T. echinococcus_ infests one-third to one-half the number of dogs examined; a fact connected with the frequency of _Echinococcus_ in man in these countries.

In sheep the most noteworthy and dangerous parasite is _Coenurus cerebralis_ (or the cystic stage of the dog-taenia, _T. coenurus_), which gives rise to the disease known as "gid" or "staggers." It is found in various parts of the brain or spinal cord, and the symptoms differ according to the position of the parasite. If this presses upon one hemisphere the sheep describes circles and finally falls: if on the optic lobes, the eyes are affected: if the pressure affects the cerebellum the movements of the sheep are uncertain and incoordinated. Four or six weeks after the appearance of the symptoms, death results from cerebral paralysis, or from general debility, and the loss of sheep incurred by this disease (happily less frequent in England than formerly) has been calculated by Youatt at a million for France annually; at 35 per cent of the flocks for England in bad seasons; and about 2 per cent for Germany. Besides sheep, which are most subject to "gid" during their first year, various ruminants—Goat, Ox, Moufflon, Chamois, Roe, Antelope, Reindeer, Dromedary—are attacked in the same way. A similar form, _Coenurus serialis_ Baill., is common in the wild rabbit in this country, and in Australia in the hare and squirrel. It forms large swellings in the connective tissue of various parts of the body, but usually does not affect the health of the host. It is not known in what carnivore _Taenia serialis_ Baill. normally occurs. Experiments have, however, shown that it develops rapidly in dogs.

The preventive measures which are steadily diminishing the prevalence of the Cestode parasites in man in some parts of Western Europe cannot be dealt with here, but it may be noticed that the Jewish observance with regard to swine is the surest preventive measure against taeniasis and trichinosis. Careful inspection of meat and general cleanliness, are the leading measures that in these hygienic matters secure the greatest immunity from disease.

{83}TABLE OF THE LIFE-HISTORIES OF THE PRINCIPAL CESTODES OF MAN AND THE DOMESTIC ANIMALS.

Cestode. Final host. Larva. Intermediate host.

_Taenia serrata_ _Cysticercus Rabbit, Hare, Mice
Goeze Dog pisiformis_ (liver and peritoneum)
Zed.

_T. marginata_ _Cyst. Monkeys, Ruminants,
Batsch Dog, Wolf tenuicollis_ ungulates (in
Rud. peritoneum)

_T. saginata_
Goeze (= _T. Man _Cyst. bovis_ Ox, Giraffe
mediocanellata Cobb. (in muscles)
_Küch.)

_Cyst.
_T. solium_ Man cellulosae_ Pig, Man, Monkeys,
Rud. Rud. (? _Cyst. Bear, Dog, Cat, Black
acanthotrias_ Rat (in various
Weinl.) organs)

_T. crassicollis_ Cat and other _Cyst. fasciolaris_
Rud. Felidae, Rud. Rat, Mouse, Bat (liver)
Stoat

_T. coenurus_. Dog, Arctic _Coenurus Brain of Sheep, Ox,
Küch Fox cerebralis_ Goat, Dromedary, Camel,
Rud. Antelope, Horse

_T. serialis_ ? Dog _Coenurus Rabbit (connective
Baill. serialis_ Baill. tissue)

_Echinococcus Man, Monkeys, many
_T. echinococcus_ Dog, Dingo, polymorphus_ Carnivores, Rodents,
v. Sieb. Jackal, Wolf Dies.(incl. Ungulates, Ruminants,
_E. multilocularis_ and Marsupials; also
found in Man) in Turkey and other
birds
_Moniezia expansa_ Sheep, Ox, Unknown
Rud. Goat, etc.

_Thysanosoma Sheep, Unknown
fimbriata_ Dies. Cervidae

_Stilesia
globipunctata_ Sheep Unknown
Riv.

_Anoplocephala
perfoliata_ Horse Unknown
Goeze

_Dipylidium Cysticercoid larva
caninum_ L. Man, Dog, (Fig. 43), Body-cavity of
(= _Taenia Cat _Cryptocystis_ _Trichodectes_ and
cucumerina_ trichodectis_ _Pulex_ of Dog
Bloch = _T. Vill.
elliptica_ Batsch)

_Hymenolepis_ _Cercocystis_ Usually absent
_murina_ Duj. Mouse, Rat Vill.[104] (develops
in parental host)

_H. nana_ Man Unknown
v. Sieb.

_H. diminuta_ Meal-moth, _Asopia_
Rud.(= _Taenia Man, Mouse, _Cercocystis_ (_Pyralis_)
flavopunctata_ Rat Vill. _farinalis_; also
Weinl.) certain Orthoptera
and Coleoptera

_Drepanidotaenia_ Duck, Goose, _Cercocystis_ The Ostracods {84}
_gracilis_ Zed. Wild Duck Vill. _Candona rostrata_ and
_Cypris compressa_,
and also
_Cyclops viridis_

_D. anatina_ Duck " " _Cypris incongruens_,
Krabbe and also Perch

_D. setigera_ Goose " " _Cyclops brevicaudatus_
Fröh.

_D. infundibuliformis_
Goeze Common Fowl " " House-fly

_Dicranotaenia Duck " " _Cypris ovum_
coronula_ Duj.

_Davainea proglottina_ " " ? _Limax cinereus_,
Dav. Fowl _L. agrestis_

_D. madagascariensis_ Unknown
Dav. Children

_D. friedbergeri_ Unknown ? Ants
v. Linst. Pheasant

_Mesocestoides
lineatus_ Dog Unknown
Goeze

Plerocercoid, Probably first enters
_Bothriocephalus Man, Dog, i.e. solid, an Invertebrate host,
latus_ Brems. ? Cat elongate larva, which is eaten by
with no bladder Pike, Perch, Trout,
etc.

STRUCTURE AND DEVELOPMENT OF CESTODA.[105]—Of the unsegmented Cestodes, _Caryophyllaeus mutabilis_, from the intestine of carp and other Cyprinoid fishes, is the most easily accessible form. _Triaenophorus nodulosus_, which is very useful for the study of the excretory system, occurs mature in the pike. In the body-cavity of the Stickleback (Fig. 40) a large, broad, yellow worm may sometimes {85}be found, the larva of _Schistocephalus solidus_ Crepl., which occurs in the intestine of Terns, Storks, Mergansers, and other birds. Species of _Ligula_ are found in the same birds. The intestine of a _Lophius_ or _Cyclopterus_ ("lump-fish") contains, usually, the early and intermediate stages of various Cestodes, while the alimentary canal of Elasmobranchs often contain many peculiar Tetrarhynchidae and other forms. For the study of development, the _Taenia anatina_ from the duck may be used. The ripe proglottides are collected, and the eggs placed with _Cypris ovum_ in an aquarium, with the probability that some of the embryos will enter the Ostracod, and the peculiar Cysticercoid may be bred.[106] _Cysticercus pisiformis_ and _Coenurus serialis_, which occur commonly in rabbits, are also suitable objects for examination.

A Cestode such as _Echinobothrium_ (Fig. 36) is divisible into head and proglottides. Moniez has suggested that the head is really the morphologically hinder end of the body, in which case the formation of proglottides would closely resemble the mode of segmentation of an Annelid larva. The close similarity, however, between the Cysticercoid larva (Fig. 43, F) and the Cercaria of a liver-fluke, seems to show that the anterior end is the same in both cases, and since it bears the central part of the nervous system, we may reasonably call it the "head." Moreover the hinder end of a Platyhelminth usually possesses the chief excretory pore. Another difficulty is the determination of dorsal and ventral surfaces. Authors are agreed,—on the analogy of Trematodes, in which the testes are usually dorsal and the ovaries ventral,—that the dorsal and ventral aspects of a Cestode are determined by the position of these organs, although the often radially formed "head," the lateral or superficial position of the genital apertures, and the variability of these features, render it a matter of considerable doubt whether "dorsal" and "ventral" are more than useful conventional terms. The suckers and hooks are borne on a muscular cap, the "rostellum," which is only slightly developed in the _Ichthyotaeniae_. The body is solid, and is divisible into an outer muscular coat—enveloped in a (possibly epidermal) investing membrane—and an inner parenchymatous tissue containing the chief part of the excretory, nervous, and reproductive systems. One or two pairs {86}of longitudinal excretory vessels are present, usually connected by transverse ducts and opening by a single terminal pore. Occasionally a regularly paired arrangement of lateral or secondary pores is present (Figs. 38 and 41, _for.sec_). Flame-cells occur at the end of the fine tubules (Fig. 38), and the whole system is well developed, but may undergo degenerative changes in the older proglottides. The central nervous system varies according to the degree of differentiation of the rostellum; and, owing to the difficulty of staining the nerves and the contradictory statements of authors, we do not yet possess a fully reliable account of the nervous system even of the commoner Taeniae. Free nerve endings and other sensory terminations have been recently stated to exist in the cuticle of Cestodes and Trematodes. If true, this would tend to show that the parasitic mode of life of these animals demands a complex nervous system comparable with that of the Turbellaria.

The reproductive organs, unlike the preceding systems, are discontinuous from one proglottis to the next. The male and female organs and their mutual connexions, especially in the unsegmented Cestodes, may be compared in detail with those of Trematodes, but the difference between the arrangement of the generative organs of various Cestodes is very great.[107] The penis (Fig. 41, _cs_) is evaginated through the male pore (Fig. 41, ♂), and inserted far into the vagina (♀, _vag_) of the same or another segment of the tape-worm.

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From this fact and the anatomical relations of the vagina, it is becoming increasingly probable that the so-called uterus of Trematodes is an organ corresponding to the vagina of Cestodes, and not to the uterus of Cestodes. The latter opens to the exterior in _Schistocephalus_, _Bothriocephalus_, and some other Cestodes of fishes by a special pore (Fig. 41, _uto_). Through this, some of the eggs (which in these genera give rise to ciliated larvae) are enabled to escape, and need not wait for the detachment of the proglottis, as must happen in the Taeniidae, where the uterus is closed. This uterus, a true physiological one, is probably the homologue of the "canal of Laurer" ("Laurer-Stieda canal," or "vagina") of Trematoda. The fertilised ovum and yolk are brought together into the "ootype," where the shell-gland forms the egg-shell around them (Fig. 41, _sh.gl_) and the egg is then passed into the uterus. The ovum segments to form a minute six-hooked larva, which may (Bothriidae, Fig. 42) or may not (Taeniidae) be ciliated. Thus in _Taenia serrata_ the proglottides are shed with the faeces of the host (dog), and they protect the young from the desiccating influence of the surroundings. If inadvertently eaten by a rabbit along with herbs, the proglottis and larval envelope are digested, and by its six hooks the tiny larva bores through the gastric wall into the portal vein, and so into the liver. Here the hooks are thrown off, and the solid mass of cells becomes vacuolated.

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{89}At one pole an invagination occurs, at the bottom of which the rostellum, suckers, and hooks are gradually formed, but inside out as compared with the head of the _Taenia serrata_. At this stage the larva (_Cysticercus pisiformis_) has usually issued from the liver and attached itself to the omentum. The invagination projects into the cavity of the bladder, within which a watery fluid accumulates. Thus the "bladder worm" is formed, the head of which is evaginated if the larva be introduced into the digestive system of a dog. The bladder and neck of invagination are digested, while the head, protected by these, remains, and forms the neck, from which the proglottides are afterwards segmented off. In _Taenia_ (_Hymenolepis_) _murina_ the whole development may take place in the parental host, the larva living in the villi, the adults in the cavity of the same rat's intestine (Grassi). The different forms of Cestode larvae depend largely upon the presence and degree of development of the caudal vesicle or bladder, which in _Scolex polymorphus_ (Fig. 38) (the young stage of _Calliobothrium filicolle_ Zsch.) is practically absent. If the bladder be small, the larva is known as a Cysticercoid. For example, the common _Dipylidium caninum_, which lives in the dog, has such a larva, the development of which is explained and illustrated by Figs. 43 and 44. The bladder becomes exceeding capacious in _Coenurus_ and _Echinococcus_.

TABLE FOR THE DISCRIMINATION OF THE MORE USUAL CESTODES OF MAN AND DOMESTIC ANIMALS.[108]

I. Scolex in most cases with hooks; uterus with a median and lateral
branches; yolk-glands simple, median; genital pore single; dorsal
excretory vessel narrower than the ventral, without a circular
commissural trunk; eggs without pyriform apparatus (processes of the
ovarian membrane) Gen. TAENIA L. (s. str.)

A. Genital ducts pass on the ventral side of the nerve and of the two
longitudinal excretory vessels _T. crassicollis_ Rud.

B. Genital ducts pass between the dorsal and ventral longitudinal
vessels.

_a._ Nerve present on dorsal side of genital ducts.

α. Head armed _T. solium_ Rud.

β. Head unarmed _T. saginata_ Goeze.

_b._ Nerve on ventral side of genital ducts.

DOG-TAENIAE[109] {90}

Head armed; genital pore marginal and
— Single

Many proglottides; strobila several centimetres long; small
hooks with guard.

Bifid hooks, which are
— 230µ-260µ long[110]; genital pore very distinct
_T. serrata_ Goeze.
— 136µ-157µ long; genital pore not very salient
_T. serialis_ Ball.

Entire large hooks, which are
— 180µ-220µ long; length of mature segments double their width
_T. marginata_ Batsch.
— 150µ-170µ long; length of mature segments treble their width
_T. coenurus_ Küch.

3-4 segments; a few mm. long _T. echinococcus_ v. Sieb.

— Double and bilateral _Dipylidium_ caninum L.

Head unarmed; two genital pores on ventral surface
_Mesocestoides lineatus_ Goeze.

II. Scolex without hooks; one or two transverse uteri present; one or two
genital pores and yolk-glands, the latter never median; genital ducts
pass on the dorsal side of the nerve; eggs with pyriform apparatus.

A. One transverse uterus present.

_a._ Uterus with bullate egg-sacs; pyriform apparatus without horns;
genital ducts between dorsal and ventral vessels
THYSANOSOMA Dies.

α. Head large (1.5 mm.); square lobed testes in median field;
posterior margin of segments fimbriated; genital pore double
_T. fimbriata_ Dies.

β. Head small; no fimbriae; pore rarely double
_T. giardii_ Riv.

_b._ Uterus without saccular dilatations; segments short, thick, and
slightly imbricate ANOPLOCEPHALA E. Blanch.

HORSE-TAENIAE.

α. Head very large

— No posterior lobes _A. plicata_ Zed.

— Four posterior lobes _A. perfoliata_ Goeze.

β. Head small, without posterior lobes
_A. mamillana_ Mehl.

B. Two uteri and two genital pores present; horns of pyriform apparatus
well developed; genital ducts pass on the dorsal side of the
longitudinal vessels MONIEZIA R. Bl.

_a._ Interproglottidal glands[111] arranged in linear series
(_planissima_ group)
_M. planissima_ S. and H. _M. benedeni_ Mz. _M. neumani_ Mz.

_b._ Interproglottidal glands saccular (expansa group)
_M. expansa_ Rud. _M. oblongiceps_ S. and H.
_M. trigonophora_ S. and H.

_c._ Interproglottidal glands absent (_denticulata_ group)
_M. denticulata_ Rud. _M. alba_ Perr.

C. Uterus single or double, without spore-like egg-sacs; eggs with a
single shell; genital pores irregularly alternate; strobila narrow;
testes absent from median part of the field STILESIA Raill.

_a._ A transverse uterus in middle part of median field; {91}
head 2 mm. diameter _S. centripunctata_ Riv.

_b._ Two lateral uteri in each segment; head less than 1 mm. in
diameter _S. globipunctata_ Riv.

III. Scolex almost invariably provided with hooks; genital pores on left
border of segment; eggs with three shells but no cornua. Segments
broader than long; posterior angles salient. HYMENOLEPIS Weinl.

_a._ Scolex with a single series of 24-30 hooks, each 14-18µ long
_H. nana_ v. Sieb. _H. murina_ Duj.

_b._ Scolex very small, unarmed _H. diminuta_ Rud.

IV. Scolex provided with two elongated muscular pits. Body segmented;
three genital apertures in middle of ventral surface
BOTHRIOCEPHALUS Rud.

Body 2-20 metres in length _B. latus_ Brems. _B. cristatus_ Dav.
(doubtful species). _B. cordatus_ Leuck. _B. mansoni_ Cobb.
(= _B. liguloides_ Leuck.)

CLASSIFICATION OF CESTODES.—The following classification, which, so far as the Taeniidae are concerned, follows that employed by Railliet, Blanchard, and most recent writers, includes only a few representative genera:—

1. Fam. CESTODARIIDAE Mont. (MONOZOA Lang).

Gen. _Caryophyllaeus_, _Archigetes_, _Gyrocotyle_, _Amphilina_.

2. Fam. BOTHRIOCEPHALIDAE.

Sub-Fam. 1. Bothriocephalinae. Gen. _Bothriocephalus_, _Schistocephalus_,
_Triaenophorus_ (= _Tricuspidaria_).

Sub-Fam. 2. Ligulinae. Gen. _Ligula_.

Sub-Fam. 3. Solenophorinae. Gen. _Solenophorus_, _Duthiersia_.

Sub-Fam. 4. Diphyllinae. Gen. _Echinobothrium_.

3. Fam. TETRARHYNCHIDAE.

Gen. _Tetrarhynchus_.

4. Fam. TETRAPHYLLIDAE.

Sub-Fam. 1. Phyllobothrinae. Gen. _Phyllobothrium_, _Echeneibothrium_,
etc.

Sub-Fam. 2. Phyllacanthinae. Gen. _Calliobothrium_, _Anthobothrium_,
etc.

5. Fam. TAENIIDAE.

Sub-Fam. 1. Cystotaeninae. Gen. _Taenia_ s. str.

Sub-Fam. 2. Anoplocephalinae. Gen. _Moniezia_, _Thysanosoma_, _Stilesia_,
_Anoplocephala_.

Sub-Fam. 3. Cystoidotaeninae. Gen. _Dipylidium_, _Hymenolepis_,
_Drepanidotaenia_, _Dicranotaenia_, _Echinocotyle_, _Davainea_.

Sub-Fam. 4. Mesocestoidinae. Gen. _Mesocestoides_, _Dithyridium_.

Sub-Fam. 5. Ichthyotaeninae. Gen. _Ichthyotaenia_, _Corallobothrium_.

{92}CHAPTER IV

MESOZOA

DICYEMIDAE—STRUCTURE—REPRODUCTION—OCCURRENCE: ORTHONECTIDAE— OCCURRENCE—STRUCTURE: TRICHOPLAX: SALINELLA.

The Mesozoa are an obscure group, the position of which in the animal kingdom is still doubtful. The name Mesozoa was given to the group by its discoverer, E. van Beneden,[112] as he concluded that they were intermediate between the Protozoa and the higher Invertebrates. Recent authors, however, have called attention to the resemblance existing between them and the "sporocysts" of Trematodes, and though we still are ignorant of certain important points in their life-histories, the Mesozoa are most conveniently (and probably rightly) considered as an appendix to the Platyhelminthes.

The animals composing this group are minute and parasitic, and are composed of a small number of cells. They may be divided into two families: the _Dicyemidae_, which occur exclusively in the kidneys of certain Cephalopods (cuttle-fish); and the _Orthonectidae_, which live in the brittle-star _Amphiura squamata_, the Nemertine _Nemertes lacteus_, or the {93}Polyclad _Leptoplana tremellaris_. In addition to the undoubted Mesozoa, certain anomalous forms—_Trichoplax adhaerens_ and _Salinella salve_—may be referred to this group.

DICYEMIDAE.—If the kidney of _Eledone moschata_, a Cephalopod common on our south-western shores, be opened, a number of fine, yellowish, hair-like filaments may be seen attached at one end to its inner surface, floating in the fluid contained in the renal cavity. These may be _Dicyemennea eledones_ Wag., although another form, _Dicyema moschatum_ Whit., also occurs in the same host. _D. eledones_ (Fig. 46) is 7 to 9 mm. long, transparent, and is composed of one large inner cell with a simple nucleus (Fig. 46, _n.end_), and of an outer layer of ciliated cells, nine of which form the "calotte" or pole by which the animal is attached. Within the former (endodermal) cell the formation of urn-shaped "infusoriform embryos" takes place (B and C), the fate of which is not known, but they are possibly the males. The individual which produces these larvae is called a {94}"Rhombogen." Other individuals which produce a more elongated larva ("vermiform larva," Fig. 45) are called "Nematogens," and Whitman has described a third kind, which produce first infusoriform, and then vermiform, larvae (Secondary Nematogens).[113]

The occurrence of the known species of Dicyemids (a group which has not been investigated on our coasts) is as follows:—

Species. Host.

_Dicyema typus_ van Ben. _Octopus vulgaris._
_D. clausianum_ van Ben. _O. macropus._
_D. microcephalum_ Whit. _O. de Filippi._
_D. moschatum_ Whit. _Eledone moschata._
_D. macrocephalum_ van Ben. _Sepiola rondeletii._
_D. truncatum_ Whit. _Rossia macrosoma_, _Sepia elegans_,
_S. officinalis._
_D. schultzianum_ van Ben. _S. biseralis, Octopus vulgaris._
_Dicyemennea eledones_ Wag. _Eledone moschata, E. aldrovandi._
_D. mülleri_ Clap. _E. cirrosa._
_D. gracile_ Wag. _Sepia officinalis._
_Conocyema polymorphum_ van Ben. _S. officinalis, Octopus vulgaris._

ORTHONECTIDA.[114]—Two species of Orthonectids are fairly well known, _Rhopalura giardii_ Metschn. from _Amphiura squamata_, and _R. intoshii_ Metschn. from _Nemertes lacteus_. The latter appears to be very rare, the former occurring in 2 to 5 per cent of the number of hosts examined. The parasites occur in a granular "plasmodium," the nature of which is uncertain. Metschnikoff regards it as formed by the Orthonectids, and he considers that the cellular envelope, by which it is sometimes enclosed, is developed from the neighbouring tissue of the host. These granular, sometimes nucleated, plasmodial masses, which can perform active amoeboid movements in sea-water, occur attached to the ventral part of the body-cavity of _Amphiura_, and between the gut-branches and body-wall in _Nemertes_. Should these hosts be infected by great numbers of the Orthonectids, their sexual organs degenerate (as is the case with pond-snails attacked by sporocysts[115]), and it is possible that the remains of these organs may constitute the "plasmodia" (Braun).

{95}_Rhopalura giardii_ is of distinct sexes. Either males or females are found in one _Amphiura_. Two kinds of females, flattened unsegmented, and cylindrical segmented forms, are known. They consist of a ciliated ectodermal layer enclosing an endodermal mass of eggs, between which is a fibrillar layer usually considered to be of a muscular nature. The cylindrical female gives rise to eggs which develop, probably exclusively, into males. The flattened female produces eggs from which females alone arise, though the origin of the two forms of this sex is not well ascertained. The males contain spermatozoa which fertilise the eggs of the cylindrical female, whereas the ova of the flat form probably develop parthenogenetically.

TRICHOPLAX.[116]—This anomalous animal has only been found in aquaria, originally in the marine aquarium at Graz by {96}Schulze. It has the appearance of a large, flattened, ciliated _Amoeba_ (1.5-3 mm. in diameter), but is distinguished by its structure. The upper surface is composed of a flattened epithelium. The lower surface is made up of cylindrical ciliated cells, which pass imperceptibly into the branched cells, embedded in a hyaline matrix, which compose the middle layer of the body. No distinct organs, and beyond simple fission, no mode of reproduction, have been observed. One species, _T. adhaerens_, is known, but has never been met with in a free state.

SALINELLA.[117]—This is another aquarium-animal, found by Frenzel in the Argentine, in an artificial saline solution with which he filled some aquaria. It measures .2 mm. in length, and has a somewhat flattened, barrel-shaped appearance. A single layer of ciliated cells bounds a central cavity opening at each end. Fission, and conjugation followed by encystment, have been observed. One form, _S. salve_, is known from salines taken from Cordova.

NEMERTINEA

BY

LILIAN SHELDON

Staff Lecturer in Natural Science, Newnham College, Cambridge.

{99}CHAPTER V

NEMERTINEA

INTRODUCTORY—EXTERNAL CHARACTERS—ANATOMY—CLASSIFICATION—DEVELOPMENT—HABITS— REGENERATION—BREEDING—GEOGRAPHICAL DISTRIBUTION—LAND, FRESH-WATER, AND PARASITIC FORMS—AFFINITIES

The Nemertinea form a compact group, the affinities of which have not been at present clearly determined. Several species were mentioned and described in the works of various naturalists during the latter half of the eighteenth century, though their anatomy was not understood until considerably later. The first mention of any member of the group was made by the Rev. W. Borlase in his _Natural History of Cornwall_, published in 1758. He gives a short description and a rough figure of _Lineus marinus_. From that time the increase in the knowledge of the group was very gradual. New species were from time to time described, but few of the descriptions could boast of much completeness, and many erroneous views were held until comparatively recent years. The group was very variously classified, but the general arrangement in early times seems to have been to unite it with the Planarians. Valuable contributions to the history of the development were made in 1848 and the few subsequent years by Desor,[118] Gegenbaur,[119] Krohn,[120] and Leuckart and Pagenstecher[121]; and more recently by Metschnikoff[122] and Salensky.[123]

{100}Nemertines for the most part closely resemble one another in all essential points, though they differ considerably in size, colour, and external details. They vary in length from less than an inch to thirty yards, this extreme size being attained by _Lineus marinus_.

Nemertines are common on the British coasts; about forty species have been recorded from this area. On turning over a stone on a sandy or muddy shore in a pool left by the receding tide, there may often be seen a coiled mass, having the appearance of a uniform slimy string twisted into a complicated knot. If it be carefully removed, the ends can generally be made out, one bluntly rounded and the other slightly tapering (Fig. 48, _a_ and _b_). Occasionally there may be seen attached to the blunter end a fine thread, which moves about freely. This thread may, by an instantaneous movement, be drawn into the body, no trace of its existence being left except at the tip of the head, where a small pore is visible; this is the orifice through which it was withdrawn. Shortly afterwards the thread may be again shot out, the process being instantaneous and often accomplished with {101}great force. This thread (Fig. 50, _p_) is the proboscis, a very important and characteristic organ in Nemertines.

Most Nemertines are marine; they are mostly indifferent to climate and to the nature of the soil on which they live.

A few forms live on land (e.g. _Tetrastemma agricola_,[124] _Geonemertes palaensis_,[125] and _G. chalicophora_[126]) or in fresh water (e.g. _Tetrastemma aquarum dulcium_[127] and _T. lacustre_[128]) in various parts of the globe. There are also parasitic forms; the best known of which is _Malacobdella_.[129] A pelagic form, _Pelagonemertes_,[130] has been described by Moseley.

EXTERNAL CHARACTERS.—A typical Nemertine possesses an elongated worm-like body (Fig. 49), which is usually thrown into numerous close coils (Fig. 48). In section it may be either round or more or less flattened, with the lateral edges in some cases quite thin and almost fin-like. One or two broad, flattened, and leaf-shaped forms are known, but such a condition is exceptional, and the forms in which it occurs have probably assumed it owing to the adoption of special modes of life.

In the ordinary forms the posterior end of the body is pointed either bluntly or sharply. The head is somewhat broader than the rest of the body, and often assumes a spatulate form. Eyes (Fig. 51, _e_) are usually present either in one or several pairs, or in symmetrically-arranged groups on each side of the head. The mouth (Fig. 58, _m_) is situated near the front end of the body on the ventral surface, and is usually rendered conspicuous by being surrounded by thick tumid lips. It varies in form from being slit-like to elliptical. At the anterior end of the body a {102}small terminal pore occurs; this is the external opening of the proboscis (Fig. 51, _p.p_).

Nemertines are often very diversely and brilliantly coloured, the hues most commonly found being white, yellow, green, deep purple, and various shades of red and pink. The ventral surface is usually paler in colour than the dorsal, and the latter is often marked by longitudinal and transverse stripes (Fig. 59) in contrasting colours.

The whole animal is enveloped in a layer of mucus, which sometimes becomes hardened to form a tube, and this may be still further strengthened by an admixture of particles of sand or earth.

The body is capable to a great extent of contraction and extension, a Nemertine many inches long being apt, when irritated or alarmed, to contract itself to the length of not more than half an inch. Hence, unless the animal is kept and carefully watched, a very erroneous idea may be conceived as to its size.

ANATOMY.—The body-wall consists of several layers (Fig. 52), which in a typical highly-developed Nemertine are as follows:—

1. An external epidermic layer (_ep_), consisting of ciliated cells, among which are placed numerous unicellular glands. These glands probably secrete the mucus in which the Nemertine is usually enveloped; their contents when in the body are very highly refracting. The epidermis rests on a basement membrane (_b.m_).

2. The two or three muscular layers, arranged as either an external circular and an internal longitudinal, or an inner and an outer circular separated by a longitudinal layer, or, as in the figure (_c.m_ and _l.m_), two longitudinal separated by a circular layer.

3. A fairly thick connective-tissue layer often found between the epidermis and the muscles, into which latter it gradually merges (_s.t_).

{103}THE DIGESTIVE SYSTEM.—The mouth is placed on the ventral surface near the anterior end of the body (Figs. 53, 58, _m_). It leads into a straight oesophagus (Fig. 53, _oes_), whence passes off the intestine (_int_), which is continued as a straight non-convoluted tube to the anus (_a_), situated terminally at the posterior end of the body. The intestine is thrown out throughout the greater part of its course into paired lateral pouches.

The alimentary canal is lined throughout by a ciliated epithelium. The oesophagus has, in addition to this layer, an outer thick coat of large granular cells, which probably have a glandular function.

PROBOSCIS.—The most characteristic organ of the Nemertines is the proboscis (Figs. 50, 53, 54). For many years its disposition and function were misunderstood, and it was supposed to be a portion of the digestive system. The proboscis, which lies dorsal to the alimentary canal, opens at the extreme anterior end of the body by a small pore (Figs. 51, 53, 58). When retracted it is sometimes considerably folded, and lies in a long pouch or sheath. To the walls of this sheath it is attached round its anterior {104}end; and strong muscles unite its posterior extremity to the sheath a short distance from the posterior end of the latter.

The proboscis seems to be exclusively a tactile and protective and defensive organ, for which functions it is eminently fitted by the great ease and rapidity with which it is everted or thrust out from the body. It consists of two distinct regions (Fig. 54, _g.p_ and _m.p_). In the retracted state the anterior part is a hollow tube with very thick muscular walls made up of several layers. At the base of this part in many of the Nemertines there is situated a sharp-pointed spine projecting forward into the lumen, and several smaller stylets situated in a pair of vesicles close to the base of the central spine. The position of the spines in the everted proboscis is shown in Fig. 57. The posterior part of the proboscis is also a tube, but instead of being muscular, its walls are glandular. This posterior glandular part is never everted.

The eversion is effected by a turning inside out of the anterior part of the proboscis (Fig. 54). The process whereby the proboscis is retracted has been very aptly compared to the effect which would be produced by the inversion of the finger of a glove, accomplished by pulling a string attached to its tip on the inside, the anterior muscular part being comparable to the finger and the glandular part to the string. It is thus obvious that in the everted condition the stylet will form the anterior tip of the {105}proboscis, and will there be in a position for offence or defence (Fig. 57, _s_).

NERVOUS SYSTEM.[131]—The brain is composed of two ganglionic masses (Fig. 53, _n.g_) lying at the anterior end of the body, one on each side of the proboscis, and united by commissures passing round it (Fig. 55, _d.c_ and _v.c_). Each ganglionic mass is often partially divided into a dorsal and ventral lobe (_n.g.d_ and _n.g.v_). From the brain a pair of cords pass off backwards along the sides of the body (_n.c_); these cords, which have no ganglionic swellings, in some forms unite with one another above the anus. Anteriorly nerves are given off from the brain to the eyes and front part of the head (_a.n_). A nerve to the proboscis is given off from the commissure which unites the two halves of the brain dorsal to the proboscis (_d.n_).

In two out of the three groups into which the Nemertines are divided, the lateral nerve-cords are in connexion with a network or plexus of nerves lying between the muscular layers of the body-wall (Fig. 52, _n.l_), and in some forms constituting a comparatively thick layer. In these two groups there are no definite {106}nerve branches except the anterior ones to the head. In the third group of Nemertines the lateral nerve-cords lie within the muscular layers of the body-wall, and in this case paired nerve branches are given off at definite intervals throughout the whole length of the body. These branches divide up among the organs to which they pass, and no nerve plexus is present.

The lateral cords vary in position in different cases. Sometimes they lie laterally, at others the cords tend to approximate to one another in the median dorsal or in the median ventral line, though in every case they remain distinctly separated.

SENSE ORGANS.—Sense organs are usually present in the form of eyes arranged at the sides of the head (Fig. 51, _e_), sometimes as a single pair and sometimes in one or more groups on each side. The structure of the eyes varies from a simple pigment spot to an organ which receives a special nerve-supply from the brain, and possesses a refracting body answering to a lens, and behind this a pigment layer and a layer of rods. Some forms are devoid of all traces of eyes.

A pair of simple auditory capsules has been found in some of the _Hoplonemertea_, where they occur as small vesicles on the brain.

The whole surface of the body appears to be remarkably sensitive. In a few forms small tufts of tactile hairs are said to be present in the region of the head, while in others there {107}are a few long hairs scattered sparsely among the cilia of the epidermis.

_Frontal Organ._—In many Nemertines there is present at the anterior tip of the head a disc-shaped group of cells bearing long hairs or bristles. On this disc open the secreting ducts of a number of gland cells lying in the head. It seems possible that this frontal organ may function as an organ of taste.

_Side Organs._—In the Carinellidae there is a pair of circular epithelial patches lying one on each side of the body in the region of the excretory pore. The cells composing them are richly ciliated and provided with a plentiful nerve-supply. The function of these epithelial patches is not known, but it has been suggested that they may be auditory organs.

_Cephalic Slits and Cerebral Organs._—In most Nemertines there is a peculiar pair of organs (Figs. 50, 53, _c.s_), situated in the head and in close connexion with the brain. The function of these organs is not known. Hubrecht has suggested that they may be respiratory, while Bürger[132] conjectures that they may be organs which are used for discriminating the condition of the surrounding medium. In an external examination of the head, the cephalic slits may usually be seen as a pair of lateral furrows or pits. Their form and direction vary considerably; they may take the form of shallow circular depressions, or they may lie longitudinally and be slit-like in shape (Fig. 50), or the slit may lie at right angles to the long axis of the body and be beset with short transverse furrows. In some forms these slits are merely superficial depressions, but in others they are continued into ciliated ducts, which pass inwards and penetrate into special lobes, consisting of glandular tissue and ganglion cells, in close connexion with the brain. These lobes are called the cerebral organs.

In many forms the nervous system is charged with haemoglobin, which gives to it a bright red colour.

CIRCULATORY OR BLOOD-VASCULAR SYSTEM.—The circulatory system consists of three main longitudinal vessels, a median dorsal and a pair of lateral ones. These are connected together posteriorly by a transverse trunk, and also throughout the whole length of their course by branches, which are given off at regular intervals. Anteriorly the three longitudinal vessels {108}either all unite and form a collar (Fig. 55, _v.s_) round the oesophagus, or they break up into a number of lacunar or open spaces in free communication with one another.

The blood is usually colourless, but in some cases the corpuscles are coloured red by haemoglobin.

EXCRETORY SYSTEM.—Max Schultze[133] found in _Tetrastemma obscurum_, on the outer side of, but near to the lateral blood-vessels, a pair of canals. He observed ciliary movements in the canals, but could not discover flame cells. Further contributions to our knowledge of the excretory system were made by Semper,[134] von Kennel,[135] Hubrecht,[136] and Oudemans.[137] The latter states that the excretory system consists of a pair of canals situated laterally near the anterior end of the body. Each canal communicates with the exterior by one or more ducts having lateral regularly-arranged apertures. In some cases he was unable to make out any communication with the vascular system, but in others {109}a direct communication, by means of open connexions with the lacunar blood spaces, is said to occur.

Silliman[138] in _Tetrastemma aquarum dulcium_ describes the excretory vessels as ending in numerous capillary branches, at the blind terminations of which cilia are present. He states that there is no important difference between the excretory systems of Rhabdocoeles and Nemertines.

Bürger,[139] as the result of recent investigations on the excretory system in Nemertines, finds that the minute branches end in flame-cells (Fig. 56, B) lying on and among the blood-vessels, but having no open connexion with them.

GENERATIVE SYSTEM.—The Nemertines are for the most part dioecious, only a few certainly hermaphrodite species having been described, e.g. _Tetrastemma_ ("_Borlasia_") _kefersteinii_ Mar.[140]

The generative products in both cases are contained in sacs (Figs. 52, 53, _g_) which lie in the lateral region of the body between the pouches of the alimentary canal. The ova and spermatozoa are conveyed to the exterior by short ducts. Most species are oviparous, though a few viviparous species are known (e.g. _Prosorhochmus claparedii_).

CLASSIFICATION.—Nemertines were divided by M. Schultze[141] into:—

1. _Enopla_, in which the proboscis is armed with stylets.

2. _Anopla_, in which the proboscis is unarmed.

Although this classification was fairly correct as far as it went, since many other distinctive features were correlated with the presence or absence of armature in the proboscis, still there are several primitive forms belonging to the _Anopla_, which possess characters such as render it necessary to class them together in a separate group.

For this reason Hubrecht divided the Nemertinea into three Orders—_Hoplonemertea_, _Schizonemertea_, _Palaeonemertea_; the first of these Orders corresponding with the _Enopla_, and the other two with the _Anopla_.

{110}ORDER I. HOPLONEMERTEA.

The proboscis is armed. The epidermis rests on a thick layer of connective tissue plentifully supplied with glands, below which is a prominent basement membrane. The muscular layers of the body are two in number, an outer circular and an inner longitudinal. The nerve-trunks lie within the muscular layers of the body and give off regularly-arranged branches. There is no nerve plexus. Each of the cephalic slits generally opens by a pore situated in the centre of a transverse groove, which is beset along one side by a row of shorter grooves at right angles to it. The apparatus consists of a ciliated duct surrounded by nerve tissue, and passing into lobes of tissue which are connected with the brain by thick nerve-cords. The mouth opens rather far forward in front of the brain. The intestinal pouches are symmetrically arranged. Auditory organs are said to exist in some forms, consisting of vesicles containing otoliths. The vascular trunks are connected anteriorly by closed vessels and not by lacunar spaces.

The principal British genera and species[142] are:—

_Amphiporus bioculatus_ M‘Int., _A. dissimulans_ Riches, _A. hastatus_
M‘Int., _A. lactifloreus_ M‘Int., _A. pulcher_ Johnst.

_Drepanophorus rubrostriatus_ Hubr. (= _A. spectabilis_ Qtrf.).

_Tetrastemma ambiguum_ Riches, _T. candidum_ O. F. Müll., _T. dorsale_
Abildg., _T. flavidum_ Ehrenb., _T. immutabile_ Riches, _T.
melanocephalum_ Johnst., _T. nigrum_ Riches, _T. robertianae_ M‘Int., _T.
vermiculatum_ Qtrf.

_Prosorhochmus claparedii_ Keferstein.

_Nemertes carcinophila_ Köll., _N. gracilis_ Johnst., _N. neesii_ Oerst.

_Malacobdella grossa_ O. F. Müll.

{111}ORDER II. SCHIZONEMERTEA.

The proboscis is unarmed. The epidermis is separated from the layer of connective tissue by a thin basement membrane, hence the glands in the connective tissue are more deeply situated and have long ducts. The muscular layers are three in number, an outer and an inner longitudinal layer between which lies a layer of circular muscles. The lateral nerve-cords lie between the outer longitudinal and the circular muscle layers. They are connected throughout the body by a nerve plexus, the only definite nerve branches given off being those to the brain, oesophagus, and proboscis. The cephalic slits are a pair of deep longitudinal grooves at the sides of the head. From each groove a canal passes inwards into a posterior brain-lobe. The mouth opens behind the brain, and is an elongated slit bounded by corrugated lips. Auditory organs have not been observed. The longitudinal vascular trunks are connected anteriorly by lacunar spaces, and not by closed vessels.

Principal British genera and species:—

_Lineus bilineatus_ Ren., _L. lacteus_ Mont., _L. marinus_ Mont. (= _L.
longissimus_ Gunnerus), _L. gesserensis_ O. F. Müll. (= _L. obscurus_
Desor and _L. sanguineus_ M‘Int.).

_Borlasia elizabethae_ M‘Int.

_Cerebratulus angulatus_ O. F. Müll., _C. fuscus_ M‘Int., _C.
pantherinus_ Hubr.

_Micrura aurantiaca_ Grube, _M. candida_ Bürger, _M. fasciolata_ Ehrenb.,
_M. purpurea_ J. Müll.

_Meckelia asulcata_ M‘Int.

ORDER III. PALAEONEMERTEA.

The proboscis is unarmed. The epidermis and connective tissue form one layer, below which is the basement membrane. The muscular layers are three in number, two circular separated by a longitudinal layer. The nerve-cords lie altogether external {112}to the muscular layers, and are connected together throughout by a plexus. No nerve branches are given off. The brain is not divided into lobes. The cephalic slits are only represented by a shallow depression on each side of the head, and no canals have been observed leading from them. The intestine is straight, and the pouches are usually absent or rudimentary. The circulatory system is largely made up of lacunar spaces, the closed system being but little developed.

Principal British genera and species:—

_Carinella annulata_ Mont., _C. linearis_ (Mont., MS.) M‘Int., _C.
macintoshi_ Bürger (Fig. 59), _C. polymorpha_ Ren.