fig39Fig. 39.—A,Taenia saginataGoeze. Nat. size. (From a specimen in the Cambridge Museum.) The approximate lengths of the portions omitted in the drawing are given. At * (after Leuckart) the branched uterus and the longitudinal and transverse excretory vessels are shown. The genital apertures are seen as a lateral opening on each of the larger proglottides.B, Head (scolex) ofT. soliumRud. × 12. (After Leuckart.)
Fig. 39.—A,Taenia saginataGoeze. Nat. size. (From a specimen in the Cambridge Museum.) The approximate lengths of the portions omitted in the drawing are given. At * (after Leuckart) the branched uterus and the longitudinal and transverse excretory vessels are shown. The genital apertures are seen as a lateral opening on each of the larger proglottides.B, Head (scolex) ofT. soliumRud. × 12. (After Leuckart.)
Fig. 39.—A,Taenia saginataGoeze. Nat. size. (From a specimen in the Cambridge Museum.) The approximate lengths of the portions omitted in the drawing are given. At * (after Leuckart) the branched uterus and the longitudinal and transverse excretory vessels are shown. The genital apertures are seen as a lateral opening on each of the larger proglottides.B, Head (scolex) ofT. soliumRud. × 12. (After Leuckart.)
Generally speaking, "a tape-worm" in Western Europe will prove to beTaenia saginataGoeze (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 soliumRud. (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 renderT. soliuma 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 asCysticercus 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 abnormalCysticercusof this species is probably theTaenia(Cysticercus)acanthotriasWeinl. (see, however, Leuckart,loc. cit.p. 711).
Taenia(Hymenolepis)nanav. 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. diminutaRud. (flavopunctataWeinl.), normally found in Rodents, has been rarely recorded in man.Taenia(Dipylidium)caninumL. (=T. cucumerinaBloch =T. ellipticaBatsch), the commonest parasite of pet cats and dogs, andT.(Davainea)madagascariensisDavaine, 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. echinococcusv. Sieb.) which lives when mature in the dog.
Echinococcusis 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 duringpost-mortemexaminations 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 theCysticercus, 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 echinococcusandT. coenurusare interesting, since they exhibit anindubitable alternation of asexual generations in the larval state, with a sexual adult stage.
Bothriocephalus latusBrems., 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. cordatusis very common in the dog, and probably also in man, though few cases have been recorded.B. mansoniCobb. (=B. liguloidesLeuck.) was, till recently, known only in the larval state from China and Japan. Iijima, however, has found older specimens in the latter country.B. cristatusDav. 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 ofTaenia(T. serrata,marginata,coenurus,echinococcus,krabbei, and possiblyT. serialis),Dipylidium caninum(the commonest form),Mesocestoides lineatus, and three or four species ofBothriocephalushave 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 ofTaeniaare found. Hence the prevalence ofT. serratain a given locality is connected with the abundance there of the rabbit and hare, in which the larva (Cysticercus pisiformis) occurs.Bothriocephalus cordatusdevelops from the young stage present in the fish which the Icelanders give to their dogs. In Iceland and certain parts ofAustraliaT. echinococcusinfests one-third to one-half the number of dogs examined; a fact connected with the frequency ofEchinococcusin man in these countries.
In sheep the most noteworthy and dangerous parasite isCoenurus 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 serialisBaill., 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 carnivoreTaenia serialisBaill. 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.
Table of the Life-Histories of the principal Cestodes of Man and the Domestic Animals.
Drepanidotaenia gracilisZed.
fig40Fig. 40.—A, Stickleback (Gasterosteus aculeatus) infested by an advanced larva ofSchistocephalus solidusCrepl.B, The larva. All × 1½. (From specimens in the Cambridge University Museum.)
Fig. 40.—A, Stickleback (Gasterosteus aculeatus) infested by an advanced larva ofSchistocephalus solidusCrepl.B, The larva. All × 1½. (From specimens in the Cambridge University Museum.)
Fig. 40.—A, Stickleback (Gasterosteus aculeatus) infested by an advanced larva ofSchistocephalus solidusCrepl.B, The larva. All × 1½. (From specimens in the Cambridge University Museum.)
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 sometimesbe found, the larva ofSchistocephalus solidusCrepl., which occurs in the intestine of Terns, Storks, Mergansers, and other birds. Species ofLigulaare found in the same birds. The intestine of aLophiusorCyclopterus("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, theTaenia anatinafrom the duck may be used. The ripe proglottides are collected, and the eggs placed withCypris ovumin an aquarium, with the probability that some of the embryos will enter the Ostracod, and the peculiar Cysticercoid may be bred.[106]Cysticercus pisiformisandCoenurus serialis, which occur commonly in rabbits, are also suitable objects for examination.
A Cestode such asEchinobothrium(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 theIchthyotaeniae. 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 pairsof 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.
fig41Fig. 41.—Diagrammatic transverse section ofSchistocephalus solidusCrepl., from the Wild-duck, illustrative of the Cestodes with uterine aperture (uto). × 12.cs, Cirrus-sac;for.sec, one of the paired lateral openings of the excretory vessels;ln, longitudinal nerve;ov, ovary;ovd, oviduct;par.m, parenchymatous muscles;r.sem, receptaculum seminis;sh.gl, shell-gland;te, testes;ut, uterus;uto, uterine pore;vag, vagina;vd, vasa deferentia;yd, yolk-duct;yg, yolk-glands (black); ♂, male, ♀, female genital aperture. (After Riehm.)
Fig. 41.—Diagrammatic transverse section ofSchistocephalus solidusCrepl., from the Wild-duck, illustrative of the Cestodes with uterine aperture (uto). × 12.cs, Cirrus-sac;for.sec, one of the paired lateral openings of the excretory vessels;ln, longitudinal nerve;ov, ovary;ovd, oviduct;par.m, parenchymatous muscles;r.sem, receptaculum seminis;sh.gl, shell-gland;te, testes;ut, uterus;uto, uterine pore;vag, vagina;vd, vasa deferentia;yd, yolk-duct;yg, yolk-glands (black); ♂, male, ♀, female genital aperture. (After Riehm.)
Fig. 41.—Diagrammatic transverse section ofSchistocephalus solidusCrepl., from the Wild-duck, illustrative of the Cestodes with uterine aperture (uto). × 12.cs, Cirrus-sac;for.sec, one of the paired lateral openings of the excretory vessels;ln, longitudinal nerve;ov, ovary;ovd, oviduct;par.m, parenchymatous muscles;r.sem, receptaculum seminis;sh.gl, shell-gland;te, testes;ut, uterus;uto, uterine pore;vag, vagina;vd, vasa deferentia;yd, yolk-duct;yg, yolk-glands (black); ♂, male, ♀, female genital aperture. (After Riehm.)
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.
fig42Fig. 42.—A, Free-swimming, six-hooked larva ofBothriocephalus latusBrems. (the broad tape-worm of Man), still enclosed in a ciliated (possibly cellular) double membrane or mantle. In this condition it may continue to live in water for a week or more, but eventually throws off its ciliated coat (as inB) and commences to creep about vigorously by the aid of its hooks, in search of its first host, which is at present unknown. (After Schauinsland.) × 600.
Fig. 42.—A, Free-swimming, six-hooked larva ofBothriocephalus latusBrems. (the broad tape-worm of Man), still enclosed in a ciliated (possibly cellular) double membrane or mantle. In this condition it may continue to live in water for a week or more, but eventually throws off its ciliated coat (as inB) and commences to creep about vigorously by the aid of its hooks, in search of its first host, which is at present unknown. (After Schauinsland.) × 600.
Fig. 42.—A, Free-swimming, six-hooked larva ofBothriocephalus latusBrems. (the broad tape-worm of Man), still enclosed in a ciliated (possibly cellular) double membrane or mantle. In this condition it may continue to live in water for a week or more, but eventually throws off its ciliated coat (as inB) and commences to creep about vigorously by the aid of its hooks, in search of its first host, which is at present unknown. (After Schauinsland.) × 600.
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 inSchistocephalus,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 inTaenia serratathe 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.
fig43Fig. 43.—Stages in the development ofDipylidium caninumL. (=Taenia ellipticaBatsch,T. cucumerinaBloch), the commonest of the Dog-Taeniae; compare Fig. 44.A, Six-hooked larva (now often spoken of as an "Onchosphaera");B, larva elongating; formation of a central lacuna;C, larva further advanced;D, distinction between body and tail is visible;E, invagination of the rostellum is commencing;F, Cysticercoid larva with four suckers, invaginated rostellum, and excretory vessels.c, Calcareous concretions in cells of the larva;ex.o, excretory aperture;ex.v, excretory vessels;inv, invagination commencing;rost, rostellum;sc, suckers. (After Grassi and Rovelli; highly magnified.)
Fig. 43.—Stages in the development ofDipylidium caninumL. (=Taenia ellipticaBatsch,T. cucumerinaBloch), the commonest of the Dog-Taeniae; compare Fig. 44.A, Six-hooked larva (now often spoken of as an "Onchosphaera");B, larva elongating; formation of a central lacuna;C, larva further advanced;D, distinction between body and tail is visible;E, invagination of the rostellum is commencing;F, Cysticercoid larva with four suckers, invaginated rostellum, and excretory vessels.c, Calcareous concretions in cells of the larva;ex.o, excretory aperture;ex.v, excretory vessels;inv, invagination commencing;rost, rostellum;sc, suckers. (After Grassi and Rovelli; highly magnified.)
Fig. 43.—Stages in the development ofDipylidium caninumL. (=Taenia ellipticaBatsch,T. cucumerinaBloch), the commonest of the Dog-Taeniae; compare Fig. 44.A, Six-hooked larva (now often spoken of as an "Onchosphaera");B, larva elongating; formation of a central lacuna;C, larva further advanced;D, distinction between body and tail is visible;E, invagination of the rostellum is commencing;F, Cysticercoid larva with four suckers, invaginated rostellum, and excretory vessels.c, Calcareous concretions in cells of the larva;ex.o, excretory aperture;ex.v, excretory vessels;inv, invagination commencing;rost, rostellum;sc, suckers. (After Grassi and Rovelli; highly magnified.)
fig44Fig. 44.—Schematic longitudinal sections through the larvae ofDipylidium caninumL. All these stages are passed in the body-cavity of the Dog-flea (Pulex serraticeps). (Compare Fig. 43 for further details.)A, Six-hooked larva with developing rostellum (shaded) and suckers (black). In this species the invagination (C,invag.) occurs after the formation of these organs, and not, as in most Taeniae, before it.B, Invagination commencing; the hooks are developing above the rostellum, while beneath it the nervous system (dotted) is seen.C, The invagination has now carried the suckers inwards. The tail has become distinct, and the whole larva at this stage is known as a Cysticercoid.hk, Larval hooks;invag, mouth of the invagination;n, central nervous system;rost, rostellum and hooks;sc, suckers, of which only two can be seen in a longitudinal section; four are really present. (After Grassi and Rovelli.)
Fig. 44.—Schematic longitudinal sections through the larvae ofDipylidium caninumL. All these stages are passed in the body-cavity of the Dog-flea (Pulex serraticeps). (Compare Fig. 43 for further details.)A, Six-hooked larva with developing rostellum (shaded) and suckers (black). In this species the invagination (C,invag.) occurs after the formation of these organs, and not, as in most Taeniae, before it.B, Invagination commencing; the hooks are developing above the rostellum, while beneath it the nervous system (dotted) is seen.C, The invagination has now carried the suckers inwards. The tail has become distinct, and the whole larva at this stage is known as a Cysticercoid.hk, Larval hooks;invag, mouth of the invagination;n, central nervous system;rost, rostellum and hooks;sc, suckers, of which only two can be seen in a longitudinal section; four are really present. (After Grassi and Rovelli.)
Fig. 44.—Schematic longitudinal sections through the larvae ofDipylidium caninumL. All these stages are passed in the body-cavity of the Dog-flea (Pulex serraticeps). (Compare Fig. 43 for further details.)A, Six-hooked larva with developing rostellum (shaded) and suckers (black). In this species the invagination (C,invag.) occurs after the formation of these organs, and not, as in most Taeniae, before it.B, Invagination commencing; the hooks are developing above the rostellum, while beneath it the nervous system (dotted) is seen.C, The invagination has now carried the suckers inwards. The tail has become distinct, and the whole larva at this stage is known as a Cysticercoid.hk, Larval hooks;invag, mouth of the invagination;n, central nervous system;rost, rostellum and hooks;sc, suckers, of which only two can be seen in a longitudinal section; four are really present. (After Grassi and Rovelli.)
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 theTaenia 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. InTaenia(Hymenolepis)murinathe 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 inScolex polymorphus(Fig. 38) (the young stage ofCalliobothrium filicolleZsch.) is practically absent. If the bladder be small, the larva is known as a Cysticercoid. For example, the commonDipylidium 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 inCoenurusandEchinococcus.
Table for the Discrimination of the more usual Cestodes of Man and Domestic Animals.[108]
Dog-Taeniae[109]
Head armed; genital pore marginal and
— Single
Bifid hooks, which are
— 230µ-260µ long[110]; genital pore very distinct
Bifid hooks, which are
— 230µ-260µ long[110]; genital pore very distinct
Entire large hooks, which are
— 180µ-220µ long; length of mature segments double their width
Entire large hooks, which are
— 180µ-220µ long; length of mature segments double their width
Horse-Taeniae.
α. Head very large
— No posterior lobes
a.A transverse uterus in middle part of median field; head 2 mm. diameter
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:—
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.
fig45Fig. 45.—A,B,C, Stages in the development of the vermiform larva inDicyema typusvan Ben. (After Ed. van Beneden.)cal, "Calotte";gc, germinal cell;n, nucleus of endodermal cell.
Fig. 45.—A,B,C, Stages in the development of the vermiform larva inDicyema typusvan Ben. (After Ed. van Beneden.)cal, "Calotte";gc, germinal cell;n, nucleus of endodermal cell.
Fig. 45.—A,B,C, Stages in the development of the vermiform larva inDicyema typusvan Ben. (After Ed. van Beneden.)cal, "Calotte";gc, germinal cell;n, nucleus of endodermal cell.
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: theDicyemidae, which occur exclusively in the kidneys of certain Cephalopods (cuttle-fish); and theOrthonectidae, which live in the brittle-starAmphiura squamata, the NemertineNemertes lacteus, or thePolycladLeptoplana tremellaris. In addition to the undoubted Mesozoa, certain anomalous forms—Trichoplax adhaerensandSalinella salve—may be referred to this group.
fig46Fig. 46.—Dicyemennea eledonesWag., from the kidney ofEledone moschata.A, Full-grown Rhombogen with infusoriform embryos (emb);B, one of the latter developing;C, fully formed;D, calotte, composed of the upper nine cells shown in the figure. (After Ed. van Beneden and Whitman.)emb, Infusoriform embryo;g, part of endoderm-cell where formation of these embryos is rapidly proceeding;n.ect, nucleus of ectoderm-cell;n.end, nucleus of endoderm-cell;p, "calotte."
Fig. 46.—Dicyemennea eledonesWag., from the kidney ofEledone moschata.A, Full-grown Rhombogen with infusoriform embryos (emb);B, one of the latter developing;C, fully formed;D, calotte, composed of the upper nine cells shown in the figure. (After Ed. van Beneden and Whitman.)emb, Infusoriform embryo;g, part of endoderm-cell where formation of these embryos is rapidly proceeding;n.ect, nucleus of ectoderm-cell;n.end, nucleus of endoderm-cell;p, "calotte."
Fig. 46.—Dicyemennea eledonesWag., from the kidney ofEledone moschata.A, Full-grown Rhombogen with infusoriform embryos (emb);B, one of the latter developing;C, fully formed;D, calotte, composed of the upper nine cells shown in the figure. (After Ed. van Beneden and Whitman.)emb, Infusoriform embryo;g, part of endoderm-cell where formation of these embryos is rapidly proceeding;n.ect, nucleus of ectoderm-cell;n.end, nucleus of endoderm-cell;p, "calotte."
Dicyemidae.—If the kidney ofEledone 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 beDicyemennea eledonesWag., although another form,Dicyema moschatumWhit., 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"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:—
Orthonectida.[114]—Two species of Orthonectids are fairly well known,Rhopalura giardiiMetschn. fromAmphiura squamata, andR. intoshiiMetschn. fromNemertes 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 ofAmphiura, and between the gut-branches and body-wall inNemertes. 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).