Development of the CestodaFig. 96. Diagrams of various stages in the development of the Cestoda.(From Huxley.)A. Cysticercus. B. and C. Cysticerci in the everted (B) and inverted (C) condition. D. Cœnurus. E. and F. Diagrams of Echinococcus. It is most probable that Tænia heads are not developed directly from the wall of the cyst as represented in the diagram. G. Six-hooked embryo.
Fig. 96. Diagrams of various stages in the development of the Cestoda.(From Huxley.)
A. Cysticercus. B. and C. Cysticerci in the everted (B) and inverted (C) condition. D. Cœnurus. E. and F. Diagrams of Echinococcus. It is most probable that Tænia heads are not developed directly from the wall of the cyst as represented in the diagram. G. Six-hooked embryo.
Before proceeding to describe their further history, the close resemblance between the first developmental stages of Cestoda, especially in the case of the ciliated larvæ, and those of Trematoda, may be pointed out.
In both there is a ciliated larva, and in both there is developed within the ciliated skin a second larva, which becomes freed by the stripping off of the ciliated skin.
The type of development has moreover many analogies with that of the Nemertine larva of Desor,p.163 (cf.Metschnikoff), and is probably like that an abbreviated record of a long history.
The suitable host for the six-hooked embryo to enter israrely the same as the host for the sexual form. The embryos having become transported into the alimentary canal of such a host, and become free, if previously invested by the egg-shell, soon make their way, apparently by the help of their hooks, through the wall of the alimentary tract, and are transported in the blood or otherwise into some suitable place for them to undergo their next transformation. This place may be the liver, lungs, muscles, connective tissue, or even the brain (e.g.Cœnurus cerebralisin the brain of sheep).
Here they become enclosed in a granular deposit from the surrounding tissues, which becomes in its turn enclosed in a connective-tissue coat. Within lies the solid embryo, the hooks of which in many cases disappear or become impossible to make out. In other forms,e.g.Cysticercus limacis, they remain visible, and then mark the anterior pole of the worm (fig. 98,c.). The central part of the body next becomes transformed into a material composed of clear non-nucleated vesicles. Accompanying these changes the embryo grows rapidly in size; a cuticle is deposited by its outer layer, in which also an external layer of circular muscular fibres and an internal layer of longitudinal fibres become differentiated, and internal to both there is formed a layer of granular cells.
With the rapid growth of the body a central cavity is formed, which becomes filled with fluid, and the embryo assumes the form of a vesicle. At the same time a system of excretory vessels, sometimes opening by a posterior pore, becomes visible in the wall of the vesicle.
The embryo has now reached a condition in which it is known as a cystic- or bladder-worm, and may be compared in almost every respect with the sporocyst of a Trematode (Huxley).
Cysticercus cellulosæFig. 97. Cysticercus cellulosæ.(From Gegenbaur, after von Siebold.)a.Caudal vesicle.c.Anterior part of body.d.head.
Fig. 97. Cysticercus cellulosæ.(From Gegenbaur, after von Siebold.)
a.Caudal vesicle.c.Anterior part of body.d.head.
Cysticercus with small vesicleFig. 98. Cysticercus with small caudal vesicle.A. Head involuted. B. Head everted.a.Scolex.b.caudal vesicle.c.(in A) six embryonic hooks.
Fig. 98. Cysticercus with small caudal vesicle.
A. Head involuted. B. Head everted.
a.Scolex.b.caudal vesicle.c.(in A) six embryonic hooks.
The next important change consists in the development of a head, which becomes the head of the adult Tænia. This is formed in an involution of the outer wall of the anterior extremity of the cystic worm. This involution forms a papilliform projection on the inner surface of the wall of the cystic worm, with an axial cavity opening by a pore on the outer surface. The layer of cells forming the papilla soon becomes divided into two laminæ, of which the outer forms a kind of investing membrane for the papilla. The papilla itself now becomesmoulded into a Cestode head, which however is developed in an inverted position. The suckers and hooks (when present) of the head are developed on a surface bounding the axial lumen of the papilla, which is the true morphological outer surface, while the apparent outer surface of the papilla is that which eventually forms the interior of the (at first) hollow head. Before the external armature of the head has become established, four longitudinal excretory vessels, continuous with those in the body of the cystic worm, make their appearance. They are united by a circular vessel at the apex of the head. The development is by no means completed with the simple growth of the head, but the whole inverted papilla continues to grow in length, and gives rise to what afterwards becomes part of the trunk. The whole papilla eventually becomes everted, and then the cystic worm takes the form (fig. 97) of a head and unsegmented trunk with a vesicle—the body of the cystic worm—attached behind. The whole larva is known as aCysticercus. The termscolex, which is also sometimes employed, may be conveniently retained for the head and trunk only. The head differs mainly from that of the adult in being hollow.
There are great variations in the relative size of the head and the vesicle of Cysticerci. In some forms the vesicle is very small (fig. 98),e.g.Cysticercus limacis; it is medium-sized inCysticercus cellulosæ(fig. 97), and in some forms is much larger. The embryonic hooks, when they persist, are found at the junction of the trunk and the vesicle (fig. 98A,c). Though the majority of cystic worms only develop one head, this is not invariably the case. There is a cystic worm found in the brain of the sheep known asCœnurus cerebralis—the larva ofTænia cœnurus, parasitic in the intestine of the dog—which forms an exception to this rule. There appears, to start with, a tuft of three or four heads, and finally many hundred heads are developed (fig. 96D). They are arranged in groups at one (the anterior?) pole of the cystic worm.
A still more complicated form of cystic worm is that known as Echinococcus, parasitic in the liver, lungs, etc. of man and various domestic Ungulata. In the adult state it is known asTænia echinococcusand infests the intestine of the dog. The cystic worm developed from the six-hooked embryo has usually a spherical form, and is invested in a very thick cuticle (fig. 96E and F, andfig. 99). It does not itself directly give rise to Tænia heads, but after it reaches a certain size there are formed on the inner side of its walls small protuberances, which soon grow out into vesicles connected with the walls of the cyst by narrow stalks (figs. 96F and99C). In the interior of these vesicles a cuticle is developed. It is in these secondary vesicles that the heads originate. According to Leuckart, they either arise as outgrowths of the wall of the vesicle on the inner face of which the armature is developed, which subsequently become involuted and remain attached to the wall of the vesicle by a narrow stalk, or they arise from the first as papilliform projections into the lumen of the vesicle, on the outer side of which the armature is formed. Recent observers only admit the second of these modes of development. The Echinococcus larva, in addition to giving rise to the above head-producing vesicles, also gives rise by budding to fresh cysts, which resemble in all respects the parent cyst. These cysts may either be detached in the interior (fig. 96F) of the parent or externally. They appear to spring in most cases from the walls of the parent cyst, but there are some discrepancies between the various accounts of the process. In the cysts of the second generation vesicles are produced in which new heads are formed. As the primitive cyst grows, it naturally becomes more and more complicated, and the number of heads to which one larva may give rise becomes in this way almost unlimited.
Cysticerci may remain a long time without further development, and human beings have been known to be infested with an Echinococcus cyst for over thirty years. When however the Cysticercus with its head is fully developed, it is in a condition to be carried into its final host. This takes place by the part of one animal infested with cysticerci becoming eaten by the host in question. In the alimentary canal of the final host the connective-tissue capsule is digested, and then the vesicular caudal appendage undergoes the same fate, while the head, with its suckers and hooks, attaches itself to the walls of the intestine. The head and rudimentary trunk, which have been up to this time hollow, now become solid by the deposition of an axial tissue; and the trunk very soon becomes divided into segments, known as proglottides (fig. 99A). These segments are not formed in the same succession as those of Chætopods; theyoungest of them is that nearest to the head, and the oldest that furthest removed from it. Each segment appears in fact to be a sexual individual, and is capable of becoming detached and leading for some time an independent existence. In some cases,e.g.Cysticercus fasciolaris, the segmentation of the trunk may take place while the larva is still in its intermediate host.
Echinococcus veterinorumFig. 99. Echinococcus veterinorum.(From Huxley.)A. Tænia head or scolex.a.hooks.b.suckers.c.cilia in water vessel.d.refracting particles in body wall.B. single hooks.C. portion of cyst.a.cuticle.b.membranous wall of primary cyst.c.ande.scolex heads.d.secondary cyst.
Fig. 99. Echinococcus veterinorum.(From Huxley.)
A. Tænia head or scolex.a.hooks.b.suckers.c.cilia in water vessel.d.refracting particles in body wall.B. single hooks.C. portion of cyst.a.cuticle.b.membranous wall of primary cyst.c.ande.scolex heads.d.secondary cyst.
The stages in the evolution of the Cestoda are shortly as follows:
1. Stage with embryonic epidermis either ciliated (Bothriocephalus, etc.) or still enclosed in the egg-shell. This stage corresponds to the ciliated larval stage of the Trematoda.
2. Six-hooked embryonic stage after the embryonic epidermis has been thrown off. During this stage the embryo is transported into the alimentary tract of its intermediate host, and boring its way into the tissues, becomes encapsuled.
3. It develops during the encapsuled state into a cystic worm, equivalent to the sporocyst of Trematoda.
4. The cystic worm while still encapsuled develops a head with suckers and hooks, becoming a Cysticercus. In some forms (Cœnurus,Echinococcus) reproduction by budding takes place at this stage. The head and trunk are known as the scolex.
5. The Cysticercus is transported into the second and permanent host by the infested tissue being eaten. The bladder-like remains of the cystic worm are then digested, and by a process of successive budding a chain of sexual proglottides are formed from the head, which remains asexual.
TetrarhyncusFig. 99 a. Tetrarhyncus.(From Gegenbaur; after Van Beneden.)A. Asexual state.B. Sexual stage with ripe proglottides.
Fig. 99 a. Tetrarhyncus.(From Gegenbaur; after Van Beneden.)
A. Asexual state.B. Sexual stage with ripe proglottides.
The above development is to be regarded as a case ofcomplicated metamorphosis secondarily produced by the necessities of a parasitic condition, to which an alternation of sexual and gemmiparous generations has been added. The alternation of generations only occurs at the last stage of the development, when the so-called head, without generative organs, produces by budding a chain of sexual forms, the embryos of which, after passing through a complicated metamorphosis, again become Cestode heads.
In the case of Cœnurus and Echinococcus two or more asexual generations are interpolated between the sexual ones. It is not quite clear whether the production of the Tænia head from the cystic worm may not be regarded as a case of budding. There are some grounds for comparing the scolex to the Cercaria of Trematodes,cf.Archigetes.
As might be anticipated from the character of the Cestode metamorphosis, the two hosts required for the development are usually forms so related that the final host feeds upon the intermediate host. As familiar examples of this may be cited the pig, the muscles of which may be infested byCysticercus cellulosæ, which becomes theTænia soliumof man. Similarly a Cysticercus infesting the muscles of the ox becomes theTænia mediocanellataof man. TheCysticercus pisciformisof the rabbit becomes theTænia serrataof the dog. TheCœnurus cerebralisof the sheep’s brain becomes theTænia cœnurusof the dog. The Echinococcus of man and the domestic herbivores becomes theTænia echinococcusof the dog.
Cystic worms infest not only Mammalian forms, but lower Vertebrates, various fishes which form the food of other fishes, and Invertebrates liable to be preyed on by vertebrate hosts. So far the Cestodes (except Archigetes) are only known to attain sexual maturity in the alimentary tracts of Vertebrata.
The rule that the intermediate host is not the same as the final host does not appear to be without exception. Redon[96]has shewn by experiments on himself that aCysticercus(cellulosæ) taken from a human subject develops intoTænia soliumin the intestines of a man. Redon took four cysts of a Cysticercus from a human subject, and after three months passed some proglottides, and subsequently the head ofTænia solium.
Some important variations of the typical development are known.
The so-called head or scolex may be formed without the intervention of a cystic stage. In Archigetes (Leuckart,No.227), which infests, in the Cysticercus condition, the body cavity of various invertebrate forms (Tubifex, etc.), the six-hooked embryo becomes elongated and divided into two sections, one forming the head, while the other, with the six embryonic hooks, forms an appendage, homologous with the caudal vesicle of other Cysticerci.
The embryo ofTænia ellipticasimilarly gives rise to a Cysticercus infesting the dog-louse (Trichodectes canis), without passing through a vesicular condition; but the caudal vesicle disappears, so that it forms simply a scolex. These cases may, it appears to me, be probably regarded as more primitive than the ordinary ones, where the cystic condition has become exaggerated as an effect of a parasitic life.
In some cases the larva of a Tænia has a free existence in the scolex condition. Such a form, the larva of Phyllobothrium, has been observed by Claparède[97]. It was not ciliated, and was without a caudal vesicle; and was no doubt actively migrating from an intermediate host to its permanent host.
Scolex forms, without a caudal vesicle, are found in the mantle cavity of Cephalopoda, and appear to be occupying an intermediate host in their passage from the host of the cystic worm to that of the sexual form.
Archigetes, already mentioned, has been shewn by Leuckart (No.227) to become sexually mature in the Cysticercus state, and thus affords an interesting example of pædogenesis. It is not known for certain whether under normal circumstances it reaches the mature state in another host.
Amphilina.The early stages of this interesting form have been investigated by Salensky (No.229), and exhibit clear affinities to those of the true Cestoda. An embryonic provisional skin is formed as in Cestodes; and pole cells also appear. Within the provisional skin is formed an embryo with ten hooks. After hatching the provisional skin is at once thrown off, and the larva, which is then covered by a layer of very fine cilia, becomes free. The further metamorphosis is not known.
Bibliography.
Turbellaria.
(181)Alex. Agassiz. “On the young stages of a few Annelids” (Planaria angulata).Annals Lyceum Nat. Hist. of New York,Vol.VIII.1866.(182)Dalyell. “Powers of the Creator.”(183)C. Girard. “Embryonic development of Planocera elliptica.”Jour. of Acad. of Nat. Sci.Philadelphia. New Series,Vol.II.1854.(184)Alex. Götte.“Zur Entwicklungsgeschichte d. Seeplanarien.”Zoologischer Anzeiger,No.4, 1878.(185)P. Hallez.Contributions à l’histoire naturelle des Turbellariés. Thésis à la faculté des Sciences p. le grade d. Docteur ès-sciences naturelles, Lille, 1879.(186)Knappert.“Bijdragen tot de Ontwikkelings-Geschiedenis der Zoetwater-Planarien.”Provinciaal Utrechtsch Genootschap van Kunsten en Wetenschappen.Utrecht, 1865.(187)W. Keferstein.“Beiträge z. Anat. u. Entwick. ein. Seeplanarien von St. Malo.”Abh. d. könig. Gesell. d. Wiss. zu Göttingen.Bd.XIV.1868.(188)El. Metschnikoff.“Untersuchungen üb. d. Entwicklung d. Planarien.”Notizen d. neurussischen Gesellschaft d. Naturforscher.Odessa,Bd.V.1877.VideHoffman and Schwalbe’sBerichtfor 1878.(189)H. N. Moseley. “On Stylochus pelagicus and a new species of pelagic Planarian, with notes on other pelagic species, on the larval forms of Thysanozoon, etc.”Quart. Journ. of Micr. Science.Vol.XVII.1877.(190)J. Müller.“Ueber eine eigenthümliche Wurmlarva a. d. Classe d. Turbellarien, etc.”Müller’sArchiv f. Anat. u. Phys.1850.(191) ——“Ueber verschiedene Formen von Seethieren.”Müller’sArchiv f. Anat. und Phys.1854.
Nemertea.
(192)J. Barrois.“l’Embryologie des Némertes.”An. Sci. Nat.Vol.VI.1877.(193)O. Bütschli.Archiv f. Naturgeschichte, 1873.(194)A. Krohn.“Ueb. Pilidium u. Actinotrocha.”Müller’sArchiv, 1858.(195)E. Desor. “Embryology of Nemertes.”Proceedings of the Boston Nat. History Society,Vol.VI.1848.(196)G. Dieck.“Entwicklungsgeschichte d. Nemertinen.”Jenaische Zeitschrift,Vol.VIII.1874.(197)C. Gegenbaur.“Bemerkungen üb. Pilidium gyrans, etc.”Zeitschrift für wiss. Zool.,Bd.V.1854.(198)C. K. Hoffmann.“Entwicklungsgeschichte von Tetrastemma tricolor.”Niederländisches Archiv,Vol.III.1876, 1877.(199) ——“Zur Anatomie und Ontogenie von Malacobdella.”Niederländisches Archiv,Vol.IV.1877.(200)W. C. McIntosh.British Annelids. The Nemerteans.Ray Society, 1873‑4.(201)Leuckart u. Pagenstecher. “Untersuchungen üb. niedere Seethiere.”Müller’sArchiv, 1858.(202)E. Metschnikoff.“Studien üb. die Entwicklung d. Echinodermen u. Nemertinen.”Mém. Acad. imp. Pétersbourg,VII.Ser., Tom.XIV. No.8, 1869.
Trematoda.
(203)T. S. Cobbold.Entozoa.Groombridge and Son, 1864.(204) ——Parasites; a Treatise on the Entozoa, etc. Churchill, 1879.(205)Filippi.Mém. p. servir à l’histoire génétique des Trématodes. Ann. Scien. Nat.4th Series,Vol.II.1854, andMem. Acad. Torino, 1855‑1859.(206)R. Leuckart.Die menschlichen Parasiten,Vol.I.1863, p.485, et seq.(207)H. A. Pagenstecher.Trematoden u. Trematodenlarven.Heidelberg, 1857.(208)C. Th. von Siebold.Lehrbuch d. vergleich. Anat. wirbelloser Thiere.Berlin, 1848.(209)J. J. S. Steenstrup.Generationswechsel.1842.(210)R. v. Willemoes-Suhm.“Zur Naturgeschichte d. Polystomum integerrimum, etc.”Zeit. f. wiss. Zool.Vol.XXII.1872.(211) ——“Helminthologische NotizenIII.”Zeit. f. wiss. Zool.Vol.XXIII.1873.Videthis paper for a summary of known observations and literature.(212)G. R. Wagener.Beiträge zur Entwicklungsgeschichte d. Eingeweidewürmer.Haarlem, 1855.(213)G. R. Wagener.“Helminthologische Bemerkungen, etc.”Zeit. f. wiss. Zool.Vol.IX.1850.(214)G. R. Wagener.“Ueb. Gyrodactylus elegans.”Archiv f. Anat. u. Phys.1860.(215)E. Zeller.“Untersuchungen üb. d. Entwicklung d. Diplozoon paradoxum.”Zeit. f. wiss. Zool.Vol.XXII.1872.(216)E. Zeller.“Untersuchungen ü. d. Entwick. u. Bau d. Polystomum integerrimum.”Zeit. f. wiss. Zool.Vol.XXII.1872.(217)E. Zeller.“Weitere Beiträge z. Kenntniss d. Polystomen.”Zeit. f. wiss. Zool.Vol.XXVII.1876.
Cestoda.
(218)Ed. van Beneden.“Recherches sur la composition et la signification d. l’œuf.”Mém. cour. Acad. roy. Belgique.Vol.XXXIV.1868.(219)P. J. van Beneden.“Les vers Cestoïdes considérés sous le rapport physiologique embryogénique, etc.”Bul. Acad. Scien. Bruxelles.Vol.XVII.1850.(220)T. S. Cobbold.Entozoa.Groombridge and Son, 1864.(221) ——Parasites; a treatise on the Entozoa, etc.Churchill, 1879.(222)Th. H. Huxley. “On the Anatomy and Development of Echinococcus veterinorum.”Proc. Zool. Soc.Vol.XX.1852.(223)J. Knoch.“Die Naturgesch. d. breiten Bandwürmer.”Mém. Acad. Imp. Pétersbourg,Vol.V.Ser.7, 1863.(224)F. Küchenmeister.“Ueber d. Umwandlung d. Finnen Cysticerci in Bandwürmer (Tænien).”Prag Vierteljahrsschr.1852.(225) ——“Experimente üb. d. Entstehung d. Cestoden.2oStufe zunächst d. Cœnurus cerebralis.” Günsburg,Zeitsch. klin. Med.IV.1853.(226)R. Leuckart.Die Menschlichen Parasiten,Vol.I.Leipzig, 1863.Videalso additions at the end of the 1st and 2nd volume.(227)R. Leuckart.“Archigetes Sieboldii, eine geschlechtsreife Cestodenamme.”Zeit. f. wiss. Zool.,Vol.XXX.Supplement, 1878.(228)El. Metschnikoff.“Observations sur le développement de quelques animaux (Bothriocephalus proboscideus).”Bull. Acad. Imp.StPétersbourg,Vol.XIII.1869.(229)W. Salensky.“Ueb. d. Bau u. d. Entwicklungsgeschichte d. Amphilina.”Zeit. f. wiss. Zool.,Vol.XXIV.1874.(230)Von Siebold.Burdach’sPhysiologie.(231)R. von Willemoes-Suhm.“Helminthologische Notizen.”Zeit. f. wiss. Zool.,Vol.XIX. XX. XXII.1869, 70 and 73.
[89]I.Turbellaria.1. Dendrocœla.2. Rhabdocœla.II.Nemertea.1. Anopla.2. Enopla.III.Trematoda.1. Distomeæ.2. Polystomeæ.IV.Cestoda.[90]It is probable, though it has not been observed, that the growth of the layer of small cells is assisted by the formation of fresh cells from the hypoblast spheres.[91]The account of Metschnikoff’s observations on Stylochopsis ponticus given in the German abstract is too obscure to be placed in the text, but the following are the more important points which can be gleaned from it.The ovum becomes first divided into eight segments. By further division along the equatorial zone, a ring of small cells is formed which becomes the epiblast. The two poles are at this time formed of large cells. At one pole four small cells appear, which are compared by Metschnikoff to the pole cells of the Diptera (videChapter on the development of the Insecta). At the opposite pole a blastopore is formed leading into a small segmentation cavity. The epiblast also now gradually grows over the large cells. At the blastopore pole the large cells give rise to the hypoblast and the small cells at the opposite pole assist in forming the epiblast. The blastopore disappears, and with it the segmentation cavity, while the hypoblast, forming a solid mass, becomes divided into two halves (Cf.Planaria Neapolitana). The embryo becomes ciliated and begins to rotate; and the eyes, and somewhat later (?) the nervous ganglion make their appearance.In the interior a wide cavity develops between the hypoblast cells, which becomes ciliated and is placed in communication with the exterior by an invaginated stomadæum which forms the pharynx.The larva now, as in Planaria Neapolitana, takes on a Pilidium-like form. Lateral lobes and an anterior lip grow out from the under surface, and become covered with long cilia, while at the upper pole a long flagellum makes its appearance.[92]Bütschli for Pilidium regards these pits as formed by invaginations of the epiblast, but Metschnikoff’s statements are in accordance with those in the text.[93]This is the view of both Metschnikoff (No.202) and Leuckart and Pagenstecher (No.201), and is further confirmed by Barrois, but Bütschli (No.193), though he has not observed the earliest stages of their outgrowth, believes them to be invaginations of the Nemertine skin.[94]O. Bütschli,“Bemerkungen üb. d. excretorischen Gefässapparat d. Trematoden.”Zoologischer Anzeiger,1879,No.42.[95]Videfor list of such forms at present known Willemoes Suhm,No.231.[96]Annal. d. Scien. Nat., 6th Series,Vol.VI.1877.[97]Beobachtungen üb. Anat. u. Entwick. Wirbell. Thiere.Leipzig, 1863.
[89]
I.Turbellaria.
1. Dendrocœla.
2. Rhabdocœla.
II.Nemertea.
1. Anopla.
2. Enopla.
III.Trematoda.
1. Distomeæ.
2. Polystomeæ.
IV.Cestoda.
[90]It is probable, though it has not been observed, that the growth of the layer of small cells is assisted by the formation of fresh cells from the hypoblast spheres.
[91]The account of Metschnikoff’s observations on Stylochopsis ponticus given in the German abstract is too obscure to be placed in the text, but the following are the more important points which can be gleaned from it.
The ovum becomes first divided into eight segments. By further division along the equatorial zone, a ring of small cells is formed which becomes the epiblast. The two poles are at this time formed of large cells. At one pole four small cells appear, which are compared by Metschnikoff to the pole cells of the Diptera (videChapter on the development of the Insecta). At the opposite pole a blastopore is formed leading into a small segmentation cavity. The epiblast also now gradually grows over the large cells. At the blastopore pole the large cells give rise to the hypoblast and the small cells at the opposite pole assist in forming the epiblast. The blastopore disappears, and with it the segmentation cavity, while the hypoblast, forming a solid mass, becomes divided into two halves (Cf.Planaria Neapolitana). The embryo becomes ciliated and begins to rotate; and the eyes, and somewhat later (?) the nervous ganglion make their appearance.
In the interior a wide cavity develops between the hypoblast cells, which becomes ciliated and is placed in communication with the exterior by an invaginated stomadæum which forms the pharynx.
The larva now, as in Planaria Neapolitana, takes on a Pilidium-like form. Lateral lobes and an anterior lip grow out from the under surface, and become covered with long cilia, while at the upper pole a long flagellum makes its appearance.
[92]Bütschli for Pilidium regards these pits as formed by invaginations of the epiblast, but Metschnikoff’s statements are in accordance with those in the text.
[93]This is the view of both Metschnikoff (No.202) and Leuckart and Pagenstecher (No.201), and is further confirmed by Barrois, but Bütschli (No.193), though he has not observed the earliest stages of their outgrowth, believes them to be invaginations of the Nemertine skin.
[94]O. Bütschli,“Bemerkungen üb. d. excretorischen Gefässapparat d. Trematoden.”Zoologischer Anzeiger,1879,No.42.
[95]Videfor list of such forms at present known Willemoes Suhm,No.231.
[96]Annal. d. Scien. Nat., 6th Series,Vol.VI.1877.
[97]Beobachtungen üb. Anat. u. Entwick. Wirbell. Thiere.Leipzig, 1863.
For many reasons a complete knowledge of the ontogeny of the Rotifera is desirable. They constitute a group which retain in the trochal disc an organ common to the embryos of many other groups, but which in most other instances is lost in the adult state. In the character of the excretory organs they exhibit affinities with the Platyelminthes, while in other respects they possibly approach the Arthropoda (e.g.Pedalion ?). The interestingTrochosphæra æquatorialisof Semper closely resembles a monotrochal polychætous larva.
Up to the present time our embryological knowledge is mainly confined to a series of observations by Salensky onBrachionus urceolaris, and to scattered statements on other larval forms by Huxley, etc.
In many cases Rotifers lay summer and winter eggs of a different character. The former are always provided with a thin membrane, and frequently undergo development within the oviduct. They are hatched in the autumn. The winter eggs are always provided with a thick shell.
The summer eggs are of two kinds,viz.smaller eggs which become males, and larger, females. On the authority of Cohn (No.232) they are believed to develop parthenogenetically. Males are not found in summer, and only seem to be produced from the summer eggs. Cohn’s observations, especially onConochilus volvox, are however not quite satisfactory. Huxley (No.234) came to the conclusion that the winter eggs of Lacinularia developed without previous fertilization.
The following are the more important results of Salensky’s observations (No.236) onBrachionus urceolaris.
The ovum is attached by a short stalk to the hind end of the body of the female, in which position it undergoes its development. It will be convenient to treat separately the development of the female and male, and to commence with the former. Thefemale ovum divides into two unequal spheres, of which the smaller in the subsequent stages segments more rapidly than the larger. The segmentation ends with the formation of an epibolic gastrula. The solid inner mass of cells derived from the larger sphere constitutes the hypoblast, and is more granular than the epiblast. The evolution of the embryo commences with the formation of a depression on the ventral surface, at the bottom of which the stomodæum is formed by an invagination. At the hinder part of the depression there rises up a rounded protuberance which eventually becomes the caudal appendage or foot. Immediately behind the mouth is formed an underlip.
On the sides of the ventral depression are two ridges which form the lateral boundaries of the trochal disc. They appear to unite with the under lip.
In a later stage the anterior part of the body becomes marked off from the posterior as a præoral lobe, and the hypoblast is at the same time confined to the posterior part. The supra-œsophageal ganglion is early formed as an epiblastic thickening on the dorsal side of the præoral lobe.
The first cilia to appear arise at the apex of the præoral lobe. At a later period the lateral ridges of the trochal disc meet dorsally and so enclose the præoral lobe. They then become coated by a ring of cilia, to which a second ring, completing the double ring of the adult, is added later.
Embryo of Brachionus urceolarisFig. 100. Embryo of Brachionus urceolaris shortly before it is hatched.(After Salensky.)m.mouth;ms.masticatory apparatus;me.mesenteron;an.anus;ld.lateral gland;ov.ovary;t.tail,i.e.foot;tr.trochal disc;sg.supra-œsophageal ganglion.
Fig. 100. Embryo of Brachionus urceolaris shortly before it is hatched.(After Salensky.)
m.mouth;ms.masticatory apparatus;me.mesenteron;an.anus;ld.lateral gland;ov.ovary;t.tail,i.e.foot;tr.trochal disc;sg.supra-œsophageal ganglion.
In the trunk an indication of a division into two segments makes its appearance shortly after the development of the præoral lobe. Before this period the proctodæum is established as a shallow pit immediately behind the insertion of the foot. The latter structure soon becomes pointed and forked (fig. 100,t).
The complete establishmentof the alimentary canal occurs late. The stomodæum (fig. 100) gives rise to the mouth (m), œsophagus and masticatory apparatus (ms). The mesenteron is formed from the median part of the hypoblast; the lateral parts of which appear to give rise to the great lateral glandular structures (ld) which open into the stomach, and to the ovaries (?) (ov) etc. The proctodæum becomes the cloaca and anus (an). The origin of the mesoblast is not certainly known. The shell is formed before the larva is hatched—an occurrence which does not take place till the larva closely resembles the adult.
The early developmental stages of the male are closely similar to those of the female; and the chief difference between the two appears to consist in the development of the male being arrested at a certain point.
The larvæ of Lacinularia (Huxley,No.234) are provided with a præoral circlet of cilia containing two eye-spots[98], and a peri-anal patch of cilia. They closely resemble some telotrochal polychætous larvæ.
Salensky has compared the larva of Brachionus to that of a cephalophorous Mollusc, more especially to the larva of Calyptræa on which he has made important observations. The præoral lobe, with the ciliated band, no doubt admits of a comparison with the velum of the larva of Molluscs; but it does so equally, as was first pointed out by Huxley, with the ciliated præoral lobe of the larvæ of many Vermes. It further deserves to be noted that the trochal disc of a Rotifer differs from the velum of a Mollusc in that the eyes and ganglia are placed dorsally to it, and not, as in the velum of a Mollusc, within it. The larva of Lacinularia appears to be an exception to this, since two eye-spots are stated to lie within the circlet of cilia. More important in the comparison is the so-called foot (tail), which arises in the embryo as a prominence between the mouth and anus, and in this respect exactly corresponds with the Molluscan foot.
If Salensky’s comparison is correct, and there is something to be said for it, the foot or tail of Rotifers is not a post-anal portion of the trunk, but a ventral appendage, and the segmentationwhich it frequently exhibits is not to be compared with a true segmentation of the trunk. If the Rotifers, as seems not impossible, exhibit crustacean affinities, the ‘foot’ may perhaps be best compared with the peculiar ventral spine of the Nauplius larva ofLepas fascicularis(videChapter on Crustacea) which in the arrangement of its spines and other points also exhibits a kind of segmentation.
Bibliography.
(232)F. Cohn.“Ueb. d. Fortpflanzung von Räderthiere.”Zeit. f. wiss. Zool.Vol.VII.1856.(233)F. Cohn.“Bemerkungen ü. Räderthiere.”Zeit. f. wiss. Zool.Vol.IX.1858, andVol.XII.1862.(234)T. H. Huxley.“Lacinularia socialis.”Trans. of the Microscopical Society, 1853.(235)Fr. Leydig.“Ueb. d. Bau u. d. systematische Stellung d. Räderthiere.”Zeit. f. wiss. Zool.Vol.VI.1854.(236)W. Salensky.“Beit. z. Entwick. von Brachionus urceolaris.”Zeit. f. wiss. Zool.Vol.XXII.1872.(237)C. Semper.“Zoologische Aphorismen. Trochosphæra æquatorialis.”Zeit. f. wiss. Zool.Vol.XXII.1872.
[98]In Leydig’s figure of the larva,Zeit. f. wiss. Zool.Vol.III.1851, the eye-spots lie just outside the ciliated ring.
[98]In Leydig’s figure of the larva,Zeit. f. wiss. Zool.Vol.III.1851, the eye-spots lie just outside the ciliated ring.
Although the majority of important developmental features are common to the whole of the Mollusca, yet at the same time many of the subdivisions have well-marked larval types of their own. It will for this reason be convenient in considering the larval characters to deal successively with the different subdivisions, but to take the whole group at once in considering the development of the organs.
Formation of the layers and larval characters.
Odontophora.
Gasteropoda and Pteropoda.There is a very close agreement amongst the Gasteropoda and Pteropoda in the general characters of the larva; but owing to the fact that the eggs ofthe various species differ immensely as to the amount of food-yolk, considerable differences obtain in the mode of formation of the layers and of the alimentary tract.
The spheres at a very early stage of segmentation[100]become divided into two categories, one of them destined to give rise mainly to the hypoblast, the other mainly to the epiblast. According as there is much or little food-yolk the hypoblast spheres are either very bulky or the reverse. In all cases the epiblast cells lie at one pole, which may be calledthe formative pole, and the hypoblast cells at the opposite pole. When the bulk of the food-yolk is very great, the number of hypoblast spheres is small. Thus in Aplysia there are only two such spheres. In other cases, where there is but little food-yolk, they may be nearly as numerous as the epiblast cells. In all these cases, however, as was first shewn by Lankester and Selenka, a gastrula becomes formed either by normal invagination as in the case of Paludina (fig. 107), or by epibole as inNassa mutabilis(fig. 105). In both cases the hypoblast becomes completely enclosed by the epiblast.The blastopore is always situated opposite the original formative pole.In the large majority of cases (i.e.Marine Gasteropoda, Heteropoda, and Pteropoda) the blastopore becomes gradually narrowed to a circular opening which eventually occupies the position of the mouth. It either closes or remains permanently open at this point. In some cases the blastopore remains permanently open and becomes the anus. The best authenticated instance of this isPaludina vivipara, as was first shewn by Lankester (No.263).
In some instances the blastopore assumes before closing a very narrow slit-like form, and would seem to extend along the future ventral region of the body from the mouth to the anus. This appears, according to Lankester (No.262), to be the condition in Lymnæus, but while Lankester believes that the closure proceeds from the oral towards the anal extremity, other investigators hold that it does so in the reverse direction. Fol (No.249) has also described a similar type of blastopore. In an undetermined marine Gasteropod, with an embolic gastrula, observed by myself at Valparaiso, the blastopore had the same elongatedform as in Lymnæus, but the whole of it soon became closed except the oral extremity; but whether this finally closed could not be determined. It is probable that the typical form of the blastopore is the elongated form observed by Lankester and myself, in which an unclosed portion can indifferently remain at either extremity; and that from this primitive condition the various modifications above described have been derived[101].
Before the blastopore closes or becomes converted into the oral or anal aperture, a number of very important embryonic organs make their appearance; but before describing these it will be convenient to state what is known with reference to the third embryonic layer or mesoblast.
This layer generally originates in a number of cells at the lips of the blastopore, which then gradually make their way dorsalwards and forwards, and form a complete layer between the epiblast and hypoblast. The above general mode of formation of the mesoblast may be seen infig. 107, representing three stages in the development of Paludina.
In some cases the mesoblast arises from certain of the segmentation spheres intermediate in size between the epiblast and hypoblast spheres. This is the case inNassa mutabilis, where the mesoblast appears when the epiblast only forms a very small cap at the formative pole of the ovum; and in this case the mesoblast cells accompany the epiblast cells in their growth over the hypoblast (fig. 105).
In other cases the exact derivation of the mesoblast cells is quite uncertain. The evidence is perhaps in favour of their originating from the hypoblast. It is also uncertain whether the mesoblast is bilaterally symmetrical at the time of its origin. It is stated by Rabl to be so in Lymnæus[102].
In the case of Paludina the mesoblast becomes two layersthick, and then splits into a splanchnic and somatic layer, of which the former attaches itself to the hypoblast, and gives rise to the muscular and connective-tissue wall of the alimentary tract, and the latter attaches itself to the epiblast, and forms the muscular and connective-tissue wall of the body and other structures. The two layers remain connected by protoplasmic strands, and the space between them forms the body cavity (fig. 107). In most instances there would appear to be at first no such definite splitting of the mesoblast, but the layer has the form of a scattered network of cells between the epiblast and the hypoblast. Finally certain of the cells form a definite layer over the walls of the alimentary canal, and constitute the splanchnic mesoblast, and the remaining cells constitute the somatic mesoblast.