Comparative anatomy / by C.Th. v. Siebold and H. Stannius ; translated from the German, and edited with notes and additions recording the recent progress of the science by Waldo I. Burnett.

  • Karl Theodor Ernst von Siebold
Date:
1854
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Licence: Public Domain Mark

Credit: Comparative anatomy / by C.Th. v. Siebold and H. Stannius ; translated from the German, and edited with notes and additions recording the recent progress of the science by Waldo I. Burnett. Source: Wellcome Collection.

Provider: This material has been provided by The University of Glasgow Library. The original may be consulted at The University of Glasgow Library.

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    end they completely replace. When this has taken place, there is a mass of extremely small cells, which, being covered with a delicate epithelium, form a round or oval embryo, upon one extremity of which there are grad- ually formed six small horny hooks.<S) The embryos of the Acanthocephali are perhaps developed in the same manner, but they have only four hooks.(4) The Trematodes are developed exactly like the Cestodes, excepting that their oval embryos have usually ciliated epithelium, and there is an oral sucker in place of the hooks.(5) Beside this first period of development, or embryonic state, there are other more advanced or larval states, during which many Helminthes have been described and figured as separate species in the science.*6' Among these may be especially noticed two forms of the Trematodes — the cylindrical and the cercarian larvae. The first (the germinative tubes of Baer), form one of the phases of the alternate generation, and have a more or less complete organization. In the cavity of their body, germinative corpuscles are formed; these consist of a vesicular, granular substance, and resemble eggs neither by their structure nor mode of development. These corpuscles produce larvae of a cylindrical or cercarian form, which, deprived of their tail, are changed into perfect animals which have genital organs; and thus the series of metamorphoses is terminated.<7) 3 For the embryonic development of Bothrioce- phalus, and Taenia, see Siebold (Burdach's Phys. loc. cit. p. 200), Dujardin (Ann. d. Sc. Nat. X. 1838, p. 29, PI. I. fig. 10, also XX. 1843, p. 341, Pl. XV., and his Hist. d. Helm. PI. IX.- XII.), and Kolliker (Muller's Arch. 1843, p. 91, Taf. VII. fig. 44-56). The small hooks which the cestoid embryos so actively protrude and retract, somewhat resemble those which are circularly arranged with the adult Taenia* 4 As yet, with Echinorhynchus gigas alone have I succeeded in liberating the embryos from the egg by compression. The four hooks of these embryos resemble, by their form and position, those of the Cestoid embryos. It does not appear, how- ever, that the embryos of all Echinorhynchus have them ; at least Dujardin has not found them with those of Echinorhynchus transversus, and globocaudatus (Hist. d. Helm. PI. VII.). 5 For the embryonic development of Monosto- mum, and Distomum, see Siebold (Burdach's Phys. loc. cit. p. 206), and Kolliker (Muller's Arch. loc. cit. p. 99). The embryos which swim about like Infusoria by means of ciliated epithelium, and which escape the egg while yet in the uterus, have been observed of Distomum hians, by Meh- lis (Isis 1831, p. 190) ; of Distomum nodulosum and globiporum, by Nordmann and Creplin (Microgr. Beitr. Hft. 2, 139, and in Ersch and Gruber's Encyclop. XXIX. 1837, p. 324) j of Distomum cygnoides, longicolle, Amphisto- mum subclavatum, and Monostomum mutabile, by mj'self (Wiegmann's Arch. 1835, I. p. 66, Taf. I.). See also Dujardin, in the Ann. d. Sc. Nat. VIII. 1837, p. 303, PI. IX. fig. 3. I have seen the embryos of Distomum tereticolle, and Aspido- gaster conchicola, without the ciliated epithelium. * f § 118, note 3.] The history of all our best embryological studies shows that the segmentation of the vitellus is the invariable preface to the be- ginning of development with all true ova. In the case of the Cestodes, if, as above mentioned, there is no such process, it is highly probable that such Those of Distomum. longicolle, cygnoides, Mon- ostomum mutabile, and Aspidogaster conchi- cola:, have an oral sucker. In this last species, there is another sucker also, at the posterior ex- tremity of the body (Dujardin, Hist. d. Helm. p. 325). 6 In this category are the genera Cercaria, Histrionella, Bucephalus and others, which as yet have been founded only upon different species of Trematode larvae. The Helminth described by Leblond (Ann. d. Sc. Nat. VI. 1836, p. 289, PI. XVI. fig. 3) as Ampliistomum ropalo'ides, is only a larva of a Tetrarhynchus. The species forming the genus Scolex are certainly only imperfect Bothriocephalus; and the Gryporhynchus pu- si.llus of Nordmann (Micr. Beitr. Hft. I. p. 101, Taf. VIII. fig. 6, 7), is probably only a young Taenia. There may also be a doubt here, if the Cystici can be considered as real species. It is very probable that they are imperfect Ces- todes whose genital organs are to be afterwards developed, as with Cysticercus fasciolatus, while the Rodents in which it lives are devoured by car- nivorous animals. Taenia crassicollis is, per- haps, to Cysticercus fasciolar is, what Bothrioce- phalus nodosus is to Bothriocephalus solidus-, see Creplin, Nov. Observ. &c. p. 90. 7 The cylindric larvae of the Trematodes have been termed by Steenstrup (loc. cit. p. 50) nurses (Ammen). They are yet known only as living parasitically upon Mollusks, as for instance, upon Paludina, Lymnaeus, Planorbis, Ancylus, Suc- cinea, Anodonta, and XJnio; also upon Helix pomatia, and Tellina baltica, according to Boja- nus, Baer, Carus, Steenstrup, and n^'self. The cylindric larvae of Bucephalus polymorphic, are very long tubes, varicose here and there, some- times ramified, and which do not exhibit any development is not from true eggs but rather from buds, a view which is the more worthy of attention from the recent developments made by Siebold with Gyrodactylus; see below, my note under § 118, note 7. — Ed.