^118. 



THE HELMINTHES. 



129 



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. *^^ 



The embryos of the Acanthocephali are perhaps developed in the same 

 manner, but they have only four hooks. ^^' 



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. *^' 



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.® 

 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.'"* 



3 For the embryonic development of Bothrioce- 

 pkalus, and Taenia, see Siebold {Burdack^s 

 Phys. loc. cit. p. 200), Dujardin (Ann. d. Sc. 

 Nat. X. 1838, p. 29, PI. I. fig. 10, also XX. 1843, 

 p. 341, PI. XV., and his Hist. d. Helm. PI. IX.- 



• Xn.), and KuUiker {Mailer'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 aU Echinorhynclius 

 have them ; at least Dujardin has not found them 

 with those of Echinorhynchus transversus, and 

 globocaudatus (Hist. d. Helm. PI. A'll.). 



■5 For the embryonic development of Monos to- 

 mum, and Distomum, see Siebold (JiurdacVs 

 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 o{ Distomum hians, by Meh- 

 lis (Isis 1831, p. 190) ; ot Distomum nodulosum 

 and globiporum, by Nordmann and Creplin 

 (Microgr. Beitr. Hft. 2, p. 139, and in Ersch and 

 Gruber's Encyclop. XXIX. 1837, p. 324) ; of 

 Distomum cygnotdes, longicolle, Amphisto- 

 mu?nsubclavatum,a.nil Monostomum mutabile, 

 by myseM {rV(egmann''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 ot" Distomum tereticoUe, and Aspido- 

 gaster conchicola, without the ciUated epithelium. 



* f § 118, note 3.] The history of all oui- 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). 



fi In this category are the genera Cercaria, 

 Jfistrionella, 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 Amphistomum ropaloides, is only 

 a larva of a Tetrarhynchus. The species forming 

 the genus Scolex are certainly only imperfect 

 Bothriocephalus ; and the Grypoi hynchus pu- 

 sillus 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,v/hi\e 

 the Rodents in which it lives are devoiu-ed by car- 

 nivorous animals. Taenia crassicollis is, per- 

 haps, to Cysticercus fasciolaris, 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 hy Steenstrup (loc. cit. p. 50) nurses 

 (Ammen). They are yet known only as Uving 

 parasitically upon Mollusks, as for instance, upon 

 Paludina, Lymnaeus, Planorbis, Ancylus, Suc- 

 cinea, Anodonta, and Unio ; also upon Helix 

 pomatia, and Tellina baltica, according to Boja- 

 nus, Baer, Cams, Steenstrup, and myself. The 

 cylindric larvae of Bucephalus polymorphus, 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. 



