128 THE HELMINTHES. § 118. 



The few observations hitherto made upon the genital organs of the Gor- 

 diacei have shown that they are wholly tubular as in the Nematodes. But 

 their intimate structure, and the development of their spermatic particles 

 are so strikingly different, that this point alone would justify their separ- 

 ation from the Nematodes. ^^°^ 



§ 118- 



With the exception of the Nematodes, and Gordiacei, the development of 

 all Helminthes, which reproduce by means of genital organs and eggs, is 

 metamorphotic. A complete series, from beginning to end of these meta- 

 morphoses has yet never been observed with any species. From the separate 

 parts of it here and there which have been observed, there appears the 

 remarkable fact, that the embryos after escaping the egg, are not always 

 changed at the end of the metamorphosis, into individuals like the parent, but 

 appear as larva-like animals, capable in their turn of producing other larvae. 

 These last larvae alone, change into individuals, which are like the parent. 



This particular kind of transformation and development which is quite 

 common among the Trematodes, has received the name oi Alternate Gene- 

 7'ation.^'^^ Whether it occurs among the Cestodes and Acanthocephali, 

 cannot now bo stated positively, for as yet we are unacquainted with the 

 first period of their metamorphosis, — the embryo as it escapes from the 

 egg.'-' In many Cestodes and Trematodes, the embryos are developed 

 before the eggs are cast, and in some of the last order, they make their 

 escape while the eggs are in the uterus. 



The development of the Cestodes occurs as follows : After the disap- 

 pearance of the germinative vesicle, large, transparent embryonic cells 

 appear in the midst of the vitellus, which undergoes fissuration. These 

 multiply by division, increasing at the expense of the vitellus, which in the 



tightly to the vulva of the female in this act, that opening is always at the posterior extremity of the 



they cannot disengage themselves {Siebold and body. The testicular tubes of Gordius aquaticus 



Natliusius, in Wiegmann^s Arch. 1836, 1, p. 105, contain anteriorly, cell-like bodies ; but posteriorly 



Taf. III. 1S37, I. p. 60, 66). With many other there are others, staff-like, and which, being found 



species of Strongylus, and Ascaris, it is not rare among the eggs in the uterine tube, I have regarded 



to find a brownish gum about the vulva, and in as perfect spermatic particles. The genital open- 



which there is, sometimes, the very distinct impress ing of the male Gordius is between the two more 



of the male caudal valve (jl/eAto,Isis,1831, p. 87).* or less prominent lobes of the caudal extremity, 



111 In the genus Mermis formed by Dujardin, and is without copulatory organs. The simple, 

 the tubular uterus, the muscular vagina, and the round, colorless eggs, are bound together at the 

 vulva situated far from the caudal extremity, — all posterior pai't of the uterus by an albuminous sub- 

 remind one much of the Nematodes. The eggs of stance, and are deposited in a very long row. It 

 Mermis nigrescens, like those of Ascaris den- is this row of eggs which Li!on Dufour has 

 ta<a, have long fibrillated appendages {Dujardin described as Filaria Jilariae (Ann. d. Sc. Nat. 

 Ann. d. Sc. Nat. 1842, XVIII. p. 133, PI. VI., and XIV. 1828, p. 222, PI. XII. fig. 4). 

 Sieliold, in fFiegmann''s Arch. 1843, II. p. 309) ; 1 See Steenstrup, Ueber den Generationswech- 

 and at the caudal extremity of the males of Mermis sel, &c., 1842. 



albicans, raihi (Entom. Zeit. 184:^, p. 79), there - In various marine fish theft; is a trematode 



are, as in most Nematodes, two horny penises. larva of a Tetrarhynckus (Miescher, Bericut 



But with Gordius, the structure of the genital ueber die Verhandl. d. Naturforsch. Gesellsch. ii^ 

 organs is very ditl'erent (see Siebold, and Dujar- Basel. 1840, p. 29, and in IViegmann''s Arch. 

 din, loc. cit.). In both sexes the cavity of the 1841, II. p. 302), which would lead one to con- 

 body is completely filled with a double genital elude thiit alternate generation exists also with tlie 

 tube, straight, and simple posteriorly, the sides Cestodes.t 

 of which aie formed of large cells. The genital 



* [§ 117, note 9.] For many details of the re- the alternating generation of the Cestodes, has 



productive organs of Ascaris infecta,^i\h beauti- recently been confirmed most thoroughly by Sie- 



ful illustrative figures, see Leidy, A Flora and bold, who has treated the subject in a most com- 



Fauna, &c., loc. cit. 4 B. PI. VII. 14, 16, b. 19. — prehensive manner, in a Memoir in Siebold and 



Ed. Kiilliker^s Zeitsch. II. 1850, p. 198. — Ed. 



t 1 § 118, note 2.] The view here suggested of 



