$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. 
8 For the embryonic development of Bothrioce- 
phalus, and Taenia, see Siebold (Burdach’s 
Phys. loc. cit. p. 200), Dujardin (Ann. d. Se. 
Nat. X. 1838, p. 29, Pl. I. fig. 10, also XX. 1843, 
P 341, Pl. XV., and his Hist. d. Helm. Pl. 1X.- 
II.), and Kwlliker (Miller'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. Pl. VII.). 
5 For the embryonic development of Monosto- 
mum, and Distomum, see Siebold (Burdach’s 
Phys. loc. cit. p. 206), and Kélliker (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- 
dis (Isis 1831, p. 190) ; of 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 cygnoides, longicolle, Amphisto- 
mum subclavatum, and Monostomum mutabile, 
by myself (Wiegmann’s Arch. 1835, I. p. 66, Taf. 
I.). See also Dujardin, in the Ann. d. Sc. Nat. 
III. 1837, p. 303, Pl. LX. fig. 3. I haveseen the 
embryos of Distomum tereticolle, and Aspido- 
gaster conchicola, without the ciliated epithelium. 
*[§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. 
by 
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, Pl. 
XVI. fig. 3) as Amphistomum ropalotdes, is only 
a larva of a Tetrarhynchus. The species forming 
the genus Scolex are certainly only imperfect. 
Bothriocephalus ; and the Gryporhynchus 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, while 
the Rodents in which it lives are devoured 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 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 Unio; also upon Helix 
pomatia, and Tellina baltica, according to Boja- 
nus, Baer, Carus, 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. — Ep. 
