180 
THE HELMINTHES. 
§ 119. 
§ 119. 
With the Nematodes, of which very many are viviparous, the embryos 
are developed within the egg in two different ways: Hither the embryo- 
movements (Baer, in the Nov. Act. Acad. Leop. 
XIII. pt. 2, p. 570, Tab. XXX.). Those of Dis- 
tomum duplicatum have simple, oval, and rigid 
germinative utricles (Baer, Ibid. p. 558, Tab. 
XXIX.). Those of Cercaria ephemera, are also 
very simple, but.of a cylindrical form (Sieboéd, in 
Burdach’s Phys. loc. cit. p. 187, and Steenstrup, 
loc. cit. p. 78, Taf. III. fig. 1-6), Those of Cer- 
caria furcata are simple and cylindrical, but very 
jong and endowed with quite active peristaltic 
motions (Baer, loc. cit. p. 626, Tab. XX XI. fig. 6). 
The curious animal, Leucochloridium paradox- 
um, consisting of only a cylindrical sac with a 
tail, is only a trematode larva (Carus, in the Nov. 
Act. Acad. Leop. XVII. pt. 1, p. 85, Tab. VII.). 
ith the slow-moving, cylindrical, orange-colored 
- nurses of Cercaria ephemera, there may be easily 
seen a mouth, a pharynx, and a simple coecal 
intestine (Siebold, in Burdach’s Phys. loc. cit. p. 
187). Those of Cercaria echinata, are similar, 
but they have also two short oblique prolongations 
in front of the obtuse caudal extremity (Baer, loc. 
cit. p..629, Tab. 31, fig. 7,and Steenstrup, loc. cit., 
p. 51, Taf. II. fig. 2-4). The germinative bodies 
from which Cercaria is developed, have nothing 
comparable to a chorion or germinative vesicle. 
Their larvae have always a tail, which is simple 
{Cercaria armata, ephemera, Distomum dupli- 
catum), or bifurcated (Cercaria furcata), or 
double (Bucephalus polymorphus). The move- 
ments of this tail are very slow with Distomum 
duplicatum, but extremely lively and vortical 
with Cercaria. With Bucephalus, the two fili- 
form tails lengthen and shorten considerably, at the 
asame time jerking all about. 
‘When the larvae are developed, they leave the 
corpuscles and pass into other animals to complete 
their final metamorphoses. Many Cercariae appear 
to prefer the larvae of insects whose bodies they en- 
ter by means of their cephalic hooks. In this way 
I have seen the Cercaria armata easily enter the 
larvae of Ephemera, Nemura,and Perla. By the 
aid of its sting itcan perceive the intersegmental 
membrane of these larvae. Frequently it loses its 
tail in’ passing through a narrow opening it has 
made, 
I diately upon r the cavity of the 
body of the larva, it is surrounded by a vesicular 
membrane, in which the sting is rejected, and the 
animal enters upon its final metamorphosis. But 
I have a doubt whether it is there completed, for 
among the numerous similar parasites which I have 
found in the most different insects whose larvae are 
ni 
* [§ 118, note 7.] In this connection should be 
noticed the remarkable phenomena of reproduction 
with Gyrodactylus a8 recently observed by Sie- 
bold (Siebold and Kélliker’s Zeitsch. I. 1849, p. 
345). Individuals are here developed viviparously 
as in the so-called alternating generations, and 
Siebold has observed a mother in which was a 
daughter and in this last a grand-daughter, the 
series being therefore three-fold. These viviparous 
individuals contain no sexual organs proper, but 
the new individual is developed out of a group of 
cells situated within the body. The whole repro- 
ductive conditions which Siebold has detailed with 
his usual care appear to me to closely resemble 
those of the viviparous Aphides which I have 
aquatic, as of Libellula. Agrion, Ephemera, and 
Phryganea, I have never met with one whose - 
genital organs were in a state of advanced develop- 
ment. er 
The full development of these organs, the deli- 
cate contours of which may be seen while the par- 
asites are in the bodies of these animals, is not 
perhaps attained, until the insects have been 
swallowed by birds and other animals, — being 
thereby furnished with more proper conditions for 
their complete formation. 
Some Cercariae lose their tail and are surrounded 
with a capsule without leaving the Mollusks which 
are their first habitat. This is probably so, be- 
cause these Mollusks are liable to be eaten by 
aquatic birds, in which these parasites may prop- 
erly reach their final development. It should, 
however, be remarked that when these larvae be- 
come chrysalides, their investing capsule or cyst, 
is a secretion from their bodies, and not a product 
of the animals in which they live. It is probable 
that very many of these larvae never attain a per- 
fect state, for, in their migrations, they fail to.reach 
their destined and final habitat. 
These migrations undoubtedly occur with many 
Cestades while young ; at least Miescher (loc. cit.) 
has observed it with Tetrarhynchus. But although 
we have followed these in their migrations, and the 
transformation of many of them into Monostomum 
and Distomum has been observed, and therefore 
the completion of their metamorphoses, yet we are 
but slightly informed as to their beginning by the 
alternation of generation. 
There is yet little known as to the manner in 
which these embryos are changed into the cylin- 
dric nurses. There are now only two isolated facts 
throwing light upon this point. According to my 
own observations (Wiegmann’s Arch. 1835, I. 
p. 75, Taf. I.), each embryo of Monostomum 
mutabile .contains a germinative tube, which, at 
the death of the embryo, is freed and quite resem- 
bles the nurse of Cercaria echinata. I have also 
observed in the embryos of Amphistomum gee 
clavatum a tubular body, but I could not satisfy 
myself of its germinative nature. According to 
Steenstrup (loc. cit. p. 98), there is an animal like 
a Paramaecium, and probably an embryo of & 
Distomum, living in Muscles, and which finally 
is deprived of its epithelium, and changed into the 
rigid, germinative tube of Distomum duplicatum ; 
see upon this, my Jahresbericht in Wiegmann’s 
Arch. 1843, II. p. 300.* 
recently investigated ; and I believe this mode of 
reproduction to be only a peculiar form of gemmi- 
parity or budding suited to some ulterior, econom- 
ical purpose of the animal’s life. On a future page 
T shall speak more fully on this point and attempt 
to show that the whole set of phenomena known 
under the name of “ Alternation of Generations” 
is, when divested of its paraphernalia, only a kind 
of Gemmiparity. 
See also for further details on that curious ani- 
mal ZL hloridium par » Piper, in 
Wiegmann’s Arch. 1851, I. p. 318, but especially 
Siebold, in Siebold and Kélliker’s Zeitsch. IV. 
1853, p. 425, Taf. XVI. B. This last-named ob- 
server has shown that this animal form is only a 
