History of the Nematode Worms. 341 
more than 1] inch), and they often occur in great numbers, they 
have long been known and introduced in various parts of the 
helminthological system under different names (Filaria piscium, 
Ascaris capsularis, &c.). They belong (as indeed is proved by 
the varying form and size of the boring-tooth) to several different 
species, and may be developed, in predaceous fishes, sea-birds, 
seals, and dolphins, into well-known Ascarides (perhaps A. aucta, 
A. spiculigera, A. osculata, &e.). At any rate, the final develop- 
ment, as in Cucullanus, only takes place after their transference 
to another host. 
That the intermediate form of Cucullanus is free, and not 
encapsuled like the young Ascarides just mentioned, can hardly 
be regarded as an important distinction between them. This 
depends rather upon the nature of the host and of the organ ° 
inhabited than upon the peculiarities of the parasite, as we shall 
see immediately from another example. 
In Cucullanus (and probably also in A. incisa, &c.) a trans- 
formation takes place in the embryos during their residence in 
the intermediate bearer. They do not grow, but change their 
structure, especially that of the mouth. But this is not always 
the case throughout the Nematoda. 
We can hardly examine an example of the bleak (Leuciscus 
alburnus), so common in our brooks, without finding in the 
mesentery and liver numerous small capsules (up to 1 millim. in 
diameter), each of which encloses a young roundworm. _ It 
is probably the worm referred to in Diesing’s ‘Systema Hel- 
minthum’ as a Trichina (T. cyprinorum). It varies in size 
from 0°6 to nearly 2 millims., but otherwise presents the same 
characters—a slender body, short tail, wide mouth, with a 
strongly prominent boring-tooth on the ventral side, and a 
muscular pharynx with a long glandular cecum. The only 
difference that I could detect between the smaller and larger 
specimens consisted in the last-mentioned cecum, which is less 
developed, not only absolutely but relatively, m the smaller 
worms ; so that it may readily be supposed to have been deve~ 
loped from the posterior end of the cesophagus only after immi- 
gration into the bleak. But that the worm undergoes no other 
changes is certain; the characters above indicated are found not 
only in the largest specimens, but also in those which have died 
in their capsules, the number of which is usually by no means 
small. 
As to the course taken by the worms in their immigration 
there can be no doubt, when we find that they are by no means 
rare even in the intestine of the bleak. And we do not meet 
only with small worms in this situation, but also with half-grown 
animals of 1 millim. and more, which sufficiently proves that 
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