THREAD- WORMS. 
449 
days in the grub or maggot of the cockchafer and allied beetles. These latter are 
rooted up and devoured by pigs, which thus unconsciously swallow the worm. 
Similarly, E. proteus of various fish lives in an immature state in the intestine of 
the water-shrimp, which swallowed it while still in the egg; and E. moni- 
liferus , which occurs adult in such rodents as hamsters and voles, lives during 
the larval state in beetles. Another species, E. polymorphous, has to be transplanted 
from the body of the water-shrimp into that of a duck to reach maturity. 
Typical Thread-Worms, —Order Nematoidea. 
The illustration showing the growth and structure of one of the thread-worms 
( Nematoxys ), an internal parasite of the frog, is intended to show the mode of 
development typical of the whole group. The egg is elliptical, and contains a 
mass of granular protoplasm, the external wall of which soon becomes marked out 
into a layer of large cells. Meanwhile, there appears at the side a distinct notch 
or nick, which, shallow at first, gradually deepens, until, as shown in the figures, 
it represents the space enclosed between the head- and tail-ends of the bent-up 
embryo, which may be recognised respectively by their blunt and pointed 
front end of thread-worm, Enoplus (much enlarged). 
extremities. The external layer of cells becomes transformed into the cuticle, and 
the mouth appears as a depression at the end of the blunt head. When the 
muscular system and alimentary canal are developed, the embryo hatches in the 
form shown in the bottom right-hand figure. Most of the species lead a parasitic 
life, chiefly in animals; many, however, are free-living forms, occurring in damp 
earth, fresh water, and the sea. A genus, with marine habits, has received the 
name of Enoplus, and includes small, slim, transparent creatures, some of which 
are provided at the front end with isolated bristles (as shown in the illustration 
above), while many are furnished with a peculiar spinning - gland, opening 
beneath the tail. According to Schneider, “ as soon as the animal has fixed its 
tail to something it moves on, and draws after it the secretion in a transparent 
thread, which is often several lines long. One end of this thread sticks fast, and by 
the other the animal floats freely in the water.” The young are found in shallow 
water, and may be seen crawling on the surface of seaweeds; but the mature animals 
occur at depths of from two to three fathoms. As another example of lion-parasitic 
species, we may take the common vinegar-eel ( Anguillida ), the magnified figure 
of which shows that the body is bluntly rounded at the head-end, and narrowed 
and pointed at the tail. The greater part of the body-cavity is occupied by the 
alimentary canal, which traverses it almost from end to end. The oval particles 
contained in two tubes, which unite and open by a common orifice, are the eggs. 
This worm appears to live both in vinegar and paste, although it does not seem 
to derive its nourishment directly from either of these substances, but rather from 
vol. vi.—29 
