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. polymorphus, 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 op 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 non-parasitic 

 species, we may take the common vinegar-eel (Anguillula), 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 



