THE NEMATOIDEA. 551 



tary canal of Man, of the Pig, and other mammals ; but the 

 young, set free in the alimentary canal, bore their way through 

 its walls, and enter the fibres of the voluntary muscles, in 

 which they become encysted in the sexless state. If the flesh 

 thus trichinized be eaten, the Trichince are set free, acquire 

 their sexual state in the alimentary canal, and the thousands 

 of embryos which are developed immediately bore their way 

 into the extra-alimentary tissues of their host. 



The insect parasites, Gordius and Mermis, are sexless 

 so long as they are parasitic ; but, when they have attained 

 their full growth, they leave the body of their host, acquire 

 sexual organs, copulate, and lay eggs. From these, embryos 

 proceed, which bore their way into the bodies of insects. 



It has been stated that the JSFematoidea are, for the most 

 part, dioecious. Schneider has, however, discovered certain 

 species of the nonparasitic genera, Leptodera and Pelodera, 

 which always have the external appearance of females, but in 

 the ovarian tubes of which spermatozoa are developed, and 

 impregnation takes place. This was placed beyond doubt by 

 isolating embryos of these Nematoids, and tracing out the 

 development of the spermatozoa, which result from the sub- 

 division of the first cells developed from the rhachis. After 

 a time, the development of spermatozoa ceases, and the cells 

 separated from the rhachis become ova, which are impregnated 

 by the already formed spermatozoa. These Nemaioidea are 

 probably the most complete and necessary hermaphrodites 

 known in the animal kingdom. 



Ascaris nigrovenosa is parasitic in the lungs of Frogs and 

 Toads, and attains a length of three-quarters of an inch. It 

 has the characters of a female, and no male has ever been met 

 with, but spermatozoa are developed in the ovaries in the same 

 manner as in the preceding forms. 



The eggs of this Ascaris are discharged, and the embryos 

 find their way into the intestines of the Amphibian in which 

 they are parasitic. Here they become males and females 

 which are very much smaller than the hermaphrodite form 

 (not exceeding one-twentieth of an inch in length), and other- 

 wise different from it. They are evacuated with the fasces of 

 the frog, and passing into damp earth or mud, the females 

 give rise to a few eggs. Embryos are developed from these 

 eggs within the body of the mother, the organs of which they 

 destroy, until her cuticle forms a mere case for them. The 

 free embryos, introduced into the Frog's mouth, pass into the 

 lungs, and take on the characters of the large hermaphrodite 



