550 THE ANATOMY OF INVERTEBRATED ANIMALS. 



state only among putrefying vegetable or animal matters. 

 The sexless worms, which live in moist earth, are at once at- 

 tracted hy nutriment, such as a few drops of milk. 1 Here 

 they multiply with great rapidity as long as the store of food 

 lasts ; but, when it is exhausted, the last-hatched young- 

 wander away. In the course of their wanderings, the em- 

 bryos enter into their larval condition ; but, before doing so, 

 they become twice as large as those which attain the larval 

 state in putrefying substances. The embryonic cuticle be- 

 comes thickened, and its oral and anal apertures closed, so 

 that it forms a cyst for the larva. The larva, however, is not 

 restrained by this cyst from moving about and continuing its 

 wanderings, though, at length, it passes into a quiescent con- 

 dition. Its inner substance, at the same time, becomes dark 

 by transmitted light, in consequence of the accumulation of 

 small fatty granules ; and, if this state of things lasts long, 

 the larva dies. If the larvae should dry up, the circumstance 

 tends to their preservation. The embryonic cuticle is sepa- 

 rated, and forms a protective cyst ; and, when moistened, the 

 larvae resume their vital activity. 



Nematoid worms belonging to naturally free and nonpara- 

 sitic genera may enter, and become encysted in, worms and 

 slugs ; but they only attain their sexual state when their 

 host dies, and they are nourished by the products of its putre- 

 faction. 



Angitillula scandens, the Nematoid which infests and gives 

 rise to a diseased condition of the ears of wheat, is a true 

 parasite. The young are hatched from the eggs laid by the 

 parent in the infected ear, and there become encysted. When 

 the wheat dies down, the larvae are set free, and wander on 

 the moist earth, until they meet with young wheat plants, up 

 which they creep, and lodge themselves in the developing 

 ears. Here they acquire the sexual condition, nourishing 

 themselves at the expense of the inflorescence, which becomes 

 modified into a kind of gall. 



Most Nematoids found in the alimentary canal of animals 

 are parasitic in the sexual state, but have a longer or shorter 

 period of freedom as larvae or as eggs. But some, as Cucid- 

 Icmns elegans, are parasitic both in the sexless and the sexual 

 condition ; inhabiting Cyclo])s, while in the former state, and 

 sundry fresh-water fishes, particularly the Perch, in the latter. 



Trichina spiralis 2 acquires its sexual state in the alimen- 



1 Schneider, I. c, pp. 362-'3. 



2 Leuckart, " Untersuchungen uber Trichina spiralis," 1866. 



