PARASITIC ROUND WORMS 517 



Within the egg-shell is formed in direct fashion a minute worm 

 which on hatching displays the main features of nematode struc- 

 ture. This embryo may require weeks or months for its growth 

 and may wait within the shell for years before it is passively intro- 

 duced into a new host; or it may break out from the shell and spend 

 a period in moist earth or water awaiting the time when in one 

 way or another it is brought into a suitable host. In most cases 

 the embryo of a parasitic nematode spends a brief period at least 

 as a free-living larva, and always in an aquatic environment, but 

 this may be semi-fluid mud as well as open water. Frequently it 

 undergoes in this stage or earlier the first of the four character- 

 istic molts and within the cast cuticula of the embryonic form 

 enters upon a resting stage well protected against drying out. 

 In this condition it may be transported by wind or water, or at- 

 tached to other objects, even such living agents as the feet of 

 reptiles, birds, or mammals, and thus be carried far in attaining 

 the location where by some chance it is introduced into the body 

 of a new host. When this new host is reached it may be the same 

 as the original host in which case further molts bring the worm in 

 a short time to the adult condition. In other instances the larva 

 reaches an intermediate host in which it becomes encysted in 

 muscles or viscera and after a period of growth is ready for transfer 

 to the final host. This change involves the consumption of the 

 flesh with the encysted larva by a suitable final host, whereupon 

 digestion sets the worm free, the active development is resumed, 

 and the adult form reached after a period of growth. 



Most often the larval parasite is taken into a new host with 

 water or food. In some cases the free-living larva does not 

 depend on chance to carry it but gains entrance by its own activ- 

 ity. Thus the hookworm larva, living in moist earth, when brought 

 suitably in contact with the skin of an available host burrows into 

 it and completes its life history during its devious wanderings in 

 that host. 



As an illustration of the life history of a typical aquatic species 

 may be taken the development of Camallanus lacustris, formerly 

 often designated Cucullanus elegans. This development was worked 

 out and described by Leuckart somewhat as follows: 



