C(ELHELMINTHES 219 



and lying loosely in the coelomic cavity. In the nial(> this genital tract 

 is always single, the finer part of the tube constituting the testis, the 

 heavier remaining portion serving as a seminal vesicle and terminating 

 in the duct. The ovaries and uteri are likewise continuous structures, 

 the former being constituted by the finer portions, while the uteri are 

 usually much distended (Fig. 119). In certain forms there is but one 

 genital tube in the female, but in most all there are two which unite 

 close to the external opening to form the vagina. There is no distinct 

 vitellarium as in the flatworms, the ovary assuming the function of this 

 gland. 



The eggs are usually globular or ovoid in shape; as there is copulation, 

 they are fertilized in the uterus. Following this the development may 

 or may not take place while the eggs are retained. 



As to the terms oviparous and ovoviviparous. frequentl}' used in 

 summarizing the characteristics of parasitic groups, it may be well to 

 direct attention here to their correct application. 



The term oviparous is properly applied to the oviposition of eggs 

 which undergo incubation after they have been oviposited, or to the 

 oviposition of eggs which have been incubated within the genital cavity 

 of the female and at the time they are oviposited contain enil)ryos more 

 or less developed. 



The word ovoviviparous is commonly used hi reference to the oviposi- 

 tion of eggs containing embryos developed and ready to emerge at the 

 time the eggs are extruded, as might be in the last-stated case. It is 

 more correctly applied where the embryos, having been developed, 

 escape from the eggs while these are still within the body of the female. 



In other words, the escape of the embryos from the eggs occurs out- 

 side of the body of the parent in the oviparous method, within the parent 

 body in the ovoviviparous. 



The term viviparous, often applied in biology for ovoviviparous, 

 has reference to the typical mammalian method of giving birth, where 

 the egg is not concerned in this process, and there is consequently no 

 hatching. 



Parasitism of the Nematodes in General . 



In most of the nematode parasites there is a post-embryonic free 

 existence, the infection of the host being direct and necessary to the 

 parasite's sexual maturity. A notable exception is furnished by Trich- 

 inella, where there is no period of free life, the transfer from host to 

 host being accomplished by the ingestion of food containing the encysted 

 larvae. 



The degree of injury to their hosts by the nematodes varies consider- 

 ably and is frequently not characteristic. In general, it may be said 

 to depend upon the number of the parasites present, but the seriousness 



