426 The Animal Kingdom 



thelium. Posteriorly the sperm duct is widened into the seminal vesicle 

 which in turn becomes the ejaculatory duct at the junction with the 

 rectum. This portion of the rectum is known as the cloaca inasmuch 

 as it receives the undigested foods and also the sex products. Usually 

 the cloaca has one or two copulatory spicules which spread the female 

 sex opening during copulation. 



The female usually has two ovaries located above the digestive 

 tract, and extending toward one another from opposite ends of the 

 body. The covering of the ovary is continued as the slender oviduct 

 which soon widens into the uterus. The uterus passes into a short 

 vagina which opens to the outside through the female sex opening, the 

 vulva, which is usually near the middle of the body. A portion of the 

 uterus often serves as a seminal receptacle for storing sperm and for 

 fertilization of the eggs. The eggs may be retained during development 

 within the uterus, or they may be released soon after fertilization. The 

 first method is commonest among the free-living forms, but both 

 methods are found in both parasitic and free-living forms. If develop- 

 ment to the larval stage is within the uterus, the young are released 

 usually by the rupture of the body. 



Cleavage in the nematodes is of the determinate type, and the 

 germ layers are clearly defined. Mesoderm arises from both ecto- 

 and entodermal sources, and the embryonic mouth or stomodaeum gives 

 rise to the mouth of the adult. The larvae resemble the adults in gen- 

 eral appearance and do not pass through different stages as was typical 

 of some flatworms ; however, many of the parasitic forms have compli- 

 cated life cycles involving several hosts. 



The Classification of the Nemathelminthes. — While the mode 

 of life may vary greatly among the members of this phylum, their struc- 

 ture is so very similar that all are grouped into a single class, the 

 class Nematoda. 



Class I. Nematoda. Cylindrical worms with the characteristics of the 

 phylum. 



THE CLASS NEMATODA 



The anatomy of the various nematodes is quite similar, and the 

 details are most easily seen by a study of some small transparent, free- 

 living form such as Chiloplacus sp. 



