Phylum Nemathelminthes and Related Phyla 425 



four areas to form a middorsal, a midventral, and two lateral ridges. Be- 

 low the hypodermis is the muscular layer formed only of longitudinal 

 muscle fibers. These extend slightly into the pseudocoel, forming its 

 outer border. 



A complete digestive tract with both mouth and anus is seen in 

 the members of this phylum. The anus was foreshadowed by the 

 anal pore of some ctenophorans and flatworms, but this is the first 

 group (except for the Nemertina) to have a true anus at the posterior 

 end of the digestive tract. The mouth is terminal in these forms and 

 often is bordered by lips. The mouth leads into the buccal cavity which 

 is variable in shape and at times armed with jaws or protrusible stylets. 

 Posterior to the buccal cavity is the muscular pharynx which serves to 

 pump dissolved materials into the mouth. From the pharynx, the 

 straight intestine leads to the posterior anus. At the posterior por- 

 tion, the intestine narrows and is termed the rectum. 



While there are no special organs for respiration or circulation, 

 the roundworms do have an excretory system. This system is quite 

 unlike that of the flatworms inasmuch as there are no flame cells. 

 There are two possible types of excretory systems. The primitive type 

 found in many free-living forms consists of a glandular cell in the 

 region of the pharynx. This is termed the renette cell. From this cell 

 an elongated neck runs forward and opens at an anterior excretory pore. 

 The other type of excretory system is a series of canals arranged in an 

 H-shape. There are both lateral and longitudinal canals connected by 

 a series of transverse ones. From these, a common canal leads to an 

 excretory pore. 



Surrounding the pharynx is a nerve ring on which are located 

 some ganglionic masses. Anteriorly, a series of nerves pass from the 

 ring to sensory papillae around the mouth ; posteriorly there is a series 

 of main nerves including a middorsal, a midventral, and several lateral 

 ones. The free-living forms are well supplied with sensory bristles 

 and papillae at the anterior end of the body ; in a few fresh-water and 

 marine forms, light-sensitive structures are located at the sides in the 

 region of the pharynx. 



Unlike the flatworms, nearly all the roundworms are dioecious. 

 Usually the males are considerably smaller in size than the females, and 

 are readily distinguished by their curved posterior ends. In the male, 

 a single testis is usually present. From the free end of the testis, 

 there is a sperm duct which is continuous with the testicular epi- 



