PSEUDOCOELOMATE AND MINOR EUCOELOMATE PHYLA 



parasitic in plants or in animals only as adults. Others may be parasitic as 

 juveniles and leave the host at adulthood to become free-living. Therefore 

 the line between "free-living" and "parasitic" species is difficult to draw, 

 and it is virtually impossible to make a general statement describing the 

 "typical" life cycle of the nematodes. 



The body of a generalized nematode is a relatively simple, cylindrical tube, 

 tapered at both ends (Fig. 12.3). Externally, the body is covered by a 



Excretory 

 pore 



Vulva 



Oviduct — { — f^ 



Anus- 



Rectum 



Pharynx 



Excretory 

 pore 



Renette 

 cells 



Fig. 12.3. Class Nematoda. A, a typical nematode, Rhabditis maupasu adult female. In its 

 juvenile stages this species is parasitic in the coelom and nephridia of earthworms. B, ren- 

 ette cells of a related nematode, Rhabdias. {A, redrawn, after G. E. Johnson, from C. G. 

 Goodchild in F. A. Brown, Jr., et ai. Selected Invertebrate Types, copyright 1950 by John 

 Wiley and Sons, Inc., printed by permission; J5, redrawn, after B. G. Chitwood, from L. H. 

 Hyman, The Invertebrates: Acanthocephala, Aschelmmthes, and Entoprocta, copyright 1951 by 

 McGraw-Hill Book Co., Inc., printed by permission.) 



349 



