THE PHYLA ASCHELMINTHES AND NEMERTEA 231 



Roundworms also show the phenomenon of cell constancy in most 

 of their organs. Mitotic divisions continue throughout life only in the 

 epidermis, gastrodermis and gonads. In large nematodes, the organs with 

 cell constancy increase in size entirely by the growth of the cells, and not 

 by an increase in their numbers. This explains why the individual 

 cell bodies of Ascaris muscle fibers are so easily visible. In many parts 

 of the body the tissues tend to become syncytial. As in the rotifers, the 

 ability to regenerate is very poor. 



91. Molting 



The growth of the young nematode into an adult, when contained 

 in a heavy cuticle, presents a problem that is solved by periodically 

 shedding the cuticle and expanding rapidly before the new cuticle 

 hardens. This process is called molting. Each nematode molts four times 

 in becoming adult. When the external cuticle is shed, the cuticle lining 

 the mouth cavity, pharynx and rectum is also shed. This indicates that 

 these structures are also of ectodermal origin. 



92. Parasitism 



The roundworms have exploited endoparasitism more fully than 

 any other metazoan group. Practically all metazoa have roundworm 

 parasites that produce a wide variety of diseases. These include such 

 human diseases as pinworm, hookworm and elephantiasis. The subject 

 will be treated more fully in Chapter 39. 



93. Class Gastrotricha 



The microscopic gastrotrichs (Fig. 12.1) are common, but seldom 

 abundant, in quiet fresh and salt water. A few can be found in almost 

 any sample of pond debris. Gastrotrichs are very active, darting about 

 on the two longitudinal bands of cilia on their ventral surface, clamber- 

 ing rapidly over vegetation and debris. They feed on bacteria and algae, 

 sucking them into the anterior mouth with a triradiate pharynx very 

 similar to that of the nematodes. Cell constancy is as rigid in this class 

 as in the rotifers. In some gastrotrichs all growth is limited to the em- 

 bryonic stage; the parent produces enormous eggs, one at a time, that 

 later hatch into full grown individuals. The fresh-water species have 

 only females which reproduce parthenogenetically. 



94. Class Kinorhyncha 



These small marine worms, less than 5 mm. long, (Fig. 12.1), are 

 seldom found. They live in soft sand and mud at the bottom of shallow 

 or deep seas. Kinorhynchs resemble nematodes in two ways. They grow 

 by molting, and they have a pharynx similar to the nematodes except 

 that the muscle and epithelial layers remain distinct. The muscle fibers 

 are radially arranged, however, and produce suction by contraction. 



