THE PHYLA ASCHELMINTHES AND NEMERTEA 



227 



Figure 12.4. A desiccated rotifer from a dried-up puddle. It can absorb water and 

 become active again in a few minutes. 



point of water to well below zero. They have even survived eight hours 

 in liquid helium, where the temperature is — 272°C., just one degree 

 above absolute zero where molecular activity ceases. Rotifers, then, can 

 achieve a state of suspended animation by the mere loss of water. 

 While a few other groups of animals can also survive desiccation in 

 this way, such a water loss is usually lethal. The properties of rotifer 

 protoplasm that enable it to survive desiccation are entirely unknown. 



88. 



Class Nematoda 



The Nematoda are mostly cylindrical worms tapered toward both 

 ends, and are commonly called roundworms. The class includes many 

 parasites and a very large number of small free-living species. They are 

 common wherever there is water, even though it be but a thin film. The 

 nematode body is covered by a thick cuticle (Fig. 12.5) that is elastic 



Dorsal nczrve cord- 



Pharynx 



Pha-rynx 

 lumo^n. 



Pse-udocoel 



Cuticle 



Epidermis 



Lateral line 



Longitudinal 

 muscle fibers 



-Ventral nerve cord. 

 Figure 12.5. Cross section through the pharynx of a nematode, showing the body 

 wall and the peculiar cell structure of the pharyngeal wall. 



