Phylum Nemathelminthes and Related Phyla 441 



cavity. The testis discharges its products into a duct which can be 

 protruded dorsally as a penis. 



Most rotifers reproduce primarily by parthenogenetic eggs. These 

 thin-walled, unresistant eggs develop into parthenogenetic females 

 throughout the summer months. When a change occurs in the en- 

 vironment, such as the chilling of water with the approach of winter, 

 some of these parthenogenetic females produce small eggs which de- 

 velop into males, while others produce large female type eggs. The 

 males develop and fertilize the females internally. The fertilized eggs 

 have heavy resistant shells and are known as winter eggs. It is these 

 eggs that are blown and carried long distances and which give rotifers 

 their cosmopolitan distribution. With the advent of favorable condi- 

 tions, the eggs hatch and the population of parthenogenetic females 

 begins anew. 



The Phylum Gaslrotricha. — These small, free-living animals are 

 found in fresh and salt waters (Fig. 143,^4). They vary in size from 0.1 

 to 0.5 mm. Ventrally they are flattened and they move about by means 

 of ventral cilia. Most species have a rounded head region separated by a 

 neck constriction from the elongated trunk which terminates in a forked 

 end. The cuticle is variously provided with spines, scales, and bristles. 

 Cilia are present over various regions of the body and ordinarily there are 

 one or more pairs of cement tubes located on the forked tail. Like those 

 of the rotifers, these are used for temporary attachment. If there is an 

 excretory system, it is of the protonephridial type with flame cells. 



The gastrotrichs feed on smaller organisms such as bacteria by 

 sucking them into the mouth by action of the muscular pharynx. This 

 pharynx is quite similar to that of the nematodes. The marine gastro- 

 trichs are hermaphroditic, while only females are known in the fresh- 

 water species. Thus among the fresh-water forms, reproduction is 

 entirely parthenogenetic. Two types of eggs, however, are produced. 

 The summer eggs are thin-walled and develop rapidly, while the winter 

 eggs are heavy-walled and develop slowly. 



In some respects the gastrotrichs resemble the rotifers, that is, 

 their common possession of cilia, the protonephridial system, and the 

 structure of the integumental musculature. On the other hand, they 

 resemble the nematodes in the structure of the pharynx, the cuticular 

 structures, and the shape of the brain. 



The Phylum Kinorhyncha. — This exclusively marine phylum has 

 about 100 known species. They are small animals (Fig. 143,C), less 



