1 52 PL A TYHELMINTHES. 



anastomose in front of the mouth. The brain is a large mass lying 

 dorsally to the mouth ; it suppUes nerves to various parts of the body. 

 Ventral to the stomach is a single ovary with a large vitelline gland 

 and an oviduct opening into the cloaca. 



The male Hydatina is much smaller in size and has no alimentary 

 system. 



, PHYLUM ROTIFERA. 



The Rotifera are an important phylum of common microscopic 

 animals. They are marine and freshwater in habit, and they may be 

 active, sedentary, tubicolous or ectoparasitic. They are interesting in 

 their diversity of external form, their sexual dimorphism (with small 

 and degenerate males), their summer and winter eggs and their power 

 of resisting drought. They are three-layered in structure, but they 

 have no coelom ; the cavity of the body is an archicoele and there are 

 no nephridia, excretion being conducted by flame-cell tubules. These 

 and other characters indicate a relationship to the Platyhelminthes. 

 Hydatina is fairly typical but for the exceptional absence of eyes or 

 other simple sense-organs. In other Rotifera there is great diversity 

 in the form of the wreath and of the foot. 



IV.— ASCARIS. 



Phylum - - - Nemathelminthes. 

 Class - - - - Nematoda. 



Fig. 86. — Dissection of Female Ascaris Megalocephala 

 FROM the Dorsal Side. {Ad 7iai.) 



tory Duct. Intestine, 

 hagus, 



Vagina. 



Uterus. Ovary. 



Ascaris megalocephala is a large nematode worm 

 found commonly in the stomach of the horse. It is usually 

 known as the "maw-worm." The body is long and 

 cylindrical, tapering at each end. The female may be one 

 foot or more in length ; the male is usually less. In addition, 



