152 PLATVHELMINTHES. 
anastomose in front of the mouth. The brain is a large mass lying 
dorsally to the mouth ; it supplies nerves to various parts of the body. 
Ventral to the stomach is a single ovary with a large vitel/ine gland 
and an ovzduct 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 ccelom; the cavity of the body is an archiccele 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 Rofzfera 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 DorsAaL SIDE. (dd mat.) 
Excretory Duct. 
(Esophagus. 
Intestine, 
Oral 
Papilla. 
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, 
