554 FRESH-WATER BIOLOGY 



ciliated area or corona serves as a rule to distinguish a rotifer at 

 once from any other many-celled animal Hving in fresh water. 



The extreme diversity of form and organization in different 

 rotifers, though constituting the greatest charm of their study, 

 makes it almost impossible to give a formal definition of the group. 

 Even the most characteristic feature, — the cihated corona, — 

 is in a few cases lacking. The form of the body varies extremely, 

 from spherical in Trochosphaera (Fig. 947) to the excessively atten- 

 uated form of Rotifer neptunius (Fig. 960), the fiower-Hke shape of 

 Stephanoceros (Fig. 937), or the spiny, turtle-like figure of Poly- 

 chaetus (Fig. 905). 



Yet one can give a characterization that will be true for the great 

 majority of the rotifers. The body is as a rule somewhat elon- 

 gated, with the cihated corona at the anterior end ; it is extended at 

 the posterior end, behind and below the cloacal opening, to form a 

 stalk, or tail-like appendage known as the foot. This frequently 

 ends in two small pointed toes. There is a well-developed ali- 

 mentary canal, with a muscular pharynx, containing complex jaws. 

 There is a simple excretory system, while circulatory and respira- 

 tory systems are lacking. The nervous system consists of a prom- 

 inent brain and of certain nerves and sense organs. The sexes are 

 separate, and the male is usually a minute, degenerate creature, 

 lacking the alimentary canal. 



Rotifera may be found wherever there is fresh water. Lakes, 

 pQnds, and streams harbor them in immense number and variety. 

 Swamps and marshes swarin with them. Wayside pools, drains, 

 and even the dirty water that stands in barnyard holes about 

 manure heaps, are proKfic sources of rotifers. The mud of eave- 

 troughs, the bottoms of funeral urns, the cavities found in the axils 

 of the leaves of certain mosses, — all these are famous collecting 

 grounds for the rotifer hunter. A few rotifers are parasitic, some 

 externally, some internally. A few live in salt water, but they are 

 much less abundant in the ocean than in fresh water. 



In giving an account of the structure and life of the rotifers, it 

 will be well to have in mind at first some representative type ; then 

 the variations found in other rotifers may be traced. The typical 

 rotifers, as well as the commonest ones, are those belonging to the 



