MOLLUSCOIDA 



Branch VI. TROCHELMINTHES 



The wheel animalcules, or rotifers, mostly found in 

 fresh water, are composed of a few ill-defined segments, 

 and have on the anterior end a disk 

 which is ciliated on the edge, the 

 motion of the cilia causing the ap- 

 pearance of a rotating wheel, whence 

 the name. They are from 200 to WQ 

 of an inch long. They have a well- 

 developed digestive system, the food 

 consisting of minute organisms, and 

 a rudimentary nervous system. Roti- 

 fers have been kept for several years 

 in a dried condition and have after- 

 ward been revived (Fig. 40). 



Branch VII. 



FIG. 40. -r Rotifer, or 

 '' Wheel animalcule " 

 (Hydatina}, highly 

 magnified. 



MOLLUSCOIDA 



These ani- 

 mals have gen- 

 erally a body 

 cavity, in which lies the alimen- 

 tary canal, bent in such a man- 

 ner that the mouth and the anal 

 opening are close together. Near 

 the mouth is a curved ridge, 

 the lophophore, bearing tentacles. 

 There is a very rudimentary 

 nervous system (Fig. 41). 



The PolyZOa rCSCmble polyps 



FiG. 4 i.-DiagramofaPoly 2 oan: 



Aiophophore bearing tentacles, j n appearance, living in clusters, 



// m, mouth; a, digestive cav- ... 



ity; i, intestine: a, anus; e, each individual inhabiting a dell- 



excretory organ; b, "brain." ,, , -. , 



Much magnified. cat e cell, or tube, and having a 



