ORGANS OP SUPPORT, 



FIG. 11. 



the helminthoid classes, the rudiments of all those character- 

 istic parts of the complicated skeletons of insects, and of all 

 the higher entomoid articulata. 



VII. Rotifera. The wheel animalcules are more closely 

 allied to the helminthoid articulata than to any of the inferior 

 radiated classes, especially in their supra and infra-sesopha- 

 geal ganlia, their abdominal longitudinal nerves, their dorsal 

 vessel for circulation, their lateral maxillae, their highly deve- 

 loped genetal system, and their muscular activity. Their 

 exterior covering, though generally thin and transparent as 

 crystal, appears to possess 

 considerable firmness from the 

 numerous powerful muscles 

 inserted into it, and from the 

 transverse corrugations it 

 presents when the body is 

 drawn backwards to their 

 fixed caudal extremity. Some- 

 times it is in form of a sheath 

 enveloping the middle of the 

 body,and open both before and 

 behind, to allow the head and 

 tail to be retracted and pro- 

 tected. This firm tough 

 elastic covering of the roti- 

 fera has some resemblance in 

 its hermogenous texture to 

 that of the entozoa, and pro- 

 bably is persistent and en- 

 larges with the body. There 

 are no earthy deposits formed 

 in any part of the body of 

 these minute and active 

 animals. The densest parts of 

 their body appear to be the 

 two jaws which move trans- 

 versely by powerful muscles, 

 and are generally provided 

 with numerous sharp teeth. 

 One of these wheel- animal- 

 cules, termed hydatina senta, common in our ponds of fresh 



