371 



tion of the hooks commences, that it is thrown off, when it forms 

 a second cystic envelope. The embryo, or rather the larva, pro- 

 ceeding from it, divides very soon into two layers, a thick dermal 

 layer and an inner cell-mass, from which the other organs origi- 

 nate." The ovaries and testes are produced at a very early stage. 



V. ROTATORIA. 

 The Rotifers, by some eminent naturalists regarded as Crus- 

 tacea, are shelled worms, related to the flat-worms in many re- 

 spects. The body consists of sev- 

 eral segments, and the sexes are j**' 1 ^ v> 

 very unlike, the small males hav- 

 ing the organs more or less rudi- I 



mental, with no alimentary canal. ^^HHHSBH 



Like the lower worms they have a *' ^^^BHBHft^M 



tubes excretory in their na- g( ^^^H^^^^B^H , 



and perhaps respiratory, cor- * ^^HHH^U^H 60 

 to^the water ^^^^^HH^> 



tubes the Radiates, but ^H^H^HH^H 



comparable with s? ^HSH|BS^H ° 



mental of the Brachiopods ^|H5»kS^H egl 



and higher worms ; also pair of " ^■■B 



teeth in pharynx, as in many v ^^HE^H 



worms. The anus is situated on f 

 the back at the base of the tail. 



Sometimes the digestive canal ^^^^^^^ 

 ends in a blind sac. The distinc- 



tive organ is the retractile, ciliated, paired organ which may be 

 called the velum. Fig. 183 1 from H. J. Clark, represents Squa- 



