CILIATED PITS OF PRORHYNCHUS STAGNALIS. 1 87 



movements by raising and lowering the anterior sixth of its body. 

 Thus we have a crawling animal with ventrally disposed ciliated 

 pits which makes its exploratory movements by raising and 

 lowering the anterior end of its body. It is well to compare the 

 conduct of this animal with that of Microstoma caudatum Leidy 

 with reference to the position of their respective ciliated pits. 

 Microstoma caudatum is a free-swimming animal and has laterally 

 disposed pits. As we have shown ('12) this animal makes 

 exploratory movements by moving its anterior end from side to 

 side. Likewise, we gave experimental evidence to show that 

 these exploratory movements were made in order to test the 

 surrounding medium. Thus we see by the comparison of the 

 two rhabdocceles that the method in which they test the sur- 

 rounding water conforms to the position of their ciliated pits." 



So much for Microstoma caudatum and Prorhynchus applanatus. 

 Kepner and Taliaferro in Microstoma caudatum dealt with a 

 form that was primarily a free swimming one; in Prorhynchus 

 applanatus with a form that was primarily a creeping one. 

 Prorhynchus stagnalis usually creeps. It is, however, provided 

 with mucous cells at the posterior end of its body by means of 

 which it can attach itself to the substratum. From this station- 

 ary point it can move its body about in an almost complete circle, 

 sometimes adhering closely to the surface, but frequently lifting 

 the anterior third of its body, moving it either from side to side or 

 backwards and forwards. Therefore, it would be natural to 

 expect its special sense organs to be ventro-laterally disposed 

 and such happens to be the case. In Prorhynchus stagnalis is, 

 therefore, presented a form that sometimes creeps and sometimes 

 swims freely. So it is of interest to observe that when it is 

 creeping the anterior end, through flattening, applies the ciliated 

 pits to the substratum much as Prorhynchus applanatus does; 

 whereas when the animal moves or sways through the water, its 

 anterior end, through becoming rounded, places the ciliated pits 

 more laterally as they are always placed in Microstoma caudatum. 

 It has been thus observed that the position of the ciliated pits of 

 Prorhynchus stagnalis also conforms with the diverse habits of 

 Prorhynchus stagnalis. 



The pits are directed obliquely posteriorly and mesially, there 



being a decided curvature in the "neck" of the pit. 

 13 



