200 HYATT, 



the annular folds, a deeremental character, (3) the thick- 

 ness of the ectocyst, a jiiictuating character. The decrement 

 of the first two and the fluctuation of the last being due to 

 the increasing functional activity of the third membrane in 

 forming annular folds, which is apparently due to the same 

 character as the second series, namely, the increase in the 

 gelatinous nature of the ectocyst. 



EVAGINATOKY SYSTEM. 

 ALIMENTARY CANAL. 



Membranes fully developed. The invariability of the 

 cellular structure of the membranes of the canal is, prob- 

 ably, as in the case of the evaginable endocyst, due to the 

 similarity of the circumstances and positions occupied by 

 them with relation to the surrounding parts and fluids. 



Lies straight in the c<x,noe,cium uihen invaginated. The 

 cells in Fredericella and Plumatella are not generally of 

 suflicient capacity to admit of any doubling on the part of 

 the alimentary canal, and, since there is room enough for 

 it to lie straight, it naturally assumes that position. 



The location, also, of the bases of the retractors are at 

 such a distance from the orifice of the cell, that it is out of 

 the question for the gastric retractors to act in such a 

 manner as to double the alimentary canal on itself. 



In variety 5 of Plumatella vitrea, however, the bases 

 of the muscles are nearly opposite the coenoecial orifice, 

 and it would be inferred, from the similarity of their po- 

 sitions to those in Pectinatella, that the canal must double 

 upon itself when undergoing retraction ; but, although the 

 lophophore is very closely withdrawn, the canal floats 

 freely in the chamber (PI. 9, fig. 1). The position of 

 Pectinatella, as figured in PI. 9, figs. 12, 13, shows that 

 the canal is not only bent ventrally, but when completely 

 drawn in, the anterior end of the coecum lies between the 

 trunks of the retractors. In Cristatella this tendency is 

 expressed more strongly, the canal, passing between the 

 trunks of the retractors ventrally lies in precisely the 

 reverse position to what it did in Fredericella and Plu- 

 matella. 



