OBSERVATIONS ON POLYZOA. 201 



The length proportionally to the length of the evaginable 

 endocyst above the orifice. This character is very impor- 

 tant. It shows the gradual progress of the alimentary 

 canal out of the coenoecium, in proportion as the evagiua- 

 tion of the polypide becomes more extended. 



Although the canal is absolutely very much larger in 

 Pectinatella than in any other genus (vide synopsis, pro- 

 portion of the greatest dorso-ventral, to the greatest an- 

 tero-posterior diameter) , its position with relation to the 

 lower edge of the invaginated fold does not change as 

 long as that fold exists. When, however, there is no fold 

 as in Cristatella, the region of the sphincter crosses the 

 coecum near the end instead of its former place, just be- 

 low the intestinal valve. (PI. 14, fig. 1.) 



The length, however, with relation to the length of 

 the evaginable tube and the extent of the canal exposed 

 by its increasing evagination, is an incremental character, 

 and shows farther how the destruction of the walls of the 

 cells affects the size of the canal. The Fredericellse with 

 their long, narrow cells, have also long, narrow canals ; 

 the cells becoming wider in Plumatella, the stomachs are 

 correspondingly wider ; and in Pectinatellse, where the 

 evagination removes the invaginated fold, giving a wide 

 orifice, and where there are no cells, it attains, compara- 

 tively, an immense size. 



In Cristatella, the evagination being perfected, and the 

 amount of room in the coenoecium very much reduced, it 

 returns again to a smaller size, but retains nearly the same 

 proportions of the dorso-ventral and antero-posterior di- 

 ameter (see next character in the synopsis) which it had 

 acquired in Pectinatella. 



The consequence of this reduction in the absolute size 

 renders the whole but little longer than the fully evagin- 

 ated endocyst, and the canal may, therefore, be said to fol- 

 low the tube out of the coenoecium. 



Proportions of the dorso-ventral to the antero-posterior 

 diameter. This character has been sufficiently explained 

 above, with the exception of the measurements. These 

 were taken, the dorso-ventral just below the phyloric 



COMMUNICATIONS ESSEX INSTITUTE, VOL. V. 27 FEB., 1868. 



