ACTINOZOA. 



147 





body. In both, the nutrient system is lined by 

 a ciliated endoderm, the vibratory action of which 

 serves to maintain a circulatory motion of the 

 included fluid. The contractile tissues of the 

 ectoderm may further assist such movements. 

 And in Pleurobrachia, whose bilateral symmetry 

 is more strongly marked than that of Actinia, the 

 nutrient fluid, as Agassiz has shown, is at times 

 alternately impelled between the right and left 

 sides of the somatic cavity. 



Very many curious modifications are presented 

 by the canal system among the different genera of 

 Ctenophora, to some of which reference will be 

 made in the more particular account to be given 



of that order. 

 No manducatory apparatus exists in the Acti- 



nozoa. The oral margin may, however, be some- 

 what thicker and firmer than the surrounding 

 parts, or otherwise become altered in appearance ; 

 and the cilia of the digestive sac may also differ 

 from those which occur on other regions of the 



body. 



As in Actinia, one part of the digestive cavity 

 may undergo some amount of modification, coloured 

 granular masses appearing in its walls which have 

 been supposed to indicate a liver. Such coloured 

 cells in the Ctenophora usually arrange themselves 

 as vertical ridges which surround the innermost, 

 or stomachal, division of the otherwise transparent 

 digestive sac. 



Milne Edwards has also shown the existence in 

 Cestum of another structure whose function is 

 probably secretive. Between certain of the cili- 

 ated bands and their corresponding ctenophoral 

 canals ; parallel to, and in close connection with 



L 2 



