THE CCELENTERATA 6 1 



account of their possessing bands of cilia, fanci- 

 fully compared to the teeth of a comb. At first 

 sight most of them somewhat resemble jelly- 

 fishes, being transparent forms swimming near 

 the surface of the sea. They are carnivorous, 

 and some of them highly phosphorescent at 

 night. The gastric cavity is divided up into 

 branches. The representatives of the Cteno- 

 phores, most often seen on our own coasts, are 

 small rounded forms. 



Two remarks must be added before quitting 

 the subject of the Ccelenterata. 



Firstly, the description of them as two-layered 

 Animals is one that only applies typically and to 

 the simpler forms. In others, such as the jelly- 

 fishes, there is an intermediate layer of jelly, 

 which appears to acquire a cellular structure by 

 the immigration of cells derived from the pri- 

 mary layers. Thus we see, within the group of 

 the Crelenterata, the gradual establishment of 

 that third body-layer, which is found in all ani- 

 mals of higher structure. Scarcely indicated in 

 Hydra, as a faint trace of a boundary-line 

 (lamella) between the ectoderm and endoderm, 

 it attains a good thickness in the Jelly-fish and 

 Ctenophora. In animals of higher structure the 

 third body-layer, being now fully established, is 

 cellular from its beginning in the embryo; in the 

 Ccelenterata its gradual formation is to be traced. 



Secondly, it must be remarked that the co- 

 lonial structure and the arrangement sometimes 

 concomitant with it of " alternation of genera- 

 tions," is by no means confined to the Ccelen- 

 terata. Both are seen in other forms of life, in 

 which the units, or zooids, differ greatly in struc- 

 ture from those of this group. 



