528 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



such primitive forms as Peripatus or Amphioxus to 

 have been of considerable extent, just as is the 

 mouth of a sea-anemone ; like it, it tended to close up 

 in its middle, and to have an orifice at either end 

 (A. Sedgwick) ; and these two ends would therefore 

 represent the oral and anal orifices of the primitive 

 intestine. The fate of the blastopore is very various ; 

 it may remain as the permanent mouth, or as the 



permanent anus ; 

 or it may close 

 up, and the 

 mouth or the 

 anus be subse- 

 quently formed 

 at the point of 

 closure, or it may 

 close up and ap- 

 parently have no 

 topographical re- 

 lation to either 

 the future mouth 

 or the future 

 anus. 



In most, 

 though not all 

 (e.g. Hydra) Me- 



tazoa, a third layer (the mesoblast) appears between 

 the epiblast and hypoblast, and gives rise to the sup- 

 porting tissues, the muscles, the connective tissue, 

 and the blood and lymph ; the epiblast covers the 

 surface of the animal, and is the seat of origin of the 

 nervous system and sense organs, the hypoblast lines 

 the alimentary tract, and its appended glands. As 

 has been already pointed out, a cavity (the ccelom) 

 appears in the midst of the mesoblast (see page 45) 

 in all Metazoa above the Ccelenterata. 



The epiblast becomes divisible in all forms above 



Pig. 216. Diagram of a Gastrula. 

 o, Blastopore; hyp, hypoblast ; ep, epiblast. 



