CHAPTEE V 



COELOMATA 



In the Acoelomata there is a common cavity, the Coelenteron, 

 which is lined by endoderm cells, and which pervades various 

 parts of the body. Whatever cavities exist in these animals, 

 with the exception of certain ectodermic pits, are all diverticula 

 of this one primitive cavity, and remain in connection with it. 



The Coelomata, on the other hand, start with an Arch- 

 enteron, probably the equivalent of the Coelenteron ; but this 

 is replaced by two distinct and separate cavities — that of 

 the alimentary canal, and that of the body. The latter is 

 termed the Coelom, and is entirely shut off from the cavity of 

 the digestive system. The cavity of the alimentary canal is 

 lined by endoderm ; between the endoderm and ectoderm a 

 new layer of cells has appeared, the mesoderm, and it is in this 

 layer, and lined by it, that the coelom appears. 



The Coelomata have typically a bilaterally symmetrical 

 form, which is however in many cases lost. In front of the 

 mouth there is a prostomium, or preoral lobe. The mouth 

 and anus, when present, are lined by invaginations of the ecto- 

 derm, termed the sfomodaeum and prododaeum respectively. 



The very important cavity, the coelom, which distinguishes 

 the Coelomates from the lower animals, is characterised by the 

 following peculiarities. (i.) It develops as one or more 

 diverticula from the primitive archenteron (enterocoel), or it 

 arises as a space or spaces in the mesoblast (schizocoel), or it is a 

 remnant of the segmentation cavity, into which a lining of meso- 

 blast grows (archicoel) ; it is consequently always lined by meso- 

 blast. (ii.) Its walls give rise to the reproductive cells, which 

 are set free into a portion of the coelom, and leave the body 



