CHAPTER 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 prostomiwm, or preoral lobe. The mouth 
and anus, when present, are lined by invaginations of the ecto- 
derm, termed the stomodaeum and proctodaeum respectively. 
The very important cavity, the coelom, which distinguishes 
the Coelomates from the lower animals, is characterised by the 
following peculiarities. (1) 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 
