86 



MANUAL OF ZOOLOGY. 



The leading feature which distinguishes the Ccelcnterata, and 

 the one from which the name of the sub-kingdom is derived, is 

 the peculiar structure of the digestive system. In the Protozoa, 

 as we have seen, a mouth is only present in the higher forms, 

 and in no case is there any definite internal cavity bounded by 

 the walls of the body to which the name of " body-cavity " or 

 "somatic cavity" could be properly applied. In animals 

 higher than the Coelenterata, on the other hand, there is not 

 only generally a permanent mouth, but the walls of the body 

 usually enclose a permanent chamber or " body-cavity." Fur- 

 ther, in most cases, the mouth conducts into an alimentary 



Yig. 17 Diagrammatic vertical section of a Sea-anemone (Actinia), a Stomach ; 

 b Mesentery; c Convoluted cord or " craspedum ;" d Tentacle. The dark line 

 indicates the " ectoderm," the fine line and clear space adjacent mark the " endo- 

 derm. 



canal, which is always distinct from the body-cavity, never 

 opening into it, but usually passing through it to open on the 

 surface by another distinct aperture (the anus). In most cases, 

 therefore, the alimentary canal is a tube which communicates 

 with the outer world by two apertures a mouth and anus 

 but which simply passes through the body-cavity without in 

 anyway communicating with it. In the Ccelenterata (fig. 17) 

 there is an intermediate condition of parts. There is a distinct 

 and permanent mouth, and a distinct and permanent body- 

 cavity, but the mouth opens into, and communicates freely 

 with/ the body-cavity. In some cases (Hydrozoa) the mouth 

 opens directly into the general body-cavity, which then serves 

 as a digestive cavity as well (fig. 18). In other cases there 

 intervenes between the mouth and the body-cavity a short 

 alimentary tube, which communicates externally with the outer 

 world through the mouth, and opens below by a wide aperture 

 into the general cavity of the body (Actinozoa, fig. 17). In no 

 case is there a distinct intestinal canal which runs through the 



