IO6 . MANUAL OF ZOOLOGY. 



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 (unless it be 

 allowed that such really exists in the sponges). In animals 

 higher than the Cotlenterata, 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 

 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 

 any way communicating with it. In the Coelenterata (fig. 35) 



Fig- 35- Diagrammatic vertical section of a Sea-Anemone, a Mouth ; s Stomach ; 

 b Body-cavity ; c c Convoluted cords (" craspeda ") containing thread-cells, and form- 

 ing the free edges of the mesentery (m) ; t, t Tentacles ; o Reproductive organ con- 

 tained within the mesentery. The ectoderm (e) is indicated by the broad external 

 line, the endoderm (^) by the thin line and the space between that and the ectoderm. 



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. 37). 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 



