CffiLENTERATA. 51 



we Lave seen, a mouth ia only present in some groups, 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 most of the higher animals, on the 

 other hand, not only is a permanent mouth present, but the walls of 

 the body enclose a distinct and permanent chamber or body-cavity. 

 Further, in most cases the mouth opens into an alimentary or diges- 



Fig. 26. — Diagrammatic vertical section of a Sea-anemone, a Mouth ; s Stomach ; 

 5 Body-cavity: c c Convoluted cords ("craspeda") containing thread-cells, and 

 forming the free edges of the mesentery (m) ; 1 1 Tentacles ; o Reproductive organ 

 contained witliin the mesentery. The ectoderm (e) is indicated by the broad ex- 

 ternal line ; the endoderm (e') by the thin line and the space between that and the 

 ectoderm . 



tive tube, which is always distinct from the body-cavity, and never 

 opens into it, usually passing through it to ojjen 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 an anus — but which simply passes 

 through the body-cavity without in any way communicating with 

 it. In the Ccelenterata the condition of parts is intermediate in its 

 arrangement. There is a distinct and permanent mouth, and there 

 is a distinct and permanent cavity enclosed within the body-walls, 

 but the mouth opens into, and thus communicates freely with, the 

 latter. In some cases the mouth opens straight into the general 

 body-cavity, which then serves as a digestive cavity as well (fig. 28). 

 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 (fig. 26). In no case is 



