C(ELENTERATA. 



61 



system. Peculiar stinging organs, or ^'■thread-cells^'' are 

 usually if not always present, and in most cases there is a 

 radiate or star-like arrangement of the organs, which is 

 especially perceptible in the tentacles, which are in most in- 

 stances placed round the mouth. Distinct reproductive organs 

 exist in all. 



The leading feature which distinguishes the Ccelenterata, 

 and the one from which the name of the sub-kingdom is de- 

 rived, is the pecuKar arrangement of the digestive system. In 

 the Protozoa, as we have seen, a mouth is only very rarely 

 present, 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 digestive 

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

 opens into it, usually passing through it to open on the sur- 

 face by another distinct aperture (the anus). In most oases, 

 therefore, the alimentary canal is a tube which commimicates 

 with the outer world by two apertures — a mouth and an 

 anus — but which simply passes through the body-cavity with- 

 out in any way communicating with it. In the Ccelenterata 

 (Fig. 13) the condition of parts is intermediate in its arrange- 



FiQ. 12.— Diagrammatic vertical section of a Sea-anemone (Actinia), a Stomach; 6 Mesen- 

 tery; c Convoluted cord or 'craspedum;" d Tentacle. The dark line indicates the 

 " ectoderm," the fine line and clear space adjacent mark the " endoderm." 



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

 is a distinct and permanent body-cavity, but the mouth opens 



