CQELENTERATA, 55 
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 ts especially perceptible in the tentacles, which are in most 
instances placed round the mouth. Distinct reproductive organs 
exist in all. 
The leading feature which distinguishes the Ce/enterata, and 
the one from which the name of the sub-kingdom is derived, is 
the peculiar arrangement of the digestive system. In the Pro- 
tozoa, 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 
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i 
1 
1 
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1 
1 
1 
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Fig. 15.—Diagrammatic vertical section of a Sea-anemone (Actinia). a Stomach ; 
6 Mesentery; ¢ Convoluted cord or ‘‘craspedum;” @ Tentacle. The dark line 
indicates the ‘‘ectoderm,” the fine line and clear space adjacent mark the 
*“ endoderm.” 
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 surface by another dis- 
tinct 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 Ce/enterata the condition of parts is interme- 
diate in its arrangement. There is a distinct and permanent 
mouth, and there is a distinct and permanent body-cavity, but 
