CCELENTERATA : HYDROZOA. 79 



The leading feature which distinguishes the Cceknterata, 

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

 derived, is the pecuHar structure of the digestive system. In 

 the Protozoa, as we have seen, a rhouth 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 Cceknterata, 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." Further, '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 aper- 

 tures — a mouth and anus — ^but which simply passes through 

 the body-cavity without in any way communicating with it. 

 In the Cmknierata 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 com- 

 municates 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. 

 12). 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. 28). In no case is there a distinct intestinal 

 canal which runs through the body and opens on the surface 

 by a mouth at one end and an excretory aperture or anus at 

 the other. 



With regard to the fundamental tissues of the Cxknterata, 

 there exist two primary membranes, of which one forms the 

 outer surface of the body, and is called the " ectoderm ; " 

 whilst the other lines the alimentary canal, the general cavity 

 of the body, and the tubular tentacles, and is termed the 

 " endoderm." These membranes correspond with the primi- 

 tive serous and mucous layers of the germinal area, and be- 

 come differentiated in opposite directions, the ectoderm grow- 

 ing from within outwards, the endoderm from without inwards. 

 Each consists of numerous nuclear bodies, or " endoplasts," 

 embedded in a granular "intercellular substance" or "peri- 

 plast ; " and each may be rendered more or less complex by 

 vacuolation or fibrillation. 



