178 FOSSIL ACTINOZOA. 



opening by a large aperture into the general body-cavity 

 below, and extending about half-way between the mouth 

 and the base. The wide space between the stomach and 

 column-wall is subdivided into a number of compartments 

 by radiating vertical lamellse, termed the "primary mesen- 

 teries," arising on the one hand from the inner surface of the 

 body- wall, and attached on the other to the external surface 

 of the stomach. As the stomach is considerably shorter than 

 the column, it follows that the inner edges of the primary 

 mesenteries below the stomach are free ; and these free edges, 

 curving at first outwards and then downward and inwards, 

 are ultimately attached to the centre of the base. Besides 

 the primary mesenteries, there are other lamellse which also 

 arise from the body-wall, but which do not reach so far as 

 the outer surface of the stomach, and are called " secondary " 

 and " tertiary " mesenteries, according to their breadth. The 

 reproductive organs are in the form of reddish bands, which 

 contain ova and spermatozoa, and are situated on the faces of 

 the mesenteries. 



B. ZOANTHARIA ScLEROBASiCA. The members of this 

 group are all composite organisms, consisting of numerous 

 polypes, each of which has essentially the structure of a 

 small Sea-anemone, united together by a common organised 

 medium or "ccenosarc" (fig. 68). Each polype has, with 



Fig. 68. Part of a living stem of Antipathes anguina, of the natural size. (After Dana.) 



rare exceptions, six tentacles, and the entire organism is 

 supported by an internal skeleton or " corallum." The coral 

 is horny, and it is what is called " sclerobasic " that is to 

 say, it forms an internal axis, over which the ccenosarc is 

 spread, much as the bark encloses the wood of a tree. As 

 the polypes are sunk in the coenosarc, and as this simply 

 forms a rind for the coral, it follows that the polypes are 

 outside the corallum. In other words, the polypes take no 



