FOSSIL ACTINOZOA. 87 



leathery consistence. The two extremities of the column are 

 termed respectively the " base " and the " disc," the former 

 constituting the sucker, whereby the animal attaches itself at 

 will, whilst the mouth is situated in the centre of the latter. 

 In a few cases ( Cerianthus and Peachia) the centre of the base 

 is perforated, but the object of this arrangement is unknown. 

 Between the mouth and the circumference of the disc is a flat 

 space, without appendages of any kind, termed the " peris- 

 tomial space." Round the circumference of the disc are 

 placed numerous tentacles, usually retractile, arranged in alter- 

 nating rows, and amounting to as many as 200 in number in 

 the common Actinia. The tentacles are tubular prolongations 

 of the ectoderm and endoderm, containing diverticula from 

 the somatic chambers, and sometimes having apertures at 

 their free extremities. The mouth leads directly into the 

 stomach, which is a wide membranous tube, opening by a 

 large aperture into the general body-cavity below, and extend- 

 ing about half-way between the mouth and the base. The 

 wide space between the stomach and column-wall is sub- 

 divided into a number of compartments by radiating vertical 

 lamellae, termed the " primary mesenteries," 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 out- 

 wards and then downward and inwards, are ultimately attached 

 to the centre of the base. Besides the primary mesenteries, 

 there are other lamellae 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" mesen- 

 teries, according to their breadth. The reproductive organs 

 are in the form of reddish bands, which contain ova and sper- 

 matozoa, 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 " coenosarc" (fig. 37). Each polype has 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 coenosarc 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 



