112 



MANUAL OF ZOOLOGY. 



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 " peristomial 

 space." Round the circumference of the disc are placed 

 numerous tentacles, usually retractile, arranged in alternating 

 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 



Fig. 29. Morphology of Actinidse. a Actinia rosea; b Arachnactis albida. 

 (After Gosse.) 



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 aper- 

 ture 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 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 outwards and then 

 downwards and inwards, are ultimately attached to the centre 



