CCELENTERATA : ZOANTHARIA. 155 



Edwardsia) bury themselves more or less completely in the 

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

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



Fig. 70. Morphology of Actinidae. a Actinia rosea', b Arachnactis aiutda. 

 (After Gosse.) 



unknown. Some forms, again (Minyas, Nautactis, Oceanactis, 

 &c.), are oceanic in their habit. 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, usu- 

 ally retractile, arranged in alternating rows, and amounting to 

 as many as 200 in number in the common Actinia. The ten- 

 tacles are tubular prolongations of the ectoderm and endoderm, 

 containing diverticula from the somatic chambers, and often 

 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 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. 

 In reality the mesenteries are arranged in pairs, the chamber 

 between each pair opening above into the cavity of a tentacle. 

 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 



