150 THE OCEAN WORLD. 



in organic continuity with the polyp, which has retired thither more 

 or less completely as into a cell. 



Milne Edwards remarks that the polypidom of the Madrepora pre- 

 sent in their structure five principal modifications, due in part to the 

 fundamental number of which the chambered cells are the multiple, 

 and in part to the mode of division in the visceral chamber, and finally 

 to the manner in which its tissue is constituted. M. Edwards avails 

 himself of this peculiarity of structure in order to divide the Madrepores 

 into fixed sections ; namely, Madrepores apores, Madrepores perfores, 

 Madrepores tabules, Madrepores tuberleux, and Madrepores rugueux. 

 In the group of Aporous Madrepores, the polypidom is perhaps the most 

 highly organized. We find there a well-developed and very perfect 

 wall, and a well-developed visceral apparatus. The calyx is neatly 

 starred ; the number of rays in the earlier stages being six, which soon 

 afterwards reach from twelve to twenty-four. The cells between the 

 chambers are sometimes open in all their depth, sometimes more or less 

 shut up by transverse plates ; these, being independent of each other, are 

 never reunited in the breadth of the visceral cavity, so that they con- 

 stitute discoid plates such as we find in tabular and rugose Madrepores. 



The animals belonging to this group, which may be characterised as 

 steUiform or star-like, are very abundant in every sea, and in several 

 geological formations. They constitute many families, among which 

 may be noted the MILLEPORINA of Ehrenberg, the polypidom of which 

 Dr. Johnston describes as " calcareous, fixed, plant-like, branching or 

 lobed, with cells scattered over the whole surface, distinct, sunk in 

 little fosses, obscurely stellate, the lamellae narrow and almost obsolete." 

 (JOHNSTON'S Zoophytes, vol. i. p. 194.) In Turbinolia, the animal is 

 simple, conical, striped, furrowed externally with larger and smaller ribs, 

 the mouth surrounded by numerous tentacula, and solidified by a cal- 

 careous polypidom, which is free, conical, and also furrowed externally ; 

 attenuated at the base, but enlarged at the summit, and terminating in a 

 shallow radiated lamellar cup or cell. Several species have been dredged 

 off the coast of Cornwall, and the west coast of Scotland and Ireland. 



T. melletiana is described as coral-white, wedge-shaped, somewhat 

 compressed, with interspaces or ribs equidistant, smooth, and glossy. 

 Above, the ribs turn over the edge, and are continued into the centre 

 of the enlarged cup, forming its lamellae. " That the zoophyte must 

 have lived for some time after having become a movable thing, is 



