SPICULAR ORIGIN OF RED CORAL. 57 



In Alcyonium digitatum, the spicules differ very charac- 

 teristically from all the preceding, in their much greater profusion, 

 in their greater massiveness, and in their peculiar shape. The rod- 

 like form, so closely adhered to in A. palmatum, is here nearly 

 obliterated, and the spicules are often irregularly branched, and all 

 the limbs beset by enormous wart-spines. (Fig. 3). 



In the Organ-Pipe Coral (Tubipora) the spicules of the nrpsoglsea 

 surrounding each individual, become locked firmly together by 

 numerous minute serrations, and form perfect calcareous tubes. In 

 the Red Coral of commerce, (CoralUum rubrum), on the other 

 hand, a strong central stony axis is formed by an even more intimate 

 association of the spicules of the axis of the colony, due to an actual 

 cementing together of these tiny limy rods. In Tubipora the polyps 

 can retract within their protective tubes, but in the Red Coral 

 there is scarcely any provision for the retraction, for safety, of the 

 polyps. 



As in the Anthozoa generally, so in Alcyonium the sexes are 

 separate ; indeed even the sexes of different colonies are distinct ; 

 the individuals in any one commonwealth are thus either all males, 

 or else all females. The ova and the sperm masses are borne on 

 little stalked capsules upon the free edges of the mesenteries, and 

 development takes place outside the parent. The embryos are free 

 swimming by means of a complete investment of lashing threads 

 or cilia ; a little while they sport thus amid the waves, and then 

 affix themselves to some rock, and by continued budding produce 

 extensive colonies. 



Note the very large size of the hollow pinnate tentacles, and the 

 thickness of the walls of the oesophagus (st.) Below the latter, the 

 upper parts of the mesenteries are very apparent because of their 

 greatly convoluted and thickened margins (PL x, Fig. 1, m. /.) ; from 

 this point downwards, the mesenteries decrease rapidly in prominence, 

 and soon show us very slight ridges only. The two dorsal ones 

 are of greater length than any of the others, i.e., descending lower 

 down the gastric cavity, and whereas their thickened edges or 

 mesenterial filaments (craspeda) are composed of strongly ciliated 

 cells derived from the downgrowth of two lines of cells from the 

 lining of the oesophagus (ectoderm cells*), the thickened edges of the 

 remaining mesenteries are less ciliate and their cells are endodermal 



* Ectoderm and endoderm are terms of extreme importance. The embryo very 

 early in its history consists of two layers of cells, an outer ectoderm and an inner 

 endoderm, between which develops quickly a third, the mesoderm. All succeeding 

 tissues owe origin to one or other of these, and roughly we may take it that 

 epidermal tissues have ectodermal origin ; the alimentary canal and its offshoots 

 being lined with endoderm, while muscle, connective tissue, &c., are of mesodermal 

 origin. 



