THE STRUCTURE OP XENIA HICKSONI. 255 



of other Alcyonaria. The polyps are, for the greater part 

 of their length, bound together in bundles of about forty 

 to sixty, each bundle forming one stem of the colony. 

 The external characters of the free portion of the polyp 

 have been described above. The eight mesenteries of 

 the polyp are arranged as in typical Alcyonaria. On their 

 ventral faces they bear the retractor muscles, and on the 

 opposite faces the protractor muscles, neither of which are well 

 developed. This somewhat feeble development of retractor 

 and protractor muscles accounts for the non- retractile nature 

 of the polyps. The intermesenterial spaces are continued 

 upwards into each tentacle and into each pinnule. Tn the free 

 portion of the polyp, and particularly in the tentacles and 

 pinnules, these spaces are to a large extent filled up by 

 zooxanthellie. The specimen is a male, and in the lower part 

 of the free portion of the polyp, and in the upper part of the 

 stem, the ccelentera are crowded with sperm-sacs containing 

 spermatozoa in all stages of development, from the small 

 masses containing only two or four primitive sperm-cells to 

 the large mature sperm-sacs containing many hundreds of ripe 

 spermatozoa. 



Mesogloea, its Canals and Cells. — In the stems the 

 ccelentera of the polyps are bound together by a moderately 

 large quantity of mesogloea. The mesogloea immediately round 

 each polyp is slightly denser than that further away, so that 

 in transverse sections of the stem, especially if the sections 

 be taken through the upper part, one can distinguish the more 

 deeply staining ring of slightly denser mesogloea, which defi- 

 nitely belongs to the coelenteron within it, from the less dense 

 mass of mesogloea between these rings, which cannot be 

 assigned to any polyp or polyps (Plate 25, fig. 9). Traversing 

 the mesogloea of the stem are numerous canals, cords and 

 strands of cells, which place all the parts in intimate com- 

 munication with each other. The canals may be divided into 

 two systems — a superficial system find an internal system. 



The superficial canal system (figs. 8, 9, Sup. Can.) is 

 formed by a plexus of numerous eudodermic canals, which 



