REPORT ON CORALS — HELIOPORID^E. Ill 



the layer are to be found nematocysts. The ectoderm is prolonged to form the lining of 

 the stomachs of the polyps ; otherwise it is superficial only. 



The mesoderm consists of three different histological structures : — (1) A nearly homo- 

 geneous transparent connective tissue ; (2) layers of connective tissue cells ; and (3) masses 

 of finely fibrillar tissue. 



(1.) Beneath the ectoderm is a thick layer, of a mean thickness of about '07 mm., 

 likewise extending over the whole surface of the coral, which consists of a highly 

 transparent connective tissue, which is almost homogeneous, but in which faint lines 

 indicating slight fibrillation may here and there be seen. 



Extensions of this homogeneous layer form the central layers of the membranes lining 

 the ccenenchymal tubes and calicles, and the median plates of the mesenteries, part of 

 the wall of the stomach, &c. The layer of mesoderm immediately beneath the ectoderm 

 is pierced by the superficial system of canals and traversed by the projecting points of 

 the corallum (PL II. fig. 4). 



(2.) Imbedded within the superficial homogeneous layer of the mesoderm occur also 

 fusiform and branched connective tissue cells, which are associated together in elongate, 

 often nearly linear groups (PI. II. fig. 4). Many of these cells are branched, throwing 

 off fine filamentous processes in various directions. Layers of similar cells lie everywhere 

 next apposed to the hard tissues of the living corallum, as has already been described. 

 These cells do not compose any portion of the polyps themselves, but merely line the 

 calcareous calicles. 



(3.) In decalcified preparations of Heliopora enclosed within the layers of connective 

 tissue cells, at the places before occupied by the growing points of the corallum, occur 

 the masses of very finely fibrillar calciferous tissue already described (PL II. fig. 4, P). 

 Both this and the corallum itself belong to the mesoderm. 



Endoderm. — The endoderm consists of spherical cells, each with small transparent 

 nucleus, and contents consisting of irregular yellow pigmented masses and dark coarse 

 granules. They have a mean diameter of about "014 mm., but vary much in size. They 

 are most probably ciliated in the fresh condition, as are the closely similar endodermal 

 cells of other Alcyonarians. I have not been able to see cilia in the hardened specimens 

 which I have examined ; nor have I in these specimens been able to detect differences 

 between the endodermal cells lining the cavities of the calicles and tubes and those 

 lining the canals. Some of the cells show a division of their contents into four (PL II. 

 fig. 7, a, a). The endodermal cells form layers lining the canals, the ccenenchymal 

 tube cavities, the cavities of the calicles, and interseptal spaces. 



Soft Tissues of the Tubes and Calicles. — The ccenenchymal tubes in their upper 

 cavities are thus lined throughout by a membrane consisting of three layers, viz., an 

 outer layer of connective tissue cells, a middle layer of homogeneous connective tissue, 

 and an inner lining Layer of endodermal cells. The calicles are lined throughout in like 



