REPORT ON CORALS—HELIOPORID. 111 
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 coenenchymal 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 (PI. 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 
oranules. 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 (PI. I. 
fic. 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 coenenchymal 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 
