REPORT ON CORALS—HYDROCORALLIN. 43 
coral in small openings, which are usually seen to be quite closed by contraction of the 
surrounding superficial membrane in hardened specimens. The sacs lie loose within 
the pores of the ccenostea ; that is, they are smaller in diameter than their containing 
calcareous cavities, but they are held in place by the attached radial offsets of the 
ccenosarc, which issue from the numerous openings in the walls of the pores to join on to 
them (PI. II. fig. 1, GZ). 
The dactylozooids of Sporadopora vary much in size, the smaller being of less than 
half the dimensions of the largest. They are elongate-conical in form, and are composed 
of an ectoderm, endoderm, membranous and muscular layers. They have an axial tubular 
cavity within, which communicates directly at their bases with the larger deeply-situate 
canals of the ccenosarcal meshwork. 
The ectoderm forms, in the retracted zooids, a thick external layer, which is thrown 
by the contraction of the zooid into a series of transverse folds (Pl. III. DZ. No 
doubt, in the expanded condition of the zooid the ectoderm would appear much thinner. 
The outer surface of the layer is thickly beset with nematocysts of the smaller variety, 
which are so closely packed side by side, with their pointed ends outward, that in the 
retracted zooid no interstices between them are to be made out (Pl. X. fig. 2, E). 
Beneath this armature of nematocysts the main thickness of the ectodermal layer is 
composed of finely granular matter filled with ovoid nuclei and nematocysts, in various 
stages of development. No definite cell-structure could be determined in the layer, but 
fine lines, having a radial disposition in transverse sections of the zooid, seemed to 
indicate that the layer is composed in reality of somewhat prismatic cells, disposed 
in it radially to the central axis of the zooid. 
At the inner surface of the ectoderm is a layer of very distinctly differentiated 
muscular ships, which have a longitudinal disposition (Pl. X. fig. 2, M ; Pl. X. fig. 3). 
These muscular slips do not form a quite continuous layer, being separated from one 
another, as appears in transverse section, by a definite series of intervening spaces. 
These muscles are fine and difficult to detect towards the tips of the zooids, but increase 
in thickness towards their bases. In these regions of the zooids they are extremely 
conspicuous, and spread out in a thick layer over the large main vessels of the ccenosare 
in immediate connection with the bases of the zooids, passing beneath the ectoderm of 
these canals, and being inserted into their walls. These muscles act evidently as the 
retractors of the zooids. Since they are more highly developed in the case of the 
gastrozooids, they will be further described when these are under consideration. 
United with the muscular layer and inseparable from it, is a layer of membrane 
which is continuous with the membranous layer of the eccenosarcal canals, and forms a 
complete sac within the zooids. This basement membrane shows, in the contracted 
zooids, a transverse striation (Pl. X. fig. 6), which was at first supposed to indi- 
cate the existence of a layer of circular muscular fibres crossing the described longitu- 
