REPORT ON CORALS — HYDROCORALLI1SLE. 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 (PI. III. D Z. 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 (PI. 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 slips, which have a longitudinal disposition (PI. X. fig. 2, M ; PI. X. fig. 3). 

 These muscular slips do not form a cprite 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 sjDread out in a thick layer over the large main vessels of the ccenosarc 

 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 ccenosarcal canals, and forms a 

 complete sac within the zooids. This basement membrane shows, in the contracted 

 zooids, a transverse striation (PI. X. fig. 6), which was at first supposed to indi- 

 cate the existence of a layer of circular muscular fibres crossing the described longitu- 



