436 MR, H. N. MOSELEY ON THE STRUCTURE OF THE STTLASTEBIM. 
retracted zooids they contract gradually in diameter, to terminate at the surface of 
the coral in small openings, which are usually seen to he quite closed by contraction of 
the surrounding superficial membrane in hardened specimens. The sacs lie loose 
within the pores of the coralla ; 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 coenosarc, which issue from the numerous openings in the walls of the pores to 
join on to them (Plate 35, fig. 1 , G Z). 
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 coenosarcal 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 (Plate 36, I) Z). No 
doubt, in the expanded condition of the zooid the ectoderm would appear much 
thinner. The outer suface of the layer is thickly beset with nematocysts of the 
smaller variety, which are so closely packed side by side, with their pointed ends out- 
ward, that in the retracted zooid no interstices between them are to be made out 
(Plate 43, 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 some- 
what prismatic- shaped cells, disposed in it radially to the central axis of the zooid. 
At the inner surface of the ectodermal layer is a layer of very distinctly differentiated 
muscular slips, which have a longitudinal disposition (Plate 43, fig. 2, M ; Plate 36). 
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 intervals. 
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 
coenosarc in immediate connexion with the bases of the zooids, passing beneath the 
ectoderm of these canals, and being inserted into their walls. The musdes 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 coenosarcal canals, and forms a 
complete sac within the zooids. This basement membrane shows, in the contracted 
zooids, a transverse striation (Plate 43, fig. 6), which was at first supposed to indi- 
cate the existence of a layer of circular muscular fibres crossing the described 
longitudinal muscles. No definite circular fibres could however be detected, and the 
appearance is probably due to contraction of the membrane. 
