MR. H. iST. MOSELEY ON THE STRUCTURE OF THE STYI.ASTERIHEE. 437 
The inner surface of the membrane is lined by endodermal cells. In the contracted 
zooid, these form a layer two, three, or four cells thick. The cells are globular, clear, 
and transparent, and contain a nucleus (Plate 43, fig. 2 ; Plate 36). On the actual 
inner surface of the layer, bounding the zooid cavity, is a layer of cells similar in 
character to, but much smaller than, those composing the main mass of the layer. 
No doubt the inner surface of the cavity of the zooid is ciliated in living condition ; 
cilia were, however, not detected. Towards the base of the zooid cavity, the trans- 
parent cells are replaced in the endoderm by the spherical pigmented cells, which 
are the principal constituent of the endoderm of the coenosarc. 
The clactylozooids have a tendency to be attached by their bases to one side of the 
bottoms of their sacs, rather than to the lowest extremities of the sacs. When this is 
the case, as in Plate 3, D Z, the zooid in the retracted condition is partly doubled up 
upon itself, and not merely drawn directly in. The main retractor muscles, however, 
pass almost directly downwards to their insertion into the coenosarcal canals. In 
consequence of this arrangement the bottoms of the sacs are, when it occurs, pulled 
somewhat to one side. This form of attachment of the dactylozooids occurs mostly 
amongst the larger examples, no doubt because their greater length requires such 
an arrangement in order to allow of more complete retraction by the aid of the 
doubling of the zooid. This tendency to lateral attachment in the dactylozooids, as 
occurring in Sporado'pora, where the zooids are diffusely scattered over the coral 
surface, is of interest because the same tendency is shown by the dactylozooids in 
nearly all the Stylasteridse ; and in some, as in Cryptolidia, Allopora, &c., it is the 
normal and only method of attachment. 
Gastrozooids . — The gastrozooicls in Sporadopora dichotoma are cylindrical in form, 
with four short tentacles set on to the body equi-distantly in a single whorl. Above 
the line of origin of the tentacles rises the dome-like hypostome, which in the retracted 
condition of the zooids has a height equal to that of about one third of the entire 
height of the zooid body. 
The zooid in its inferior region is circular in section, but superiorly in the region 
where the tentacles are given off and in that of the hypostome, it assumes the form in 
section of a rectangle with the corners rounded off and the sides indented, the 
tentacles being situate at these corners of the rectangle. 
Within the zooid is a wide gastric cavity, into the axis of which, in the retracted 
condition of the zooid, the calcareous style of the gastropore protudes for two-thirds 
of the height of the cavity (Plate 36, St). 
The mouth at the summit of the hypostome appears when viewed from above as a 
cruciform opening leading directly to the gastric cavity. The gastric cavity communi- 
cates by tubular offsets with the axial cavities of the tentacles, and at its base it 
becomes at its periphery continuous with the cavities of four large canals. These canals 
subdivide almost immediately into smaller trunks which anastomose with the general 
coenosarcal meshwork. 
