44 THE VOYAGE OF H.M.S. CHALLENGER. 
dinal muscles. No definite circular fibres could, however, be detected, and the appearance 
is probably due to contraction of the membrane. 
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 (Pl. X. fig. 2; Pl. Q11.). 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 endoderm. 
No doubt the inner surface of the cavity of the zooid is ciliated in the living condition ; 
cilia were, however, not detected, Towards the base of the zooid cavity, the transparent 
cells are replaced in the endoderm by the spherical pigmented cells, which are the 
principal constituent of the endoderm of the ccenosare. 
The dactylozooids have a tendency to be attached by their bases to one side 
of the bottoms of their sacs, rather than to the lowest extremities. When this is the 
ease, as in Plate HI. DZ, 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 ccenosarcal 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 Sporadopora, 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 Stylasteride ; and in some, as in Cryptohelia, Allopora, &c., it is the 
normal and only method of attachment. 
Gastrozooids.—The gastrozooids in Sporadopora dichotoma are cylindrical in form, 
with four short tentacles set on to the body equidistantly 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, in section, 
the form of a rectangle with the corners rounded off and the sides indented, the ten- 
tacles 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 caleareous style of the gastropore protrudes for two-thirds 
of the height of the cavity (Pl. IIL. 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- 
