STRUCTURE AND RELATIONS OE CERTAIN CORALS. 
115 
liminary examination of its structure gave the following results -The histological 
structure of the corallum of Pocillopora differs from that of Heliopora and Millepora 
in that its hard tissue is composed of definite prisms of calcareous matter, the poly- 
gonal ends of which prisms can be seen when the systems in which they are disposed 
are viewed end on. The prisms further show a transverse banding, appearing some- 
thing like that of transversely striped muscular tissue. The corallum is excessively 
hard and compact. 
When the coral is decalcified, a very thin continuous membrane separates from 
the surface with the entire structure of the polyps disposed within it at intervals. The 
membrane shows little structure, consisting, as it does, of a thin epithelial ectodermic 
layer and of a layer of connective-tissue. It is devoid of thread-cells and of any canals 
or vessels. Beneath it shows small masses of concentrically wound fine fibres, where 
the projecting points of the corallum have rested, just as in Heliopora. 
The polyps have very shallow cavities, as necessitated by the form of the corallum. 
They have twelve tentacles, six intermediate ones of which are smaller than the 
others. They have the usual twelve mesenteries, and have long mesenterial filaments 
coiled up in the intermesenterial spaces, though how many of these filaments are 
present is uncertain. 
The specimen of Pocillopora examined was devoid of generative organs. The deep 
tissue of Pocillopora, when decalcified, yields a delicate gelatinous mass, showing very 
ill-defined structure, but full of the same vegetable parasite as that which attacks 
Millepora. A further examination of Pocillopora will be made at Hawaii. 
It has already been stated that Professor Verrill discovered some time ago that 
Pocillopora was a Hexactinian ; but I do not know to what extent he carried his 
examination of the coral. 
Note on the Structure of a Stylaster. 
Since Kuxth, as already stated, has referred to the covering of the mouth of the 
calicle of Cryptohelia as the analogue of the operculum of the Rugosa, 1 may state that 
I examined a specimen of a Stylasteracean dredged by the £ Challenger ’ in 500 
fathoms off the Meangis Islands on February 10th, 1875, and with the same result 
arrived at by Sars in his examination of Allopora oculina (Sars, l. c.). The Stylas- 
teracean in question is exactly similar in every respect to Cryptohelia pudica, excepting 
that it has no calcareous laminae in front of the mouth of its calicle. 
The tentacles are of a dirty green colour, simple, elongate conical in form, and, as 
in the case of Allopora oculina, they lie in the grooves between the calcareous septa and 
are retracted and protruded in these grooves. The calcareous septa are twenty-two in 
number, and the tentacles also twenty-two. 
The coral when decalcified leaves an organic residue, consisting of an extremely 
open network of tissue very similar to that of Millepora. The network forms a very 
soft and feeble mass, having the form of the hard parts, the place of which latter is 
MDCCCLXXVI. R 
