48 THE VOYAGE OF H.M.S. CHALLENGER. 
Coenosteum of Pliobothrus symmetricus. 
The ccenosteum is described and figured by Pourtalés (Deep-Sea Corals, Il. Cat. 
Mus. Comp. Zool., Harvard Coll., Cambridge, Mass., 1871, p. 57, pl. iv. figs. 7 and 8). 
He describes three kinds of pores as existing in the ccenosteum. In reality, there are only 
two kinds of true pores present, viz., the larger circular-mouthed gastropores and the 
smaller dactylopores, which open at the summits of short tubular projections from the 
general surface of the ccenosteum. The third kind is described by Pourtalés as very 
small linear pores disposed over the whole ccenenchyma, and arranged in rows. These are 
merely spaces between the trabecule of hard tissue forming the ccenenchym of the 
coenosteum, and are occupied by canals of the ccenosarcal meshwork in the recent condition 
of the coral. They do not contain any form of zooid. It is to be noted that in Pliobo- 
thrus tubulatus, a second species (Pourtalés, l.c., p. 58), the projecting tubules of the 
tubulated pores are much longer than in the case of Pliobothrus symmetricus, and thus 
form a stepping-stone in the series towards the condition existing in Errina. The pores 
of both kinds in Pliobothrus are devoid of styles. The gastropore cavities are tubular in 
form for a short depth from the surface, and then expand suddenly into a wide basin- 
shaped chamber, which lodges the similarly formed base of the gastrozooid, and from the 
margins of which proceed numerous large canals running mostly to the bases of neighbour- 
ing dactylopores. They sometimes have one or two tabule. The ccenosteum is very 
coarsely porous, otherwise the finer structure is much as in Sporadopora. The ampullee 
are, as in Sporadopora, buried beneath the surface of the ccenosteum. Pourtalés remarks 
on them as ‘‘ occasional round cavities found in the centre of the branch, filled with a 
yoke-like substance contained in a membrane.” 
Soft structure of Pliobothrus symmetricus (Pl. VIII. fig. 2). 
The ccenosarcal meshwork of Pliobothrus symmetricus is very like that of Spora- 
dopora in general arrangement, as will be seen by reference to the figure (Pl. VIII. 
fig. 2). The tubes composing it are however much finer and smaller in diameter, and 
the components generally of the coral are on a smaller scale. 
There is the usual surface layer of ectoderm present, and the nematocysts which 
occur are of the two forms found in the whole of the Stylasteride. The offsets of the 
ccenenchymal meshwork, which join the sheaths of the gastrozooids, show only a very 
indefinite trace of the radiate arrangement which is so marked in Sporadopora. A 
trace of the arrangement does, however, exist (Pl. VIII. fig. 2, X X). 
The gastrozooids are devoid of tentacles." In the contracted condition they consist 
1 In a specimen of Pliobothrus tubulatus preserved in spirit, kindly given to me for examination by Count Pourtales, 
I saw what appeared to be very short tentacles, five or six in number, on the margins of the mouths of the gastropores. 
The specimen was, however, not well hardened, and I am uncertain in the matter. 
