48 THE VOYAGE OF H.M.S. CHALLENGER. 



Ccenosteum of Pliobothrus symmetricus. 



The ccenosteurn is described and figured by Pourtales (Deep-Sea Corals, 111. Cat. 

 Mus. Corop. Zool., Harvard Coll., Cambridge, Mass., 1871, p. 57, pi. 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 Pourtales as very 

 small linear pores disposed over the whole ccenenchyma, and arranged in rows. These are 

 merely spaces between the trabecular of hard tissue forming the ccenenchym of the 

 ccenosteum, 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 (Pourtales, I.e., 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 tabular. The ccenosteum is very 

 coarsely porous, otherwise the finer structure is much as in Sporadopora. The ampullar 

 are, as in Sporadopora, buried beneath the surface of the ccenosteum. Pourtales 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 (PI. 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 Stylasteridar. 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 (PI. VIII. fig. 2, X X). 



The gastrozooids are devoid of tentacles. 1 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. 



