THE SPONGES OF THE WEST-CENTRAL PACIFIC 223 



in diameter and 2 to 5 mm apart. The oscules may be said to be represented 

 by the openings into the large cloaca. They are extremely contractile. In life 

 they were in the neighborhood of 5 to 8 mm in diameter, very irregularly 

 distributed. In some areas they almost touch each other while in other areas 

 (several cm in diameter) there may be none. The size which is mentioned 

 must be regarded with caution, because of the extreme contractility of these 

 exhalant openings. The cloaca may reach a diameter of 10 mm. 



The ectosome consists of a stiff cortex about 2 mm thick, crowded with 

 microscleres and quite a few megascleres. There is also a cortex of especial 

 thickness which lines the cloaca, but it is less than half as thick as the cortex 

 which covers the exterior of the sponge. The roots are chiefly of the type of 

 structure which is represented by the cortex. The endosome of this sponge is 

 given over to enormous canals, 10 to 15 mm in diameter (often nearer the 

 larger diameter). These may be said to arise at full maximum size immediate- 

 ly below the cortex, and they become somewhat smaller, occasionally branch- 

 ing, as they penetrate more deeply into the body of the sponge. They end 

 blindly. From them, enormous numbers of pore-like apertures emerge. These 

 are about 200 jx in diameter, and there are about 2 per square mm of the 

 canal lining. These pore-like openings lead to the relatively thin layers of 

 protoplasmic structure. The latter, often less than 1 mm thick, contain some 

 tracts, about 1 mm in diameter and crowded with spicules. The canals which 

 lead from the chamber region to the cloaca are difficult to measure because in 

 all collected specimens they are small, but they show evidence of having been 

 contracted by muscular effort. It is estimated that in life they must have 

 had a maximum diameter of at least 10 mm. 



The skeleton comprises enormous smooth oxeas, reaching a maximum 

 size of at least 44 fx by 1230 p.. Sizes of about 20 /x by 1000 /x are also com- 

 mon. There are also smaller oxeas, perhaps to be regarded as microscleres, 

 ranging from 3 jx by 18 \x to (more often) 3 \x by 60 [x and (occasionally) 

 4 jx by 45 [x in dimensions. These are covered profusely by very small spines 

 or tubercles and often are centrotylote. These are abundantly distributed 

 throughout the whole sponge, but are somewhat more abundant in the cortex. 

 There also are euasters present, especially in the cortex, which are almost 

 completely absent from the lower portions of the sponge. These have a 

 maximum diameter of about 12 /x, and each of the 10 to 15 rays is noticeably 

 microspined. 



The genus Stellettinopsis was founded by Carter, 1879, page 348, for 

 two species from south Australia. These actually appear to be conspecific, and 

 both should be known as corticata. The second, described on page 349, was 

 named simplex. Neither is well described. Corticata seems to have had spicula- 

 tion much like that of isis, but simplex had a remarkable annulate distribution 

 of the spines on the acanthoxeas or acanthostrongyles. Neither had the 

 remarkable shape which characterizes isis but were smooth lobate masses 



