THE SPONGES OF THE WEST-CENTRAL PACIFIC 199 



minate at oscules. These close so rapidly that by the time the sponge is brought 

 out of water, they are quite shut. The pores can readily be made out as nearly 

 200 /x in diameter, about 3 per each square mm of surface. 



The ectosome is characterized by the very thin translucent dermis. The 

 endosome is rather densely packed with spicules. It seems clear that the in- 

 halant canals or prosochetes, almost perpendicular to the surface, lead down 

 from the optically evident pores. In contrast, the exhalant canals or apochetes 

 appear to be represented chiefly by the river systems that are parallel to and 

 near the surface. 



The skeleton comprises large tylostyles, often about 19 /x by 500 jx to 16 fx 

 by 700 fx in dimensions. Smaller ones are probably juvenile. There are exceed- 

 ingly abundant spirasters of typical shape. The largest, particularly in the 

 Ponape region, may be as much as 80 /x in total length. In the Marshall 

 Islands, a length of 50 jx is more frequently the maximum. On the other hand, 

 the spirasters may be as short as 10 /x, and in this case are almost euasters. 

 Intermediates between the two sizes do occur, but they are probably juvenile 

 forms of the larger ones, so that this species may be described as having two 

 categories of spirasters. 



The type of this genus is Spirastrella cunctatrix Schmidt, 1868, page 17, 

 a Mediterranean species. The present species is obviously close to cunctatrix, 

 but the latter has only one size range of spiraster, considerably smaller than 

 the largest ones of potamophera. It is described as having its oscules in 

 grooves, but these do not seem to be river systems increasing in size as they 

 decrease in numbers. Instead, they seem to be somewhat parallel rifts. Another 

 closely related form was first described as Thalysias coccinea by Duchassaing 

 and Michelotti, 1864, page 84, from the West Indies, where it is an exceedingly 

 abundant species. It does not have the pronounced grooves or river system 

 and in spiculation is more like cunctatrix than it is like potamophera. 



The species name potamophera, is from a Greek work for "river" and 

 "to bear," because this species bears river systems in its structure. 



Spirastrella decumbens Ridley 



Text Figure No. 135 



This species is here represented by the following : 

 U.S.N.M. No. 22899, My No. M. 201, collected August 13, 1949, by diver 



in the west portion of Truk lagoon south of Polle Islet. The depth was 



4 meters, and the substrate was dead coral. . 

 U.S.N.M. No. 22857, My No. M. 151, collected July 7, 1949, by diver at 



Ebon Atoll from the southeast portion of the lagoon. The depth was 



2 meters, and the substrate was dead coral. 

 U.S.N.M. No. 22848, My No. M. 142, collected July 5, 1949, by diver at 



Ebon Atoll from the miniature lagoon at the south corner of the lagoon. 



