SILICIOUS AND HORNY SPONGES WILSON. 337 



D. ingalli differs from D. seychellensis in not having extensive 

 intracortical cavities, but, instead, long narrow canals of uniform di- 

 ameter each leading from a single pore and passing completely 

 through the cortex. The cortex is fibrous throughout. 



D. japonica differs from D. seychellensis in the absence of the 

 choanosomal oxyaster. Hentschel (1912, p. 317) finds intermediates. 

 Topsent (1918, p. 599) thinks perhaps Hentschel had representa- 

 tives of several species before him. But it seems probable now that 

 we shall find, if we look for them, individuals that are intermediate 

 in respect to any one of the differential characters that separate re- 

 lated forms in this and many sponge genera. 



D. maza differs from D. seychellensis in the form of the choano- 

 somal aster. It is what is often called a strongylaster and distinctly 

 smaller than the oxyaster of D. seychellensis. The rays are rough- 

 ened rather than distinctly spined whereas in D. seychellensis the}" 

 are characteristically spined; rays only rarely dichotomous whereas 

 they are characteristically so in D. seychellensis. The spicule of the 

 Albatross form (see above) may be said to vary toward the D. maza 

 type. 



In D. lyncurium the cortex is variable, sometimes of the D. 

 ingalli, sometimes of the D. seychellensis type. Smaller euasters, 

 Topsent, 1918 (p. 568), variable in details of shape but of about 

 same size, the ectosomal not differing from the choanosomal ; centrum 

 small ; rays 9-15, feebly spinose along their length or only terminally 

 so, the terminal spines frequently producing an enlargement (acanth- 

 tylaster type) ; rays cylindrical or tapering, varying towards the 

 oxyaster type, which apparently is reached or nearly reached (Top- 

 sent, 1918, fig. 18, p. 568), although Topsent says "they do not dif- 

 ferentiate into pure oxyasters"; diameter of entire spicule 13-20 jx. 



In D. multifda Carter spicules are essentially like those of D. 

 seychellensis, but the body is " membraniform, lacinulate, expanded, 

 flat or erect, fan or vase shaped, proliferous" (Sollas, 1888, p. 441). 



Since Sollas' monograph a very considerable number of forms 

 have been described, one effect of which has been to make the dis- 

 tinctions between the species recognized by Sollas less sharp. Besides 

 Sollas, Lindgren, Thiele, Hentschel, Dendy, and recently Topsent 

 (1918) have critically discussed these distinctions and deepened our 

 knowledge of the genus. 



Lindgron (1898, p. 358) rearranged and combined the recorded 

 species, reducing the number to three: D. lyncurium Authors, with 

 spherasters, and oxyasters 12-20 \i in diameter; D. ingalli Bower- 

 bank, with spherasters, tylote chiasters 6-16 xt. in diameter, and 

 oxyasters 20-52 ;x in diameter; and D. japonica Sollas, with spher- 

 asters, and tylote chiasters 8-16 jx in diameter. Of these he regarded 

 D. lyncurium as the simplest and probably the oldest form. From 

 81709—25 6 



