SILICIOUS AND HORNY SPONGES WILSON. 489 



The structure of the fibers, apart from their shape and arrange- 

 ment, is remarkable. They are either entirely, or almost entirely, 

 composed of a minute reticulum, the meshes of which are about 4 \l 

 in diameter, the strands very fine and yellowish in color. Whatever 

 surface layer a fiber has is, in many cases, only a part of the general 

 reticulum which forms the body of the fiber, not differing from it. 

 But in other cases the general reticulum is strengthened at the surface 

 of the fiber by thicker bands, having the appearance of spongin, 

 which run lengthwise but anastomose and form a superficial coarse 

 reticulum. Such a surface structure appears to be only a differ- 

 entiation of the general reticulum and not something apart from it. 

 The fibers exhibit (pi. 44, fig 3) a coarse and conspicuous concentric 

 stratification. Sometimes bits of spicules and often small spheroidal 

 groups of deeply pigmented cells occur in the interior of a fiber, 

 several of the latter often close together. The latter inclusions are 

 prehaps connected with the origin of the fiber, which possibly could 

 be worked out on this material. The fibers, as might be inferred 

 from their structure, are not hard enough, in spite of their size, to 

 offer any great resistance to sectioning. 



Holotype.— Cat. No. 21291, U.S.N.M. 



Keller's Psammaplysilla ardbica (1889) is a common species in 

 the Red Sea. Incrusting, cake-shaped, and more or less conical 

 or massive specimens occur. Sponge is firm and leathery; when 

 dried, stony hard; covered with large conuli; oscula scattered. 

 Skeleton consists of distinct compound fibers which ascend from the 

 base of sponge, each soon breaking up into a reticulum of individual 

 fibers. The individual fibers are irregular and tabulated in outline 

 (Keller, 1889, pi. 22, fig 27) and usually contain abundant sand 

 grains. They are concentrically stratified in a coarse and conspicu- 

 ous fashion and consist entirely of a finely reticulate material that 

 is similar to the medulla of the Aplysitta or Aplysina fiber. Pig- 

 ment cells are abundant in the mesenchyme, especially near the sur- 

 face. The living sponge is of a bright leather color at surface, 

 yellow inside ; in alcohol, black violet. Character of flagellated cham- 

 bers unknown. The sponge has also been recorded by Topsent, 

 1892Z>, for the Red Sea. 



Keller made this interesting sponge the type of a new genus and 

 family (Psammaplysillidae) which he derived from the Aplysillidae. 

 His material did not, however, permit of the study of the flagel- 

 lated chambers, and as Dendy (1905, p. 244) has pointed out the 

 relationship is probably with Aph/shnt and not, as Keller thought, 

 with Aply,silfa. I assume, until the sponge is studied again, that 

 this is the case. If it should turn out that the canal system is 

 really of the Aplysilla type, the striking resemblance between the 

 81709—25 15 



