242 THE SPONGES. 



the larger vestibular spaces, all of the spicules lie in a tangential or nearly 

 tangential position. These tracts lose to a greater or less extent their 

 individuahty (Fig. 2, Plate 23, surface view of part of a large vesti- 

 bular membrane. The upper, left, and lower margins of the figure repre- 

 sent cut edges. The right curved margin represents part of the edge of 

 an osculum. The left and lower margins are not far from the periphery 

 of the entire membrane), in that they become loose and fray out terminally 

 into free spicules, which are scattered in moderate number through the 

 membrane. Some of the tracts are prolonged for considerable distances 

 through the membrane as narrow stream-like bands, which eventually 

 break up into free spicules. 



Comparative. The spicules in T. vestibularis are pretty close to those of 

 T. cornigata (Bwk.) (Biemma corriigata, Topsent, 1892), a parasitic form. 

 Moreover, Topsent says the spicules at the surflice are arranged in diverg- 

 ent bunches (''en bouquets divergents"). But Bowerbank describes (1866, 

 pp. 242-3) and figures (1874, Plate XLIIL, Fig. 3) the dermal membrane 

 of this sponge {Halichondria corrugata Bwk.) as strongly reticulated, and 

 vestibular spaces such as occur in T. vestibularis are not mentioned by either 

 writer. 



Lundbeck (1902, p. 82) describes in detail a Ti/lodesma {Biemma rosea 

 Frst.) known in plate-like fragments, which bear the pores on one surface, 

 the oscula on the other. With this well-marked species, which he so excel- 

 lently describes, Lundbeck thinks it possible to identify another specimen of 

 , a very different habitus. This is a little sponge occurring as a thin incrusta- 

 tion on a Hexactinellid skeleton, and which Lundbeck regards as a young 

 individual. In the description of this specimen Lundbeck does not go into 

 details, and it may be questioned whether it belongs to T. 7vsca. Lund- 

 beck's description of T. i^osea in general would indicate that this particular 

 specimen and T. vestibularis have some points of resemblance in addition to 

 the parasitic habit. 



