SPONGES WHITELEGGE. 483 



Dermal surface finely reticulated and porous ; the oscula are 

 level with the surface, or in some cases slightly depressed ; they 

 consist of circular apertures about 1 or 1-5 mm. in diameter, each 

 of which exhibits a series of six or more sieve-like pores. 



Texture hard, tough, and when waterworn resilient. Colour 

 grey, and in appearance very like some species of Chalinopsilla. 



Skeleton consisting of a rather open network of strongly 

 developed horny fibre; the primaries are about 0"1 to 0"15 mm. 

 in diameter and from 0'25 to 0*5 mm. apart ; their course is 

 generally longitudinal, but they are very irregular in outline ; 

 this irregularity is probably due to secondary fibres, which are 

 usually short, stout, and about 0*8 to 0*1 mm. in diameter; the 

 slender connectiug fibres are not very evident even near the 

 surface. Subdermally the fibres are stoutish and support a 

 highly sandy ectoderm about 0-5 mm. in thickness. 



Megascleres : — absent C?); the whole of the fibres are more or 

 less charged with spicule fragments and small sand grains ; the 

 materials, probably all of foreign origin, are very loosely arranged, 

 yet they occupy nearly the whole of the fibres. If proper spicules 

 exist they are obscured by the abundance and scattered nature 

 of the sand grains and spicule fragments present. 



Microscleres : — birotulates with a straight shaft and equal 

 dentate ends. Size 0"02 mm. long. They are abundantly 

 scattered along the fibres and also throughout the soft parts of 

 the sponge. 



YVES I A, Topsent. 



YVESIA COMMENSALIS, sp. nov. 



(Plate xliv., fig. 9). 



Station 48. 



Sponge incrusting, forming a dense blackish-brown villous 

 covering over the whole surface of a Gorgonia {Parisis australis, 

 Wright and Studer). The latter authors in describing the 

 Gorgonia state that " they are, however, unfortunately in a very 

 bad state for description, owing to their being overrun by an 

 incrusting sponge, the projecting silicious spicules of which give 

 a roughened appearance to the colony."* 



The union between the two forms is so complete that it is 

 difficult to determine either satisfactorily. There are probably 



* Wright and Studer— Cball. Eep., Zool., xxxi., 1889, p. 183, pi. xli. 

 fig. 3 (spicules only). 



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