140 THE VOYAGE OF H.M.S. CHALLENGER. 



Skeleton. — (a) Dermal ; consisting of scattered tylota, sometimes arranged in 

 irregular brushes, (b) Main ; arranged very much as in Myxilla rosacea, var. japonica, 

 but not so firm and dense ; the meshes of the sjsicular network are triangular and the 

 side of each triangle is of one spicule's length, and usually composed of several sf)ined 

 styli lying together parallel with one another, the rudimentary fibre thus formed 

 being echinated, chiefly at the nodes, by smaller spined styli. 



Spicules. — (a) Megasclera ; of three kinds, (l) Entirely and considerably spined, 

 almost or quite straight, sharply pointed styli (PI. XXVII. figs. 9, 9a), measuring about 

 0"28 by 0"0155 mm., making up the main skeleton. (2) Much smaller, straight, very 

 sharply pointed and entirely spined styli (PL XXVII. fig. 96), measuring about 0"12 by 

 O'OOS mm., echinating the skeleton fibre. (3) Tornota or tylota (PI. XXVII. fig. 9c), 

 somewhat hastately pointed or with slightly developed, smooth, oval heads, pointed at the 

 ends; size about 0"22 by 0'0063 mm., dermal. (6) Microsclera ; (1) tridentate isochelse 

 (PI. XXVII. figs. 9cZ, 9e) of rather peculiar form, and with very strongly curved shaft ; 

 length about 0"044 mm. Numerous much smaller isochelse also occur which at first 

 sight appear to be different in shape, but they are probably merely young forms of the 

 larger. The isochelse are extremely abundant, especially in the dermal membrane, 

 forming in places an almost continuous layer. (2) Very small and very slender sigmata, 

 usually contort and much bent, length usually about 0'02 mm., very rarely up to 

 0-063 mm. 



The external appearance of the sponge and the shape of the isochelse are well- 

 marked characters of this species, and serve to distinguish it from others of the 

 genus. 



ioca^.— Station 320, February 14, 1876; lat. 37° 17' S., long. 53° 52' W.; ofi" 

 the mouth of the Eio de la Plata ; depth, 600 fathoms ; bottom, green sand ; bottom 

 temperature, 37°'2. Six or eight pieces, mostly in a rather fragmentary condition. 



Myxilla noUlis, Eidley and Dendy (PI. XXVII. figs. 15, 15a, Ibh, 15c, 15fZ; PL 

 XXX. figs. 2, 2a). 



1886. Myxilla nohilis, Eidley aad Dendy, Ann. and Mag. Nat. Hist., ser. 5, vol. xviii. \). 473. 



Sponge (PL XXX. fig. 2) massive or lobate, may be encrusting. The largest specimen 

 consists of two lobes, a large one and a small one, arising from a common base; the 

 base is about 56 mm. long by 25 mm. broad and the larger lobe is 56 mm. in height. 

 Another specimen encrusts the branches of a dead Coral. Colour in spirit greyish-yellow. 

 Texture loose, soft and spongy ; rather cavernous. Surface uneven but not hispid. 

 Dermal membrane very distinct, transparent, glabrous; easily separable from the under- 

 lying tissues. Oscula ; round, scattered openings, having their margins flush with the 

 general surface of the sponge. Pores very distinct, arranged for the most part in very 



