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



sized principals and terminals of equal length, such as occur so abundantly in Aulocystis- 

 zittelii, are here wholly absent. On the pentact dermalia the proximal radial ray is 

 much drawn out. St. Vincent, West Indies. 



Species 2. A idocystis zittelii, Marshall. 



Pear- or egg-shaj^ed form, from the size of a hen's egg to that of a man's fist. The 

 system of anastomosing thin-walled tubes, as thick as a finger, exhibits a central main 

 passage, or two may be present. From the latter, simple or slightly branched anasto- 

 mosing tubes radiate outwards, and between these there is an irregular system of wide 

 anastomosing intercanals. The whole system of tubes is covered externally with a thin 

 smooth enveloping capsule, which at the end of the principal passage and lateral tubes 

 exhibits cleft-like or irregularly stellate apertures, while the portions of the capsule above 

 the intercanals consist of a more uniformly porous plate or skin, through which the water 

 enters the sponge. The dictyonal framework supporting the walls of the tubes seems to 

 be very regularly constituted, and consists of beams with pointed tubercles, enclosing 

 meshes usually exactly square or cubical. They are united by nodes of intersection, 

 which are so surrounded by strong beam-like oblique buttresses, with tubercles but 

 without axial canal, that the edges formed from the latter are the edges of a regular 

 octahedron. The direct continuations of the beams within the octahedron are weakly 

 developed and smooth, but provided with axial canals. The loose parenchymalia include 

 small, somewhat regularly formed, oxyhexasters, various discohexasters, isolated delicate 

 graphiohexasters, and in certain regions long oxydiacts with central swelling. The 

 ordinary form of discohexaster is that with medium-sized principals and almost equally 

 long terminals, and more rarely that form with short principals and long delicate 

 terminals, or that with short principals and long strongly developed terminals, with thick 

 terminal discs or knobs. The dermal skeleton consists, like the gastral, of oxypentacts, 

 with rough ends to the rays. There is almost always a rudiment of the atrophied sixth 

 ray represented by a small rounded tubercle. Philippines ; West Indies ; Little Ki 

 Island, 140 fathoms. 



