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



small amphidiscs here are equal in abundance, and similar in form to those in the 

 outer skin. 



Even the larger afferent and efferent canals are furnished with pinuli, though in 

 rapidly decreasing size and strength. The small amphidiscs also extend for a consider- 

 able distance as canaliculana. 



In regard to the numerous radial pleuralia which project to a greater or less distance 

 beyond the surface of the sponge, the uncinates proper demand special notice. These are 

 long oxydiaets which are thickly surrounded by appressed barljs. They occur chiefly at 

 the upper end in the neighbourhood of the oscular margin, and are always radially 

 disposed so that the pointed ends of the barbs are directed inwards, and the anterior 

 point of the whole spicule outwards (PI. XLIII. fig. 2). Wyville Thomson has figured 

 a specimen of such a long (up to 8 mm.) pleural uncinate [loc. cit., pi. Ixviii. fig. 4). 

 Besides these a large number of cylindrical spicules of various calibre occur, frequently 

 in the form of thin, coiled threads, which gradually decrease in thickness, towards 

 both ends from a maximum diameter about the region of the external surface of the 

 sponge. They terminate internally in a simple point, while the outer extremity generally 

 evades observation owing to breakage. They are usually smooth, either altogether or 

 for the greater portion of their length. Although these long, smooth, pleural prostalia are 

 for the most part broken, and the external extremity only uncertainly distinguishable, a 

 few seem to be intact. I was unable to decide whether there are any, or if so how many, 

 pleuralia which run out externally into a point. Most of the long, projecting, smooth 

 spicules gradually become thinner and thinner towards the extremity, but the fact that the 

 terminal portions themselves are broken off, suggests that they do not simply form points, 

 but terminate like the shorter forms about to be described, in which the outer ends were 

 preserved. On several of the thin fibre-like pleural spicules, which are quite smooth on 

 their internal pointed portion,' one notices further outwards the ajipearance of small teeth, 

 at first very inconspicuous, but afterwards gradually more prominent and spine-like. 

 These are directed transversely or somewhat obliquely outwards. The outer or most 

 external spinous portions of these pleuralia finally terminate in a club-shaped swelling 

 with four cruciately disposed lateral elevations and a single terminal point. The 

 lateral points either project transversely, or are directed oblic[uely outward's ; more rarely 

 they are bent somewhat inwards. In some instances it was possible to recognise in this 

 terminal knob a hint of an axial canal intersection, corresponding to the five conical teeth. 

 These spinose monacts occur especially on the upper portion of the sponge-body. I 

 have repeatedly found, even in the neighbourhood of the oscular margin, simdar monacts 

 of smaller size (2 to 3 mm.), which projected but slightly from the surface of the skin, 

 and were throughout their whole extent, from the internal pointed end to the external 

 terminal knob, uniformly thickly beset with small .spines directed transversely or 

 obliquely outwards (PI. XLIIT. fig. 2). 



