REPORT ON THE HEXACTINELLIDA. 195 



running out into straight terminal rays. The former vary greatly in length in the 

 diflferent amphidiscs, often hardly exceeding the length of the hemisphere, sometimes 

 almost touching the opposite raj^s (PI. XXVII. figs. 5, 6). The axial beam is sometimes 

 comparatively smooth, sometimes knotted, frequently exhibiting at the middle point a 

 cross or a ring of conspicuous knots. I have never found in the skin transitional stages 

 between the two above described very different forms of large amphidiscs, but such inter- 

 mediate structures are observed to occur in the parenchyma. 



The gastral skeleton differs in many points from the dermal. The skin lining the 

 principal gastral and the four large basal diverticula is not supported by pentacts, but 

 by strands of large smooth diacts and monacts, which usually become gradually 

 narrower towards the extremity, terminating in a sharp apex or conical point, or 

 more rarely rounded off. The large amphidiscs, with broad, shovel-like, umbel-rays, 

 which occurred so abundantly in the external skin, are here altogether absent ; and their 

 place is occupied by irregularly scattered, medium-sized forms with eight narrow, 

 comparatively long rays, whose terminal portions are more nearly approximated to the 

 axial beam (PL XXVII. figs. 8, 9). The length of these rays is about 0'3 mm., the 

 breadth of the umbel O'l mm. The axial beam is comparatively narrow, and exhibits a 

 few irregularly disposed tubercles, and usually, at the middle point, four cruciate 

 strongly developed protrusions, which are probably to be regarded as traces of the 

 abortive rays (PI. XXVII. figs. 8, 9). The rays of the umbel resemble in general form 

 those of the large amphidiscs in the external skin ; they are, however, longer, measuring 

 usually about a third of the whole length of the amphidisc. Originating in a short 

 basal piece, they extend for the greater part of their length approximatel}' parallel to the 

 axis, or even with a slight external convexity. A comparatively large number of small 

 amphidiscs occur like those of the dermal membrane figured on PI. XXVII. figs. 5, 6, 

 as also some rather larger isolated forms of similar structure, represented in 

 fig. 4. 



As in the external skin, pentact pinuli occur in abundance in the gastral membrane 

 and in its direct continuation into the efferent passages and canals, where the lining 

 membrane is covered with them. Compared with the dermal pinules, however, they 

 are somewhat different in shape and less abundantly present. The basal cross lying in 

 the limiting membrane consists of four weakly developed, and generally straight, rarely 

 somewhat curved rays, almost twice as long as the basal rays of the dermal pinules, 

 and beset with distally directed teeth. The ray which projects freely into the lumen 

 and varies greatly in length, is likewise but weakly developed, and usually bears 

 obliquely divergent, distally directed teeth. 



The spicules which compose the basal coUar-pad of the sponge body demand special 

 notice. Besides the forms which have just been described in detail, structures occur 

 in the parenchyma and in the dermal membrane of this region, which are not found else- 



