18 SALEXIA YAIUsriXA. 



existence of a large permanent anal plate in the young of many Echini 

 among the Desmosticba. 



I was in hopes that the examination of the series of young Salenia? dredged 

 by the " Blake " might throw some light on the formation of the suranal 

 plate, and its homology with the single large anal plate of the early stages 

 of young Echini belonging to other families. On examining these young 

 Salenia? I find that the abactinal system occupies a comparatively much 

 greater space of the abactinal surface of the test than in the larger speci- 

 mens. But in all the young stages, even when not measuring more than 

 1.5 mm. in diameter, the arrangement of the plates of the abactinal system 

 does not differ from that of the older specimens, the suranal plate being 

 only proportionally somewhat smaller. In the youngest Salenia collected 

 (1.5 mm. in diameter) the sutures between the genital and ocular plates are, 

 as in all voting Echini, somewhat indistinct, and it is difficult to define the 

 exact limits of the abactinal system. As I have already shown in other 

 young Echini (Strong^ylocentrotus, Echinus, Arbacia, Diadema, etc), the 

 coronal plates of the test and the plates of the abactinal system are very 

 indistinct in the stages immediately following the resorption of the Pluteus, 

 while the actinal system is well defined. It is only later that (lie separation 

 of the ambulacra] and interambulacral areas can be traced ; at about the same 

 time the limits of the abactinal system become defined ; and still later, the 

 separation of thai system into its component plates surrounding the anal 

 system, which from the earliest stages is readily recognized. In the young- 

 est stage of Salenia the anal system is distinctly pentagonal, and covered 

 by eighl large triangular plates with rounded tips; the plates carry small 

 granules supporting minute sessile papillae similar to those found on the 

 genital and ocular plates, but smaller. 



As in other young Echini, the anal system is comparatively large; the 

 suranal plate is at first quite small, a narrow plate on the side ol" the anal 

 system opposite to the nearest ocular plate. It appears to gain in size at the 

 edges adjoining the genital plates. As fast as the plates of the genital ring 

 increase, the pentagon they form round the suranal plate enlarges, and the 

 outer edges of this plate keep pace in their growth with that of the inner 

 edges of the adjoining genital plates. The genital and ocular plates carry 

 three or four large granules irregularly placed on the plates, with a row of 

 granules forming a line round the edge of the anal system. There are as 

 yet no traces of the genital or ocular openings. The whole of the abactinal 



