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



and the like. The general colour of the test of this species is yellowish-grey, the sutui-al 

 lines of the coronal plates are distinct, and the poriferous zone and ambulacra! system 

 well defined from the ambitus to the actinostome, the amljulacral tentacles prominent, 

 swollen, tapering, and tipped with a slight sucker, while they become pointed on the 

 abactinal system and are much less prominent. The miliaries and secondaries of the 

 actinal surface are more numerous than upon the abactinal surface. In a smaller 

 specimen measuring not more than 91 mm. in diameter the difference between the tuber- 

 culation of the actinal and abactinal surfaces is not so marked (PL XIX. figs. 7, 8). 

 The scrobicular area, it is true, is already covered by the swollen integument attached 

 to the base of the spine, but there is no such striking difference as in the adult ; the greater 

 prominence of the ambulacra! zone of the actinal surface, however, already exists (PI. XIX. 

 fig. 8). The sutures of the plates are also better defined, and the coronal plates are not 

 yet liroken up into secondary plates. The line of demarcation between the actinal mem- 

 brane and the coronal plates is not so well defined, and the same is the case as to the line 

 of demarcation between the aliactinal system and the coronal plates adjoining the anal 

 system. There is no material difference in the arrangement of the plates or of the pores, 

 but the coronal plates of young specunens overlap but little, and the genital as well as the 

 ocular plates (PI. XVIII. fig. 9) are far more distinct than in the adult where the large 

 anal plates have forced their way between them. 



Thomson was the first to call attention to the peculiar structure and arrangement of 

 the internal organs of this group ;^ the observations here recorded will supplement the 

 earlier ones of Thomson. 



The genital organs are narrow ; they branch slightly, forming small clusters on each 

 side of a long median tube. They extend not more than half-way from the abactinal 

 system towards the edge of the test (PI. XIV. fig. 1). The main ambulacral tube is large 

 and specially prominent on the actinal side (PI. XIV. fig. 2) where the ambulacral pores 

 are closely crowded together, and the lateral tubes extending to them from the main tubes 

 form, as has l)een noted by Thomson, a series of parallel tubes across the median ambula- 

 cral plates (PI. XIV. figs. 2a, a'). These lateral tubes are finer and more distant in the 

 abactinal part of the test (PL XVIII.'' figs. 10, 11) forming delicate lines merely, not 

 visible without being magnified (PL XIV. fig. 1). The general arrangement of the 

 alimentary canal reminds us of that of the Diadematidse (see plate xxvii., Revis. Echini) ; 

 but in addition we find that the mesenteries attaching it to the test form in this family 

 (as has already been noticed by WjrvUle Thomson) a series of partitions in the shape of 

 large open horse shoes, extending over the ambulacral systems and confining the loops of 

 the alimentary canal to the interambulacral spaces (PL XIV. fig. 2). The extremities 

 and edges of these mesenteries are attached to the actinal and abactinal floors by ex- 

 ceedingly fine threads. The whole system of horse shoes thus forms a sort of pouch, ■^dth 



1 Echiuoidea of the Porcupine Expedition, Trans. Roy. Soc, vol. clxiv. part 2, plate xlvi. 



