STRONGYLOCENTROTUS DROBACHIENS1S. 279 



In the young Sea-urchins where the Pluteus has only been resorbed 

 recently (PI. X. f. l), we find the spines limited to the edge of the flat 

 test, not articulated. They are few in number, and remarkable for their 

 size. There is no trace of an anal system, the anus still opening by the 

 side of the mouth as in young Starfishes. The tentacles equal in length the 

 diameter of the test. The whole test is thickly covered by dark crimson 

 pigment cells. The spines, which are at first mere projections of the cal- 

 careous network of the shell, become more fan-shaped in older specimens 

 (PL X. f. 4) as they extend further towards the abactinal pole. The 

 odd tentacle expands to a remarkable extent, sometimes to three times 

 the radius of the test. The four tentacles of each ambulacral space are 

 short and stout, and capable of but limited expansion and contraction 

 (PL X.f. 3), the whole actinal membrane being covered by an open plating 

 of limestone cells. The actinal system is circular (PL IX. f. 2), without any 

 notches, large, with teeth occupying but a small part of the actinal space, 

 extending as five narrow wedges from the centre to edge of test. The 

 tubercles are also comparatively large. These embryonic features are soon 

 lost, pedicillarise begin to appear, the spines lose their fan shape, becoming 



