ON THE EMBRYOLOGY OF ECHINODERMS. 



11 



Plate IV. fig. 13, seventh Memoir, and Plate VII. fig. 2, first Memoir. The whole 

 abactinal surface is thickly covered with dark crimson pigment-cells. The younger 

 spines resemble those of the young Starfish (see fig. 16, Proc. Am. Ac, 1. c), while 

 the more advanced spines are not fan-shaped, but slightly pointed, reminding us of 

 spines of Cidaris. On turning this Sea-urchin on its actinal side, as in fig. 26, we 

 find near the base of each of the five large tentacles four others, which are not as 



advanced, and are incapable of expanding beyond the edge of the test. 



Additional spines are formed on the abactinal side of the test of older specimens (fig. 

 25), so that they cover the whole of that surface, and are no longer limited to its edcre 

 as in fig. 24; the large spines become more pointed, the tentacles grow slender, and 

 they can all expand beyond the edge of the test. The odd tentacle expands and con- 



3 to a remarkable extent, sometimes as much as three times the radius. The four 



othe 



are 



d not capable of such extensive ex pan 



noids. On _„_.. & 



and contraction ; the pair of tentacles placed nearest the mouth is the stoutest. The 

 position of the tentacles is best seen from the actinal side (fig. 26). The whole actinal 

 surface is covered with a plating of limestone cells, which leave but a small circular 

 opening, the mouth, in which the points of the teeth project. This actinal system is 

 circular ; there are no notches for the passage of the gills, as in adult Echini ; the 

 ambulacral tentacles are placed one above the other. The long spines move in every 

 direction, as they are already provided with the peculiar ball-and-socket joint of Echi- 



the spines of one of these young Sea-urchins, the great size of 

 the tubercles (», »', fig. 27) and the large circular actinal system give them an aspect 

 totally different from what we are accustomed to associate with the genus Toxo- 

 pneustes. The teeth (k, fig. 27) fill but a small part of the actinal system ; they are 

 five narrow triangular wedges, extending from the centre to the edge of the shell, 

 covered partially by the network of limestone plates. (See fig. 27.) The test thus 

 denuded of its spines resembles in all the general features that of a Cidaris. With the 

 exception of the formation of the abactinal system, which is not yet developed, the 

 striking features of the young Sea-urchin — such as the circular actinal system, its 

 large size, the great prominence of the tubercles, the position of the pores one above 

 the other — are all characters belonging to a different family from that to which the 

 adult Sea-urchin belongs. The little Sea-urchin does not long retain these anomalous 

 features ; with every day of increasing age the changes which it undergoes bring it 

 closer and closer to the condition of the adult. In a young Sea-urchin of a diameter 

 of one fifteenth of an inch (fig. 28) the spines have lost almost entirely their embry- 

 onic character, the tentacles are much more numerous, and pedicellaria have made 



