EMBRYOLOGY. 723 



of his seventh Memoir. The development of the separate parts is very different 

 in the two. The number of spines is much greater in our sea-urchin, and they 

 are of an entirely different shape. Pedicellarise are likewise present in Arba- 

 cia; these do not make their appearance till a much later period in our young- 

 sea-urchins (see PI. X. f. g). What is particularly characteristic of these 

 earlier stages of the young sea-urchins is the great size and small number of 

 the spines. Their position is also peculiar ; they are all placed on the edge of 

 the test, which is exceedingly flat. (Compare this with Podophora.) Five 

 of the tentacles are strikingly prominent, equalling in length the diameter of 

 the test ; they are also remarkable for their great thickness, and the pres- 

 ence of a calcareous ring in the sucker, which is entirely wanting in young- 

 starfishes. A similar calcareous ring is figured by Midler, 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 Emb. Starfish, PI. VI.. 

 PI. VII.), while the more advanced spines are not fan-shaped, but slightly 

 pointed, reminding us of the spines of Cidaris. On turning a young sea- 

 urchin on its actinal side (as in PI. X. f. 2), 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 (PL X f. 4), so that they cover the whole of that surface, and are 

 no longer limited to its edge, as in PI. IX. f. 1 ; 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 contracts to a remark- 

 able extent, — sometimes as much, as three times the radius of the test. 

 The four other tentacles are somewhat more stout, and not capable of such 

 extensive expansion 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 (PI. X. f. .',). 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. The actinal system is cir- 

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

 the ambulacral tentacles are placed one above the other on each side of 

 the median line. The long spines move in every direction, as they are 

 already provided with the peculiar ball-and-socket joint of Echinoids. On 

 removing the spines of one of these young sea-urchins, the great size of the 



