724 EMBRYOLOGY. 



tubercles (PI. IX. f. :;) and the large circular actinal system give them an 

 aspect totally different from what we are accustomed to associate with the 

 genus Strongylocentrotus. The teeth (PI. JX. f. g) fill but a small part 

 of the actinal system; they are five narrow triangular wedges, extendiim 

 from the centre to the edge of the actinostome, covered partially by the 

 network of limestone plates (PI. IX. f. .>). 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 devel- 

 oped, the striking features of the young sea-urchin — such as the circular 

 actinal system, its large size, the great prominence of the tubercles, the posi- 

 tion of the pores one above the other — are all characters belonging to a dif- 

 ferent 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 (PI. X. f. ,?), the spines have lost almost entirely their 

 embryonic character, the tentacles arc much more numerous, and pedicellari;e 

 have made their appearance; in the interambulacral space they are more 

 thickly scattered than in the ambulacra!, where then' are merely three or 

 four. The ahactinal system consists of a single large plate covering the 

 opening of the anus, which opens on one side of it. The additional spines and 

 plates which have been formed are all developed from the abactinal region. 

 The new coronal plates are added in a spiral manner round the anal plate 

 by additions to the limestone mass, pushing farther away from the ahactinal 

 pole the first-formed plates. The outline of the new plate is at first indicated 

 on the lower edge, which becomes somewhat undulated; then the transverse 

 divisions are made, and a spine is formed on the plate soon after that. There 

 are no spines on the last-formed plates. The spines when they first appear 

 have the same fan-shaped character as the earliest-formed spines of the 

 abactinal surface (PI. IX. f. i ; PI. X. f. .?. 4). This shape they lose soon, 

 and pass at once into spines resembling the older ones in every respect 

 except size. 



The mode of formation of new coronal plates was discovered by Professor 

 Agassiz as early as 1834, when he gave a short account of it in the Edin- 

 burgh Philosophical Journal. The spiral arrangement of the plates is still 

 very plainly visible in adult specimens. Although the sea-urchins are circu- 

 lar, we have in their mode of growth something which reminds us of the 



