12 ON THE EMBRYOLOGY OF ECHINODERMS. 



their appearance ; in the interambulacral space they are more thickly scattered than in 

 the ambulacral, where there are merely three or four. The abactinal system consists of 

 a single large plate covering the opening of the anus (a) which leads out on one side 

 of it. The additional spines and plates which have been formed are all developed from 

 the abactinal region. The new plates are added in a spiral manner round the anal 

 plate by additions to the limestone mass, pushing further away from the abactinal pole 

 the first formed plates. The outline of the new plates 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. (Figs. 24, 25.) 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 the new plates was discovered by Professor Agassiz as 

 early as 1834, when he gave a short account of it in the Edinburgh Philosophical 

 Journal. The spiral arrangement of the plates is still very plainly visible in adult 

 specimens. Although the Sea-urchins are circular, we have in their mode of growth 

 something which reminds us of the earlier embryonic stages of the Starfish. I have 

 not been, able to trace all the various stages of growth of the young Sea-urchin, how 

 it passes from fig. 28 to the condition when the pores, instead of being arranged in 

 single rows one above the other, are placed in arcs on both sides of a median ambu- 

 lacral row covered with spines. We may, however, form a tolerably accurate idea of 

 the changes which must be gone through by examining the abactinal part of the 

 ambulacral area of an adult Sea-urchin. The mode of formation of the ovarian and 

 ocular plates remains still to be traced. The oldest of the young Sea-urchins (fig. 28) 

 has advanced sufficiently to enable us to see that the subsequent changes which are 

 required to make it agree with its adult condition are by no means as great as the 

 changes which the young Sea-urchin has undergone up to the present time. It has 

 reached a condition Avhich assures us that we deal with a young Toxopneustes, and 

 nothing else. The. pigment spots, so marked in the younger stages, are smaller and 

 scattered more uniformly, the muscular band around the mouth is well developed, the 

 plate covering the actinal area has separated from the edge of the test, and is moved 

 by the muscular membrane which covers the actinal system. There are no notches as 

 yet in the actinal part of the test. The teeth have not changed their form from that 

 found in earlier stages (fig. 27) ; there are from seven to eight tubercles in each verti- 

 cal row of the ambulacral and interambulacral zones. I was unable to distinguish 



