442 



EMBRYOLOGY 



appearance by the side of the pedicellaria?. They arise as 

 evaginations of the skin in which reticulated calcareous rods 

 are deposited. The first of the dorsal plates to make its 

 appearance is the central one, which is perforated by the 

 anus. Other plates are then secreted about it, in a spiral 

 line, i.e. in such a way that the newly arising plates crowd 

 away the older ones from the anal plate, since they are 

 interpolated between it and the older plates (Agassiz). 



Fig. 215.— Young sea-urchin {Arhacia panclidnin) with parts of the Pluteus lar^'a 

 attaclied (after Colton and Garman, from Brooks's Hnndbook). a, partially de- 

 generated arms of the Pluteus ; /, feet ; St, spines. 



The internal larval organs become a part of the sea-urchin, although 

 a new oesophagus is said to be developed, which is not grown around 

 by the water- vascular rosette, but grows through the previously formed 

 water-vascular ring (Bury),— a condition, therefore, somewhat different 

 from that which we observed in the development of the other groups of 

 F.chifioderms. The dorsal pore persists, and also its connection with the 

 water-vascular ring by means of the stone canal. The outgro\vth of the 

 water-vascular rosette into the ambulacral trunks appears to take place, 

 according to Agassiz, in the same way as in the starfishes, for new feet 

 are continually interpolated between the terminal foot and the adjoining 

 pair. 



