235 



The pluteus, a nomadic stage of the echinus, is as Mr. A. 

 Agassiz states " a scaffolding in which the future sea urchin plays 

 but a secondary part, and is composed of two open spirals, the 

 one to form eventually the complicated abactinal system (the in- 

 terambulacral and ambulacral plates), the other to form the water 

 system, and holding between them the digestive cavity and 

 other organs of the pluteus, which as yet appear to have no con- 

 nection whatever with the spines of the future Echinus. Yet to- 

 wards the end of the nomadic pluteus life a few hours are sufficient 

 to resorb the whole of the complicated scaffolding, which has been 

 the most striking feature of the Echinoderra, and it passes into 



something which, it is true, we could hardly recognize as an 

 Echinus, yet has apparently nothing in common with its former 



From this time the body of the pluteus is absorbed by the 

 growing sea-urchin ; the spines and suckers of the latter increasing 

 in size and number with age, and by the time the larval body has 

 disappeared the young Echinus is more like the adult than the 

 starfish at the same period in life. Fig. 109 (tt, tentacles; s"s"', 

 spines) represents the sea urchin very soon after the resorption of 



In after life the young sea urchin with its few and large spines 

 resembles Cidaris and a number of allied forms, showing that 



