332 



ECIIINODERMA. 



The Ecliinoderma are exclusively marine, occurring in large 

 numbers even in the deepest seas. Many groups, like the Crinoids, 

 are largely bathybial, others frequent rocky coasts. At the period 

 of reproduction the urchins, starfish, and holothurians frequent 

 the shallow waters, passing their sexual cells into the sea, where 

 fertilization occurs. In some, however, the young are carried 

 about in brood cases until the earlier developmental stages are past. 



m y 



--a 



Fig. 300, — Echinoderm larvsp. (After J. Mijller.) o, anus; m, mouth ; the black line' 

 the course of the ciliated bands. J, form common to all ; //, ///. developmental 

 stages of auricularia (Holothurian) ; IV, T'. stages of the Asteroid bipinnaria; 

 I'/, pluteus of a spatangoid; Vll, larva (Bracliiolaria) of Asterias (orig.). »). 

 mouth ; \\ vent. 



Where there is no brood pouch the young escape from the egg 

 as larvae which swim at the surface, and are distinguishable from 

 the adults (fig. 30G, 1) by their soft consistency, transparency, and 

 bilateral symmetry. By the development of lobe-like processes 

 and slender arms supported by calcareous rods the larvtv assume 

 the most different and bizarre shapes (plutei of eehinoids and 

 (iphiuroids, bracliiolaria and bipinnaria of asteroids, auricularia of 

 holothurians), all of which can be referred back to a common type 

 with tri-regional alimentary tract ;iud ;i ciliated band around the 

 mouth, strikingly resembling tornaria, the larva of Bahnwrilossiis. 

 The different appetirances of the larva^ are due to the drawing out 

 of the ciliated band into lobes and arms, and also to its becoming 

 lirokeii into parts which unite themselves into comjilete rings 

 (fig. oon, T'). 



The metamorphosis of the bilateral larva into the radial adult is very 

 foniplicated. It begins early with the formation of outgrowths from the 

 archouteron (tig. 307), wliicli become separated and form the anlagen of 



