332 



ECIIINODEUMA. 



The Echinoderma 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 



-\---ct 



FIG. 306. Echinoderm larvae. (After J. Miiller.) a, anus; m, mouth; the black linei 

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

 stages of auricularia (Holothurian) ; 7F, V, stages of the Asteroid bipmnaria; 

 FI, pluteus of a spatangoid; F//, larva (Brachiolaria) of Asterias (orig.). m 

 mouth ; v, vent. 



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

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

 the adults (fig. 306, /) by their soft consistency, transparency, and 

 bilateral symmetry. By the development of lobe-like processes 

 and slender arms supported by calcareous rods the larvae assume 

 the most different and bizarre shapes (plutei of echinoids and 

 ophiuroids, brachiolaria and bipinnaria of asteroids, auricularia of 

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

 with tri-regional alimentary tract and a ciliated band around the 

 mouth, strikingly resembling tornaria, the larva of Balanoglossus* 

 The different appearances of the larvae are due to the drawing out 

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

 broken into parts which unite themselves into complete rings 

 (fig. 306, F). 



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

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

 archenteron (fig. 307), which become separated and form the anlagen of 



