294 



ECHINODERMA 



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

 lame (fig. 282, I) which swim at the surface, and are distinguishable 

 from the adults 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 bi- 

 zarre shapes (plutei of echinoids and ophiuroids (VI), brack iolaria (VII) 

 and bipinnaria (VI) of asteroids, auricularia (III) of holothurians), all 



FIG. 282. Echinoderm larvae (after J. Miiller). a, anus; m, mouth; the black 

 line, the course of the ciliated bands. 7, form common to all; II, III, developmental 

 stages of auricularia (Holothurian); IV, V, stages of the Asteroid bipinnaria; VI 

 pluteus of a spatangoid; VII, larva (brachiolaria) of Asterias (orig.). m. mouth- v 

 vent. 



of which can be referred back to a common type with tri-regional alimen- 

 tary tract and a ciliated band around the mouth, strikingly resembling 

 tornaria, the larva of Balanoglossus. The different appearances of the 

 larva? 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 (}'). 



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

 cated. It begins early with the formation of outgrowths from the archenteron 

 (fig- 283), which become separated and form the anlagen of the coelom and 

 ambulacral system. It is difficult to give a short summary of the development, 

 partly from the differences in the separate groups, partly from the contradictions 

 of authors. The following seems to be the most common. A vasoperiloneal 

 diverticulum (fig. 283, he) arises from the bottom of the archenteron; this soon 

 Iivides into right and left vesicles, the left acquiring a connection with the ex- 

 terior Jmadreporic opening). Each vesicle separates into anterior (h) and 

 posterior (c) parts, the anterior forming the anlage of the water-vascular 

 (hydrocKle) system, the others the coelom. The two coelomic sacs expand and 



