294 



ECHIXODERMA 



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

 larvx (fia;. 2S2, /) which swim at the surface, and are distinguishable 

 from the adults by their soft consistency, transparency, and bilateral 

 symmetry. By the deyelopment of lobe-like processes and slender arms 

 supported by calcareous rods the larys assume the most different and bi- 

 zarre shapes [plutci of echinoids and ophiuroids {]'!). hracliiolaria (VII) 

 and hipinnaria {VI) of asteroids, auricular id {III) of holothurians), all 



Fig. 2S2. — Echinoderm larvcr (after J. ^liillerV a. anus; ";, mouth; the black 

 line, the course of the ciliated bands. /. form common to all; //. ///. de\'elopmental 

 stages of auricularia (Holothurian); 71'. T, stages of the -\stcroid bipinnaria; TV, 

 pluteus of a spatangoid: 1'//, larva (brachiolaria) of Asterias (,orig.). ??;, mouth; v, 

 vent. 



of which can be referred back to a common t^-pe with tri-regional alimen- 

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

 tornaria, the larva of Balanoglosstis. The dift'erent appearances of the 

 larvtr are due to the drawing out of the ciliated band into lobes and arms, 

 and also to its liecoming broken into parts which unite themselyes into 

 complete rings {]'). 



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

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

 (fig. 2S3), which become separated and form the anlagen of the ccelom and 

 ambulacral system. It is dilTicuU 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 vasopcriloiical 

 di\erticulum (fig. 2S3, /;<-) arises from the bottom of the archenteron; this soon 

 divides into right and left vesicles, the left acquiring a connection with the e.x- 

 terior (madreporic opening). Each vesicle separates into anterior (/;) and 

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

 (hyJroca-lc) system, the others the ccclora. The two ccelomic sacs expand and 



