142 



PHYLUM ECHINODEEMATA. 



pluteus. Antero-lateral arms are never present. The arms 

 have muscles and are contractile, a fact which renders it diffi- 

 cult to preserve thes.e larvae without distortion. 



These are the principal types of larvae of the Echinodermata, 

 but we must not omit to mention the vermiform Ophiurid 

 larvae which are pelagic and the vermiform larvae, of those 

 Asteroids in which care of the brood occurs. The larva of 

 Asterina gibbosa may be classed with the latter. Echinoderm 

 larvae are found principally near the coast and in small waters : 

 they are not characteristic of the plankton of the high seas. 

 Only a few of the larvae are taken in more than one locality. 



In the nomenclature of Echinoderm larvae when the adult is not known, 

 the name of the kind of larva is used as that of the genus and a specific 

 name is added. In this sense there are four genera of larvae, viz. Auricu- 

 laria (confining this term to Holothurian larvae), Ophiopluteus, Echino- 

 pluteus, and Bipinnaria (including brachiolaria). When the adult is 

 known specific names are not required. 



FIG. 98. Longitudinal-horizontal sections through three successive stages of the larva of 

 Asterina gibbosa (from Korschelt and Heider, after Ludwig). A, showing the first origin 

 of the enterocoel Vp from the enteron ; B, a later stage, the enterocoel is still in communi- 

 cation with the enteron D ; C enterocoel cut off from the enteron and consisting of a 

 median unpaired portion in front the anterior body cavity, and two paired portions 

 behind, the right and left posterior body cavities. Bl blastopore; D intestine; Vp entero- 

 coel (vasoperitoneal vesicle) ; r and I right and left side of the larva. The hydrocoel has 

 not yet made its appearance. 



Development of internal organs and the metamorphosis. 



The coelom arises soon after the establishment of the gastrula 

 as a Single evagination of the enteron* (Figs. 98, 99). This soon 

 separates from the enteron, and becomes divided into a number 



* The term vaso-peritoneal is sometimes applied to the enteric pouch 



