ECHINODERMA 7 



larval stage. Some few forms are viviparous or protect their 

 brood; in these, of course, there is no free - swimming larval 

 stage, although the larval form may exceptionally be retained. 



The Asteroid larvae differ from the other echinoderm larvae, 

 partly by the anterior part of the ciliated band on the 

 ventral (oral) side being separated from the rest of the band, 

 there being thus two separate bands, partly by completely 

 lacking a larval skeleton or other calcareous bodies characteristic 

 of the larva. The Holothurioid larvae have peculiar calcareous 

 bodies (wheels), the Ophiuroid and Echinoid larvae have a 

 strongly developed calcareous skeleton, which has nothing to 

 do with the skeleton of the adult but is absorbed during meta- 

 morphosis. 



The larval mouth lies on the ventral side in a transverse 

 furrow, bordered by the ciliated band. The anterior part of 

 oesophagus is bell-shaped and wide ; the stomach is rather large, 

 globular ; the rectum generally bends towards the ventral side, 

 where the anal opening lies. At the sides of the stomach lie 

 two pairs of small vesicles, the anterior and posterior enter ocoel 

 vesicles. From the left anterior vesicle a canal — the stone 

 canal — arises, which opens to the exterior on the dorsal side 

 through a pore — the dorsal pore ; around this pore the madre- 

 porite is formed. From this enterocoel vesicle also the 

 watervascular sj^stem Qiijclrocoel) develops, at first in the shape 

 of five blunt outgrowths lying crescentically at the side of the 

 oesophagus ; gradually it forms a closed ring around the oeso- 

 phagus, the ring canal, the five outgrowths forming the radial 

 canals. Various modifications of this arrangement are met with, 

 but they may all be traced back to this simple original type of 

 development. 



The echinoderm larvae are generally small, microscopical 

 animals, only a few, like the Luidia larva, the tropical Auricularia 

 nudibranchiata and the antarctic Auricularia antarctica, reaching 

 a more considerable size, up to 35 mm. in length. They are quite 

 transparent, at most with some pigment spots, chiefl}^ at the ends 

 of the arms, and are among the most beautiful microscopical 

 objects. 



Key to the maiyi types of pelagic Echinoderm Larvae. 



1. No skeleton, at most isolated calcareous bodies ... 2 

 Skeleton present ........ 3 



2. Anterior part of ciliated band not separated from the rest of the 



band ; calcareous bodies (wheels, stars) usually present 



Holothurian larv^ae, Auricularia 



