DEVELOPMENT OF ECHINODEKMATA. 565 



have brought to light a most remarkable set of facts in regard 

 to the developmental history of the Echinodermata. The 

 details of this history vary so greatly in the different sections 

 of the group, that all which can be here attempted is a 

 general notice of its most important features. The em- 

 bryonic mass of cells, which is produced in the ordinary way 

 from the egg, is usually converted, not (as in Insects) into a 

 larva which is subsequently to attain the perfect form by a 

 process of metamorphosis, but into a peculiar being, destined 

 to a merely temporary existence, whose function seems to be 

 to give origin to the real Echinoderm by internal gemmation, 

 to obtain and prepare for it the materials of its development, 

 and to carry it to a distance from its fellows, so as to prevent 

 the spots inhabited by the several species from being over- 

 crowded by the accumulation of their progeny. These larval 

 zooids present many points of resemblance to the larvae of 

 certain Annelids ; their bodies have a bilateral not a radial 

 symmetry, the two sides being exactly alike ; each side 



Fig. 305. PENTACRI.XOID LARVA OF COMATULA. Fig. 306. COMATULA. 



is furnished with, a ciliated fringe along the whole or the 

 greater part of ifs length ; and the two fringes are united 

 by an upper and a lower transverse ciliated band, between 

 which the, mouth of the zooid is situated. Although the 

 adult Star-fish and Sand-stars have neither intestinal tube nor 



