SIGNIFICANCE OF CERTAIN LARV.E OF ECHINODERMS. 185 



and during the period when it was fixed on the bottom certain of 

 the rings continued to function as food gatherers. The two 

 rings which encircled the preoral lobe, being purely locomotor in 

 function, were lost, but the other three took up a more definite 

 relation to the mouth and formed six paths for conducting food 

 to it (see Fig. 1 1, /;). The retention of the ciliated rings among 

 directly developing larvae and the return to a condition with 

 ciliated rings among larvae which possess a more complicated 

 ciliated apparatus during their free-swimming life may, as I have 

 stated elsewhere, be explained on this ground. The entire number 

 of rings is not in every ease retained or reproduced because two, 

 Nos. i and 2, belonged entirely to the locomotor apparatus (pre- 

 oral lobe) and except in the holothurians and crinoids (for reasons 

 suggested later) are no longer needed. Only those rings are re- 

 tained which in the ancestral line had a function in feeding, and 

 which are needed for the same purpose during the metamorphosis 

 of the larvae themselves until the developing tube feet are ready 

 to assume the function. 



This ciliated feeding apparatus which had been brought over 

 by the hypothetical fixed echinoderm from its free-swimming con- 

 dition and which, in the new surroundings, had, at first, answered 

 every need in this line, became gradually inadequate to furnish it, 

 as it increased in size, with enough food. Those portions of the 

 ciliated sensory epithelium of the mouth situated between the ends 

 of the ciliated paths were then gradually developed into tentacles 

 into each of which a diverticulum of the left middle body cavity, 

 lying below, protruded (see Fig. n,). In the anterior space 

 only, did no tentacle develope. This space contained the ex- 

 ternal opening of the left middle body cavity (madreporite), the 

 left anterior body cavity (Ampulla) and possibly the reproductive 

 organ. There was hence no space in which a sixth tentacle 

 might have developed. 



In this way the pentamerous structure of the hydroccele may 

 be accounted for and I assume, with others, that the hydroccele 

 formed the basis upon which the entire radial symmetry of echin- 

 oderms was built. The ciliated tentacles, simple at first, branched 

 as they grew in length and assumed more and more the function 

 of food collecting. As the animal increased in size the space 



