SIGNIFICANCE OF CERTAIN LARVAE OF ECHINODERMS. 183 



have been found, a very definite homology can be shown to 

 exist between them, as I have endeavored to indicate by the 

 numbers which have been placed opposite the rings in each of 

 the drawings. It is conceivable that the long arms of Aitriai- 

 laria, Bipinnaria, and the various plutci may have arisen and 

 been developed from elevations of the ectoderm beneath certain 

 parts of the ciliated rings, the result of which would have been an 

 increase in their length and thereby an increase in their efficiency 

 in locomotion and feeding. The relation which the type of 

 directly developing larvae of Antcdon, Cncmnaria and Ophiura is 

 suggested to bear to the familiar types, is shown in Fig. 1 1 ; the 

 larvae with transverse ciliated rings being considered the primi- 

 tive condition from which the other larvae have been specialized 

 and carried far out of the path of phylogeny, as a result of their 

 independent life. To this type of development the specialized 

 larvae tend to return at the time when their free-swimming life is 

 given up for one on the bottom, as is indicated in holothurian 

 pupae, a certain ophiuran larva and young Mcllitas in all of which 

 the transverse ciliated rings reappear at the time of metamorphosis 

 in the form in which they were of functional importance to the 

 common ancestor during the early period of its life on the bot- 

 tom. 



All larvae have not deviated to the same extent from the direct 

 line, as is shown not only in their less complicated structures but 

 also in the less radical readjustment of organs which takes place 

 during their metamorphosis. In ophiurid plutei, for example, 

 the larval mouth and oesphagus are taken over as such into the 

 adult form, which, as has been pointed out, must have been the 

 case in phylogeny. In echinoid plutei, however, the specializa- 

 tion which has taken place in these organs has been carried so 

 far that it is impossible to readapt them to the needs of the adult 

 and new ones must be formed. 



THE ATTACHED FORM AND THE ORIGIN OF RADIAL 



SYMMETRY. 



With almost no exception, students of the embryology and 

 anatomy of the echinoderms see no other way, at present, to ac- 

 count for the peculiar asymmetry of certain of the organs and 



