ECHINODERMATA. 575 



ed band breaks up into a number of transverse ciliated bands. 

 This condition is in some instances reached directly, and such 

 larvae undoubtedly approximate to the larvae of Antedon, in 

 which the uniformly ciliated condition is succeeded by one with 

 four transverse bands, of which one is prae-oral. 



All or nearly all Echinoderm larvae are bilaterally symmetrical, 

 and since all Echinodermata eventually attain a radial sym- 

 metry, a change necessarily takes place from the bilateral to the 

 radial type. 



In the case of the Holothurians and Antedon, and generally 

 in the viviparous types, this change is more or less completely 

 effected in the embryonic condition ; but in the Bipinnaria and 

 Pluteus types a radial symmetry does not become apparent till 

 after the absorption of the larval appendages. It is a re- 

 markable fact, which seems to hold for the Asteroids, Ophiur- 

 oids, Echinoids, and Crinoids, that the dorsal side of the larva is 

 not directly . converted into the dorsal disc of the adult; but 

 the dorsal and right side becomes the adult dorsal or abactinal 

 surface, while the ventral and left becomes the actinal or ventral 

 surface. 



It is interesting to note with reference to the larvae of the 

 Echinodermata that the various existing types of larvae must 

 have been formed after the differentiation of the existing groups 

 of the Echinodermata ; otherwise it would be necessary to adopt 

 the impossible position that the different groups of Echinoder- 

 mata were severally descended from the different types of larvae. 

 The various special appendages, etc. of the different larvae have 

 therefore a purely secondary significance ; and their atrophy 

 at the time of the passage of the larva into the adult, which 

 is nothing else but a complicated metamorphosis, is easily ex- 

 plained. 



Originally, no doubt, the transition from the larva to the 

 adult was very simple, as it is at present in most Holothurians ; 

 but as the larvae developed various provisional appendages, it 

 became necessary that these should be absorbed in the passage 

 to the adult state. 



It would obviously be advantageous that their absorption 

 should be as rapid as possible, since the larva in a state of 

 transition to the adult would be in a very disadvantageous 



