No. 490] 



oirru(H;i:\ETi( ■ variatios 



609 



the adult, even after its typical characters are perfectly distinct." ' 

 And again: "... .1 would say to all young students of Embryology 

 that their next aim should he to study those intermediate phases in 

 the life of a young animal, when, liaving already a{'(juired inde- 

 pendent existence, it has not yet readied tlie coiiditlon of the adult. 

 Here lies an inexhaustible mine oi valual)le nitormation unappre- 

 ciated, from which. . . .may be gathered the evidence for the solu- 

 tion of the most perplexing problems of our science."^ 



AV.YILABILITY OF THE IVIoLUJSCA FOR THE StUDY OF 



Development.\l St.\ges 



Of all classes of organisms, the moUusks are perhaj)s the best 

 adapted for the study of ontogenetic stages between the enil)ryo 

 and the adult, since all these stages are preserved as a i)erinanent. 

 record in the form of the shell. Such a record caimot, of eouise, be 

 obtained from the soft parts, where a mnnber of individuals are 

 necessary to represent the principal stages. Moreover, a certain 

 step in development may be very definitely indicated in the form 

 and .sculpture of the shell, and yet be entirely unrecognizable in 

 the .soft parts. Not only, then, is the shell a i)ennaiieiit record 

 of the chanuv., l)ut il also a more deh< a(e re-iMer ot a Kaiiee- 

 ment than is all'onh-.l by the ....ft i>art>. In other -roup., of organ- 

 isms, the record is seldom so conipiete, ihonn-h l)ra<-hiopod. and 

 corals retain, in ])erfect specimens, all the earlier stau'es. ( )ther 

 organisms, however, such as the echinodernis and vertebrates. 



