LARVAL FORMS. 



369 



Considering the very profound differences which exist 

 between many of these larvae, it may seem that the characters 

 just enumerated are hardly sufficient to justify my grouping 

 them together. It is, however, to be borne in mind that my 

 grounds for doing so depend quite as much upon the fact that 

 A B C 



FIG. 119. LARWE OF CEPHALOPHOROUS MOLLUSCA IN THE VELIGER STAGE. 

 (From Gegenbaur.) 



A. and B. Earlier and later stage of Gasteropod. C. Pteropod (Cymbulia). 

 v. velum ; c . shell ; /. foot ; op. operculum ; t. tentacle. 



they constitute a series without any great breaks in it, as upon 

 the existence of characters common to 

 the whole of them. It is also worth 

 noting that most of the characters which 

 have been enumerated as common to the 

 whole of these larvae are not such second- 

 ary characters as (in accordance with the 

 considerations used above) might be ex- 

 pected to arise from the fact of their 

 being subjected to nearly similar con- 

 ditions of life. Their transparency is, no 

 doubt, such a secondary character, and it 

 is not impossible that the existence of 

 ciliated bands may be so also ; but it is 

 quite possible that if, as I suppose, these 

 larvae reproduce the characters of some 

 ancestral form, this form may have 

 existed at a time when all marine 

 animals were free-swimming, and that it 

 may, therefore, have been provided with at least one ciliated 

 band. 



FIG. 220. LARVA OF 

 ARGIOPE. (From Gegen- 

 baur ; after Kowalevsky.) 



m. mantle ; b. setze ; 

 d. archenteron. 



B. III. 



2 4 



