A. E. Verrill — MoUuscan Archetype. 



95 



In general, it may be stated that nearly all Gastropoda, 

 except certain terrestrial and fresh-water forms, pass through 

 veliger stages (figs. 4-11, p. 99) ; that nearly all Bivalvia, except 

 fresh water mussels, pass through veliger stages (figs. 17-19) ; 

 that the Scaphopoda pass through special forms of veligers 

 (fig. 16) ; that the Pteropoda all have veliger stages, often 

 with peculiar secondary modifications (figs. 12-15); that the 

 dibranchiate Cephalopoda, so far as known, have an abbre- 

 viated development, coincident with large eggs and a large 

 amount of yolk, and that they leave the egg with the general 

 structure of the adult, without passing through a true veliger 

 stage. Terrestrial gastropods, and some fresh-water forms, also 

 have large eggs and abbreviated metamorphoses, without pass- 

 ing through a well marked veliger stage. The littoral genus 

 O^icidiiim, among terrestrial pulmonates, affords exceptions to 

 the rule, as it has a true veliger stage. 



Figure 16. — Yeliger of Dentalium, with well formed bilobed shell (s) 



19 



Figure 17. — Pro veliger of bivalve {Teredo navalis), side view. 



Figure 18.— Subveliger of Teredo. 



Figure 19 — Yeliger of mussel {Dreissena), ventral view, showing well formed 



foot (/) capable of being used for locomotion and for attachment by secreting 



a byssus. For explanation of letters, see p. 98. 



In the ordinary proveliger stage (fig. 17), the larva of a 

 bivalve, such as a clam or oyster, differs but little in structure 

 from that of a spiral marine gastropod, or from that of a 

 nudibranch gastropod, or a pteropod (fig. 12) ; nearly all the 



