238 



ZOOLOGY. 



the preceding class, and is, by some authors, regarded as 

 the type of a separate class (Scaphopoda). The sexes of 



Fi&. 176. Development of Dentalium. A, morula ; B, trochosphere ; C. annu- 

 lated larva; 1), larva with its rudimentary shell ; z, velum ; a. shell ; E. young much 

 farther advanced, the shell or body segmented ; d, rudimentary tentacles ; j, sub- 

 oesophageal nerve-ganglia ; //", digestive canal, and liver (/') ; the foot protrudes 

 from the shell. All magnified. After Lacaze-Duthiers. 



Dentalium are distinct. The young is a trochosphere and 

 afterwards becomes segmented, and the univalve 

 shell then appears. (Fig. 176.) 



Order 2. Pteropoda. In these winged-snails 

 the head is slightly indicated and the eyes are 

 rudimentary ; while they are easily recognized by 

 the large wing-like appendages (epipodium), ne 

 on each side of the head. The shell is conical 

 or helix-like. The species are hermaphroditic. 

 Cavolina tridentata Lamarck and Styliola vitrea 

 Verrill (Fig. 178) are pelagic forms, occurring on 

 Fig. vn.-Dm- the high seas, and are occasionally taken with the 



talium Indiana- * _ T _ . .. 



nun. Used as tow-net off the southern coast ot JNew .England. 



shell money. . , -n i ^ j.i e ^ 



After Stearns. Lwwcina arctica rabr. is 01 the size 01, and 

 looks like, a sweet pea, moving up and down in the water. 

 It is common from Labrador to the polar regions. 



