238 



ZOOLOGY. 



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

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



Fiif. 176.— Development of Sentalium. A, morula ; B, trochoBphere ; C, annn- 

 lated larva ;^ D, larva with its rudimentary shell • z, velum ; a, shell ; JS, young much 

 farther advanced, the shell or hody segmented ; d, rudimentary tentacles ; J, sub- 

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

 from the shell. Airmagnifled.— 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 ; wliile 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. 

 CavoKna tridentata Lamarck and Styliola vitrea 

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

 Fig, m.-Den- the high seas, and are occasionally taken with the 



rum. Used as tow-uct Oil the Southern coast of ^ew Jingland. 



After Stearns. Umacina arcUca Fabr. is of the size of, and 



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



It is common from Labrador to the polar regions. 



