238 ZOOLOGY. 
the preceding class, and is, by some authors, regarded as 
the type of a separate class (Scaphopoda). The sexes of 
.! 
Fig. 176.—Development of Dentalium. A, morula; B, trochosphere; C, annu- 
lated larva; D, larva with its rudimentary shell; z, velum ; a, shell; #, young much 
farther advanced, the shell or body segmented ; d, rudimentary tentacles ; j, sub- 
cesophageal perveeunela {f ", digestive canal, and liver (/’); the foot protrudes 
from the shell. AI!I magnified.—After Lacaze-Duthiers. 
Dentalium are distinct. The young isa trochosphere and 
ZA 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 
Wig. 17,--Den- the high seas, and are occasionally taken with the 
tatium Indiano- 
rum. Used as tow-net off the southern coast of New England. 
shel] money. — * . . . : ~ 
After Stearns.  Limacina arctica 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. 
