278 
.COMPARATIVE ZOOLOGY. 
in having no eye-stalks, the eyes being at the base of the 
tentacles. They are obliged to come frequently to the 
surface of the water to breathe. 
4. Prosobranchs. — These are aquatic Gasteropods, 
breathing by gills situated in front of the heart. They 
are the most highly organized and the most abundant of 
the crawling Mollusks. Nearly all are marine, and all 
have a shell. 
Among the lower forms are the singular Chiton , cov¬ 
ered with eight shelly plates; Limpet (Patella), well 
known to every sea-side visitor; and the beautiful Ear- 
shell (Haliotis), frequently used for ornaments and inlaid- 
work. 
In the higher Prosobranchs, the gills are comb-shaped 
and the sexes are distinct. The group includes all the 
spiral univalve sea-shells, and a few fresh-water shells. 
Many have the aperture entire, which is closed with an 
operculum: as the dull-colored Paludina and Melania 
from fresh water, and the pyramidal Trochus , pearly Tur¬ 
bo , screw-like Turritella, common Periwinkle ( Littorina ), 
and globular Natica from the sea. Others, the highest 
of the race, have the margin of the aperture notched or 
produced into a canal, and- are carnivorous and marine: 
such are nearly all the sea-shells remarkable for their 
beautiful forms, enamelled surfaces, and brilliant tints, as 
the Cowry ( Cyproea), Volute, Olive, Cone, Harp, Whelk 
(Buccinum), Cameo-shell (Cassis), Pock-shell (Murex), 
Trumpet-shell (Triton), Spindle-shell (Fusus), and Wing- 
shell (>Strombus). 
Class III. —Cephalopoda. 
The Cephalopods stand at the head of the subkingdom. 
The head is set off from the body by a slight constriction, 
and furnished with a pair of large, staring eyes, a mouth 
armed with a rasping tongue and a parrot-like beak, and 
