252 ZOOLOGY. 
then developed. The foot gradually lengthens and forms the 
characteristic fin or keel. The velum is meanwhile ab- 
sorbed, the operculum (Carinaria), or the operculum and 
shell, are thrown off, and the larva gradually assumes the 
form and organization of the adult. The close relationship 
of the Heteropods and Gastropods is shown by the great 
similarity of their larve. Gegenbaur even goes so far as to 
include them under the Opisthobranchiata, while von Jher- 
ing unites them with Chitons and some other forms under 
the name of -Arthrocochlide ; for the present it seems best 
to retain them as a subclass. 
The fossil genus Bellerophon is closely related to the 
Atlantide. 
Cuass [I.—CEPHALOPHORA. 
Mollusks with a distinct head, with tentacles, eyes and ears in the head ¢ 
the foot forming a creeping disk; the body either naked and bilaterally sym- 
metrical, or enclosed in a spiral shell, and consequently behind the head 
asymmetrical. Mouth with pharyngeal teeth and a lingual ribbon (odon- 
tophore). Nervous system consisting of three pairs of ganglia, the brain 
well developed. The intestine usually ending near the mouth. The heart 
with usually a single auricle. Breathing by a single gill, or a lung-like 
gill ; a@ double kidney, but forming a single mass. Sexes united or sepa- 
rate. Young passing through a blastula, gastrula, sometimes a trocho- 
sphere and usually a veliger stage; in the land-snails development 
direct. 
Subclass 1. Scaphopoda.—No lead, several long thread-like tentacles ; 
foot long, trilobed. Shell long, conical, open at each end, 
A single order Solenoconchw. (Dentalium, Siphonodenta- 
lium.) 
Subclass 2. Pteropoda —Body with two wing-like expansions (velum) 
on the front part of the foot, for swimming ; body naked or 
shelled. Hermaphroditic. Larva with a velum and shell, 
Order 1. Thecostomata (Hyalea, Cleodora, Cavolina), Order 
2. Gymnosomata. (Clione). 
Subclass 3. Gastropoda.—Order 1, Prosobranchiata (Haliotis, Patella, 
Trochus, Littorina, Lunatia, Paludina, Turritella, Janthina, 
Cyprea, Strombus, Cassis, Buccinum, Nassa, Purpura.) 
Order 2. Opisthobranchiata, (Bulla, Aplysia, Eolis, Doris.) 
Order 8. Pulmonata. (Limnzus, Planorbis, Auricula, Helix, 
Bulimus, Limax). 
