272 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 larvae. 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 ArthrocoMidw ; for the present it seems best 

 to retain them as a subclass. 



The fossil genus Bellerophon is closely related to the 

 Atlantidce. 



CLASS II. CEPHALOPHORA. 



Mottusks 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 four 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 morula, gastrula, sometimes a trocho- 

 sphere and usually a veliger stage; in the land-snails development 

 direct. 



Subclass 1. Scaphopoda.No head, several long thread-like tentacles ; 

 foot long, trilobed. Shell long, conical, open at each end. 

 A single order SolenoconchcB. (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. Oymnosomata. (Clione). 



Subclass 3. Gastropoda. Order 1. Prosobranchiata (Haliotis, Patella, 

 Trochus, Littorina, Lunatia, Paludina, Turritella, lanthina, 

 Cyprsea, Strombus, Cassis, Buccinum, Nassa, Purpura.) 

 Order 2. Opisthobranchiata. (Bulla, Aplysia, Eolis, Doris.) 

 Order 3. Pulmonata. (Limnaeus, Planorbis, Auricula, Helix, 

 Bulimus, Limax). 



