00 BRANCH MOLLUSCA 



and many zoologists place them above the Arthropoda. They 

 have a well-defined circulatory system and nervous system and 

 especially highly developed eyes. They usually have a metamor- 

 phosis, some of the stages of which show indications of affinity 

 with "worms." 

 Classification. — 



Class. Examples. 



Pel'ecyp'oda. Sea-mussel, Oysters, Scallop, 



Fresh-water mussel. 

 Gas'terop'oda. Limpets, Periwinkle, Snails. 



Ceph'alop'oda. Cuttlefish, Octopus, Nautilus. 



