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TEXTBOOK OP ZOOLOGY 



Order 1. Prosobranchiata 



Mostly marine, but fresh-water and land forms are repre- 

 sented. As the name implies, the gills are situated in the 

 mantle cavity anterior to the heart. This order embraces 

 such animals as the limpets, abalones, and periwinkles, all 

 of which live in the sea; also a few fresh- water genera, such 

 as Goniohasis, Campeloma and Pleurococera; Helicina orbicu- 

 lata, a terrestrial southern species which is frequently arboreal 

 in habit, comes under this order. 



Order 2. Opisthobrmichiata 



Strictly marine. Gills, when present, are situated jjosterior 

 to the heart; shell, if present, small. Includes the sea slugs. 

 In the sea butterflies (pteropods), the foot may be modified 

 into two fins which are used in swimming. Some of the 

 heavier types have broad cephalic discs, adapted for burrow- 

 ing in the sand. Many are found in coral beds and in sea- 

 weeds, their vivid colors harmonizing with the background. 



Order 3. Pulmonata 



Mostly terrestrial and fresh-water snails. Gills are absent, 

 the mantle cavity serves as a pulmonary sac; shell usually 

 present, sometimes rudimentary or absent. 

 Suborder 1. Basommatophora 



Fresh-water species; eyes located at base of tentacles; 

 external shell present. Includes the families Lym- 

 naeidae, Physidae, Planorbidae and Ancylidae. 

 Suborder 2. Stylommatophora 



Terrestrial snails and slugs; stalked retractile eyes, 

 and one pair of retractile tentacles; shell in form of 

 elevated or depressed spire, rudimentary and concealed, 

 or absent. 

 Class IV. Scaphopoda 



Marine. Mantle edges grown together along ventral side forming 

 tube, with a shell of same shape and open at both ends. Commonly 

 known as tooth shells. Approximately 300 kno-u-n living species. 

 (Dentalium.) 

 Class v. Cephalopoda 



Marine. The most highly organized of the mollusks. A definitely 

 formed head is present which bears a pair of eyes that superficially 

 resemble the eyes of vertebrates. The foot is modified into arms 

 or tentacles. They are carnivorous animals and many of them are 

 used as food by man. (Nautilus, Loligo, Polypus.) 



Order 1. Tetrabranchiata 



The chambered nautilus (Nautilus) is a representative of 

 this order. The animal inhabits the last chamber of a flat- 

 tened spiral calcareous shell. As the name Tetrabranchiata 

 implies there are four gills; also four primitive kidneys and 



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