258 COLLEGE ZOOLOGY 



b. Gastropoda in General 



Classification. — There is considerable diversity among gas- 

 tropods both in form and structure. The chief characteristics 

 used in dividing them into groups are the structure of the 

 nervous system, the method of respiration and structure of the 

 respiratory organs, and the condition of the sexual organs. 

 There are two subclasses, each containing two orders. 



Subclass I. Streptoneura. — Dioecious Gastropoda with 

 visceral connectives usually twisted into a figure 8; the heart is 

 usually posterior to the gills. 



Order 1. Aspidobranchia. Streptoneura with usually two 

 gills, two auricles, and two nephridia. Examples: Acmcsa 

 (limpet), Haliotis (ear-shell), Margarita. 



Order 2. Pectinibranchia. Streptoneura with one kidney, 

 one auricle, and one gill. Examples: Littorina, Sycotypus (Fig. 

 186, A), Crepidula (Fig. 186, B), Urosalpinx. 



Subclass II. Euthyneura. Monoecious Gastropoda with 

 visceral connectives not twisted (Fig. 183) ; the gill when present 

 is posterior to the heart. 



Order 1. Opisthobranchia. Marine Euthyneura usually 

 with a gill and mantle. Examples: Bulla, Clione, Doris. 



Order 2. Pulmonata. Land and fresh-water Euthyneura 

 which breathe air; gill usually aborted and mantle cavity con- 

 verted into a lung. Examples: Helix, Polygyra (Fig. 1S5, C), 

 Lymncea (Fig. 185, G), Limax (Fig. 184), Physa (Fig. 185, D), 

 Planorbis (Fig. 185, B). 



Air-breathing Gastropods. — The air-breathing gastropods 

 belong chiefly to the order Pulmonata, and inhabit fresh water 

 or live on land. The slugs also live on land, but are without a 

 well-developed shell. Limax maximus (Fig. 184) is a large slug. 

 It was introduced from Europe and is now more or less of a pest 

 in greenhouses because of its fondness for green leaves. The 

 shell of Limax is a thin plate embedded in the mantle. 



Three common fresh-water snails with shells are Physa, Lym- 



