420 PH YL UM MOLL USCA. 



of the ctenidium type. The shell is usually in one piece ; but 

 it is sometimes rudimentary or absent. The foot usually 

 contains a mucus gland, and tends to be divided into three 

 regions the pro-, meso-, and meta-podium. There is a singh 

 reproductive organ and genital duct. 



Branch A. STREPTONEURA 



In the torsion of the body one limb of the visceral loop crosses the 

 other in a figure 8. 



Order i. ZYGOBRANCHIATA 



The atrophy of the primitively left-side gills and nephridia is not 

 carried out, or only partially, e.g. Haliotis (ear-shell) ; Fissurella (key- 

 hole limpet) ; Patella (limpet). 



Order 2. AZYGOBRANCHIATA 



The originally left gill and the originally left nephridium have been 

 lost. Heart with single auricle, one gill, one nephridium ; operculum 

 present. 



Periwinkle (Littorina), buckie {Buccinum, Fig. 222), dog-whelk 

 (Purpura}, lanthina, and the majority of the marine Gasteropods 

 with coiled shells, together with some fresh-water forms. The 

 pelagic Heteropods are also included here: Atlanta, shell well 



developed ; Carinaria, 

 with small shell ; Pfero- 

 trachea, with no shell. 



Branch B. EUTHYNEURA 



The visceral loop does not 

 share in the torsion of the 

 visceral hump. 



Order 3. 

 OPISTHOBRANCHIATA 



The visceral loop is euthy- 

 neural, as in snails ; the 

 single auricle lies behind the 

 ventricle ; the shell and 

 mantle are often absent. 



A. Tectibranchiata. A 

 shell is present, 

 but may be rudi- 

 mentary; there is 



FIG. 229. A Pteropod (Cymbulta a well-developed 



peronii], showing the wing-like expan- mantle fold and 



sions(pteropodial lobes) of the mid-foot. a single gill, e.g. 



