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ELEMENTARY ZOOLOGY. 



Anterior adductor muscle smaller Order Heteromya. 



Examples : Modiola (horse mussel) ; Mytilus; Meleagrina 

 (the pearl oyster). 



One adductor muscle Order Monomya. 



Examples: Ostrea (oyster); Pecten (the scallop; Fig. 73). 

 Head well developed ; shell when present univalved. 

 Foot developed into a ventral crawling disc, 



Class Gasteropoda. 



FIG. 75. Helix albolabris, a pulmonate Gasteropod. Binney's Gould. 



Questions on the Figure. What is the meaning of Helix? of albo- 

 labris? Identify the parts of the shell (see Fig. 74). Is it a right 

 or left spiral? Just what do you mean by your answer? 



FIG. 76. Umax flavus, a slug. From Binney's Gould. 



Questions on the Figure. How do the slugs differ from the other 

 gasteropods? In what external respects do they seem similar to 

 them? Compare all the figures of slugs you can find. 



Lungs; land or fresh-water habitat Order Pulmonata. 



Land snails. 



Shell ; four tentacles Family Helicidae. 



Example: Helix, Figs. 74 and 75 (with 3,000 species). 



No external shell Family Limacidae. 



Examples: Limax (the slugs, Fig. 76). 



