VENUS MERCENABIA. 99 



Order 1. Decapoda. 



Ten arms, two of which are elongated, suckers 



on stalks. (Loligo, Sepia, Spirula.) 

 Order 2. Octopoda. 



Eight arms, suckers sessile. (Octopus, Argo- 



nauta.) 

 Subclass 2. Tetrabranchiata. 



Tentacles numerous. External chambered shell. 



Funnel open along one side. Only one living 



genus. (Nautilus.) 



Brooks: The Origin of the Oldest Fossils and the Discovery of the Bottom 



of the Ocean. Smithsonian Kept., 1894. 

 Kellogg: Contribution to our Knowledge of the Morphology of Lamelli- 



branchiate Mollusks. Bui. U. S. Fish Com., 1890. 



: Shell-fish Industries. Henry Holt and Co., 1910. 



Pelseneer: Contribution a L'Etude des Lamellibranches. Arch. d. Biol., 



11, 1891. 

 : Recherches Morphologiques et Phyloge'ne'tiques sur les Mollusques 



Archaiques. Acad. roy. d. Sci. d. letters et d. beaux-arts d. Belgique, 



1899. 

 : Etude sur des Gastropodes Pulmone's. Mem. Acad. roy. d. Sci. d. 



letters et d. beaux-arts de Belgique, 1901. 

 Ridewood : On the Structure of the Gills of Lamellibranchs. Phil. Trans. 



Roy. Soc., London, B, 195, 1903. 

 Stenta: Zur Kenntnis der Stromungen im Mantelraume der Lamellibran- 



chiaten. Arb. Zool. Inst. Univ. Wien., 14, 1902. 



LAHELLIBRANCHIATA. 



VENUS MERCENARIA. (Quahog.) 1 



Animals of this species wander around over muddy bottoms 

 in rather shallow water, keeping the siphon end, at least, above 

 the surface of the mud. If possible, you should find specimens 

 in their native places and watch their movements. Specimens 

 placed in water and left undisturbed for some hours are likely 

 to protrude the siphons, and the foot may be protruded in some 

 cases. 2 Allow powdered carmine to slowly settle past the open- 



1 Points in which the fresh-water mussel differ have been noted, so the 

 directions may be used for that form. 



2 Other species of lamellibranchs are more satisfactory than Venus for 

 studying movements, as they expand quickly after being disturbed. Among 

 the common ones that may be mentioned are Ensis, Cumingia, Yoldia, 

 and Mytilus. 



