116 SEA-SHORE LIFE 



of the foot has become the tube-like "siphon," through which water 

 is ejected from the large mantle cavity. 



In all living forms except the nautilus, the shell is internal, 

 although two of the arms of the female argonaut secrete a papery 

 capsule which resembles a shell in appearance, but is only a brood 

 pouch to hold the eggs, and is in no sense comparable with the 

 shells of other moUiisks. A more detailed account of the anat- 

 omy of the Cephalopoda will l^e given in the description of our 

 common squid. All of the Cephalopoda are very active creatures, 

 capturing fishes and other marine animals by means of their sucker- 

 bearing arms, and crushing the prey in their horny beaks. They 

 usually dart backward, being propelled by forcing the water from 

 the mantle cavity out through the siphon, the opening of which is 

 directed forward, but may ])e turned so as to drive the stream 

 backward. 



In the adult form the mollusks appear to be widely separated 

 from all other groups of invertebrates; but a study of their devel- 

 opment shows, that in their earliest stages they are worm-like in a 

 number of important characters, and it seems probable that they are 

 remotely descended from worm-like ancestors. 



Good accounts of our mollusks will be found in Arnold's "Sea- 

 Beach at Ebb Tide." Excellent figures and clear descriptions of 

 the species of the New England coast are given by Gould and Bin- 

 ney, in "Report on the Invertebrates of Massachusetts," 1870, 

 Wright and Potter, Boston; and the land shells are equally well 

 described in "The Terrestrial Air-breathing Mollusks of the United 

 States," by W. G. Binney, in Bulletin of the Museum of Compara- 

 tive Zoology at Harvard College, Vol. TV, 1878. 



CLAMS, OYSTERS, AND MUSSELS. 



Lamellihranehiata. 



In this great order of mollusks the shell is divided into two 

 halves, or valves, each half shell covering a side of the body, while 

 the hinge of the valves extends down the middle of the back of the 

 animal. This hinge is an elastic cushion that tends to cause the 

 valves to gape open, but this tendency is counteracted by one or 

 two strong adductor muscles, that extend across from one valve 



