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 mollusks. A more detailed account of the anat- 
omy of the Cephalopoda will be 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 be 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 ina 
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. IV, 1578. 
CLAMS, OYSTERS, AND MUSSELS. 
Lamellibranchiata. 
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 
