MOLLUSC \ 



251 



The portion of the body at the back of the head, often called 

 the visceral hump, is homologous with the shell-bearing portion 

 of the body of the Gastropoda, which is known by the same 

 name. It is inclosed in the saclike mantle, which is attached 

 along what is the upper side of the bod)' when the animal is in 

 its normal position (Fig. 260). The mantle cavity is open to 

 the outside by a slitlike aperture surrounding the neck, and 

 through this opening a funnel-like organ projects on the lower 

 side of the body. The funnel is homologous with a portion 

 of the foot of other Mollusca. Within the mantle cavity are 

 the gills, either one or two pairs, and the openings of the 

 posterior end of the alimentary canal, of the excretory and 

 the reproductive organs (Fig. 261). When at rest or moving 

 slowlv, the water is drawn into the mantle cavity and forced 

 out again through the slitlike opening referred to, by means 

 of the alternate expansion and contraction of the walls of the 

 mantle. But when the animal wishes to swim rapidly the siphon 

 is brought into play. The water is drawn into the mantle cavity 

 as usual ; then the edge of the mantle is firmly locked to the 

 body wall, — in some cases there is a special apparatus for this 

 purpose consisting of a pair of corresponding elevations and 

 depressions on the mantle and body wall respectively ; thus the 

 mantle cavity is closed and the water is forced out through the 

 funnel, so that the body of the animal is projected backward. 

 The funnel can be bent so as to force the water out in various 

 directions, and thus the animal may move sidewise or even for- 

 ward. Some Cephalopoda have fins at the sides or end of the 

 body, which also assist in locomotion. 



In a few cases there is an external shell; in most there is 

 an internal shell which has come to occupy its present position 

 through an overgrowth of the mantle ; in some there is no shell 

 whatever. In most species the integument contains numerous 

 pigment cells or chromatophores, the size of which can be greatly 

 changed, thus producing great variations in the color of the ani- 

 mals, especially as there are chromatophores of several colors. 

 In this way remarkable plays of color are produced as the ani- 

 mals swim about. Some species also have a silvery or iridescent 

 appearance. 



Of the internal anatomy we may note two or three of the 



