MOLLUSKS IN GENERAL 223 



When completely inverted the tentacles may not be seen at all, but as 

 eversion takes place they become longer and longer until finally in the 

 case of those bearing the eyes these appear like small beads at the tips. 

 These eyes can perceive light but do not afford vision, as do those of the 

 highest mollusks. The mouth contains a chitinous plate covered with 

 teeth which is known as a radula. It serves as a grater and by a rasping 

 action can remove the epidermis from leaves or a growth of algae from 

 the surface of submerged objects. 



The visceral mass bears a mantle which secretes a shell, but since this 

 mass is not of sufficient consistency to hold the shell in a constant position 

 in the median line, its weight carries it to one side. Growth is also 

 faster toward the outside of the shell. Consequently, as the margin is 

 extended by growth it tends constantly to change its direction of inclina- 



Mar;f/e co'v/fy or 

 pa/fT70/^ary chan7ber 



^£ye 



Olfactory 

 ^ ferfac/e 



'oath 



Cen/'fa/ 

 Intest/he \ ope/o/ngr 



Open/'ng' /hto 

 pulmonary c/7t;*^^£>er 



Fig. 133. — European edible snail, Helix pomatia Linnaeus, shown in an extended 

 condition. {Modified from Schmeil, "Text-book of Zoology," by the courtesy of A. and C. 

 Black, and of Quelle and Meyer.) Somewhat diagrammatic, the positions of the alimen- 

 tary canal and mantle cavity being indicated by dotted lines. 



tion and gradually develops a spiral form. Under the mantle is a 

 mantle cavity the wall of which is kept moist by a secretion and serves in 

 respiration. 



The number of ganglia in the gastropods is much greater than that in 

 the mussel, and they lie closer together in the body (Fig. 134). The 

 sense organs include, besides eyes and olfactory organs, a pair of organs 

 of equilibrium similar in structure to those of the mussel, one lying on 

 each side of the head near the supraesophageal ganglion. 



In locomotion the body is protruded from the shell, the foot is 

 extended, and a slime gland behind the mouth secretes a mass of sHme 

 which passes back under the foot and forms a pathway upon which the 

 animal glides, its course being marked by the track of slime left behind. 

 Some snails can move with a moderate degree of rapidity. Baker ascribing 

 to them a speed of two inches per minute. 



Snails are either diecious or monecious. In case they are monecious, 

 self-fertilization does not usually occur, but in copulation there is recip- 

 rocal fertilization. 



