MUSCULAR AND SKELETAL SYSTEMS. 



237 



Fig. 152. — Snail walking. 



Gastropoda. — In snails (Fig. 152) and slugs the loco- 

 motive organ ox foot is an elongated flat disk, lying along 

 the ground, which consists of muscular fibers running in 

 all directions, some transverse, some oblique, and some 

 longitudinal. The longitudinal fibers shorten, the ob- 

 lique and trans- 

 verse constrict 

 and lengthen. 

 A snail or slug 

 seems to glide 

 slowly and con- 

 tinuously along 

 without appar- 

 ent mechanism. 

 If watched care- 

 fully, however, 

 waves of con- 

 striction and ad- 

 vance chase one another from one end of the foot to 

 the other. In the case of those having a shell, as the 

 snail, muscular fibers from the foot are attached to the 

 interior of the shell, whereby the foot is drawn into the 

 shell. Of course there are also small muscles, ring fibers, 

 and longitudinal fibers for moving the tentacles. 



Cephalopoda. — Confining ourselves to locomotion, 

 we find here, again, a new kind. The squid and cuttle- 

 fish are invested with a thick, hollow, muscular mantle, 

 within which are inclosed all the viscera, but with con- 

 siderable space between filled with sea water. Just be- 

 neath the head protrudes a conical tube [siphon), valvu- 

 larly connected with the hollow in such wise that when 

 the mantle contracts the water is forced through the 

 siphon with great force, and the animal is shot back- 

 ward with great speed. Besides this rapid locomotion 

 for escape, there is another in both the squid and cuttle- 



