240 SCHARFF : HOW DOES A SNAIL CRAWL? 



who has carefully investigated the course of the most intricate 

 system of muscles in the snail. He found, in fact, that there 

 is a net-work of muscles in the foot going in all directions. 

 There are horizontal longitudinal and horizontal cross-fibres, 

 vertical as well as horizontal and inclined oblique muscular 

 fibres. After many experiments Dr. Simroth discovered that 

 the horizontal longitudinal fibres brought about the movements 

 of the foot. These are the extensile muscles. They produce 

 the wave motion. By their action the sole of the foot is 

 elongated in front, and shortly after it is shortened behind to 

 the same extent. The effect of this is that a kind of sliding 

 motion is produced. This motion is materially assisted by the 

 intercalation of a layer of mucus between the foot and the 

 object on which the animal crawls. The fugitive snail's course 

 can always easily be tracked by the marks of slime left behind. 

 Why should it leave this slime behind ? Because the skin of 

 the snail is so exceedingly sensitive, that the contact with a 

 rough surface is apparently very repugnant to its tender feel- 

 ings. It therefore provides itself with an abundant supply of 

 mucus, which in the case of Helix aspersa, is furnished especi- 

 ally by the large foot-gland, but to a minor extent also by the 

 mucous glands. 



However, although this mode of locomotion may seem 

 very pleasant, being at any rate quite unique among animals, 

 the snail's lot is not a happy one. When the locomotary 

 muscles are once set a-going, the movement is automatic, that 

 is to say the snail can neither increase nor slacken its pace, nor 

 can it go backwards. In that respect it is like a watch which 

 may be wound up and which we can stop at will, but we cannot 

 force the wheels to change their rate of velocity. Hence when 

 the snail is pursued by an enemy, it is unable to run away or 

 rather slide away. The only possible manner to evade the 

 enemy is to stop the motion of the foot and wind up another 

 series of muscles by means of which the snail is enabled to 

 retire within its shell. 



J.C, v., Oct., 1887. 



