106 



ANIMAL MECHANISM. 



It appears tliat the cephalic region lays hold on the fixed 

 point towards which all the rest of the body is dragged for- 

 ward. In fact, in the posterior region an opposite phenome- 

 non takes place ; each new band which takes its rise there, is 

 accompanied by a backward motion of that region, which moves 

 as if it were drawn by a longitudinal retraction of the con- 

 tractile tissue. 



Other modes of creeping are not less curious ; that, for 

 example, which takes place in the interior of a solid body ; 

 as a worm, when it advances in the tubular cavity which it 

 has hollowed out in the ground. The hinder part of the body, 

 soft and extensible, is assuredly of much less size than the 

 cavity of the hole from which we endeavour to pull it, and 

 yet the worm resists the force of traction, and breaks rather 

 than be drawn out. Jhis is because, within the ground, 

 the anterior portion of the body, shortened but swollen, dilates 

 within the passage, and finds there a solid point of resistance. 

 If we let the worm go we shall see it rapidly shorten its 

 body, and withdraw the rest of it into the ground, being 

 dragged backward towards the anterior portion which has a 

 firm hold on the soil. 



By the side of the action of creeping we may naturally 

 place that of climbing^ in which the anterior limbs seek to lay 

 hold of some elevated projection, and as they bend raise the 

 rest of the body of the animal. The hinder part then fixes 

 itself in its new position, and the anterior limbs, thus set free, 

 seek, higher up, a fresh resting place to make a new effort. 

 What different types in these two modes of terrestrial locomo- 

 tion ! The varieties are so great that we can scarcely give 

 an exact idea of them, except by describing the mode of pro- 

 gression adopted by each particular animal. 



Locomotion in water presents a still greater diversity. In 

 one case, we see a fish which strikes the water with the flat 

 of its tail ; in another, a cuttle fish or a medusa, which, com- 

 pressiDg forcibly its pouch full of liquid, drives out the water 

 in one direction and propels itself in a course directly opposite ; 

 the same phenomenon is produced when a mollusk closes rapidly 

 the valves of its shell, and projects itself in the direction opposed 

 to the current of water which it has produced. The larvae of 



