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Its length, a series of curves and straight lines, in succession, from the 

 head towards thetail; but sometimes, likewise, from the tail towards the 

 head, in the serpents called amphisbcenous, in which the scales cover- 

 ing the belly are equally favourable to a retrogade motion, as to a motion 

 forward. 



The crawling of serpents is facilitated by the length of their body, by 

 the smoothness of their scales, the immense power of their muscles, and 

 the flexibility of their vertebral column. The bones which form this 

 part of the skeleton, are articulated by arthrodia, and loosely jointed, so 

 that a very slight cause destroys their union: hence a blow, with a very 

 small stick, is capable of killing the largest serpent, if applied on the 

 back. The lateral inflections of this column are very considerable; the 

 degree of extension is limited to the spinous processes, and these are 

 sometimes, of considerable size, as in the rattlesnake. Hence, notwith- 

 standing what has been stated by several authors, and although painters 

 have represented serpents moving in vertical curves, they move, inmost 

 instances, in horizontal curves. 



A serpent, to swim, is obliged to bend and unbend his body, in more 

 rapid succession; this swimming consists merely in crawling faster and 

 in moving on a less resisting plane. 



The motions of reptiles, -in swimming, surpass, in strength, and velo- 

 city, those of reptiles which crawl on the ground, in as much as the lat- 

 ter yields a more fixed point than water. If the serpent is desirous of 

 leaping, he suddenly, and at once, brings to a straight line all his curves, 

 resting, at the same time, on the extremity of that which is nearest his 

 tail : then, as I have several times observed, fce describes the smallest 

 possible number of curves, bends, into three or four greater arches 

 than usual, but never into a single one, whatever the length of his body 

 may be. 



Tortoises, frogs, lizards, salamanders, and all reptiles that have legs, 

 drag themselves along on their belly, being ill supported by their weak 

 limbs, which bear no proportion to the bulk of their body, and can 

 scarcely be said to crawl by a mechanism similar to that which has just 

 been explained. 



Caterpillars and maggots crawl much in the same manner as serpents. 

 The legs of the caterpillar, too feeble to support it, or, of themselves, to 

 carry the body forward, are used by these creatures, to obtain a hold on 

 the surface on which they move, by bending in arches, mostly vertical, 

 the parts situated between the legs, that are in pairs, at a certain distance 

 from one another. The caterpillars that have a scaly covering, crawl 

 better, the elasticity of their scales assisting the contractile action of their 

 muscular fibres. Earth worms move, at times, in undulations, as the 

 snake, and at others, by dragging themselves like slugs. This last variety 

 of crawling is performed as follows : instead of forming direct curves, 

 the contractile fibres of the reptile, shorten themselves, from the head, 

 which is fixed, towards the tail which is moveable, and the animal per- 

 forms only slight inflexions. We may compare the mode of crawling 

 peculiar to some animals, to the motion by which a man lying horizon- 

 tally, on his belly, moves forward by drawing his whole body toward his 

 arms, which are in a state of extension, and with which he has a hold of 

 some fixed object. The motion of the snail is performed almost entire- 

 ly in the same manner. 



The snail loaded with his shell, adheres to the surface on which he 



