136 PHILOSOPHY OF ZOOLOGY. 
motion resembling the action of feet. Where the organs of 
adhesion are double, one at each extremity, as in the leech, 
the mouth adheres to one part of the surface, while the tail 
is brought up towards it, and is then fixed, the body being at 
this time like an arch. The head then quitsits hold, the body 
extends itself, and, when at full length, the head is again 
attached, and the tail brought up. By these alternate move- 
ments, the leech, at every step, advances nearly the length 
of its own body. In some of the intestinal worms, the ad- 
hesion of the head takes place by means of reversed spines 
or hooks, as in the Echinorinchus. 
3. Leaping.—In the action of leaping, the whole bedy 
rises from the ground ; and, for a short period, is suspend- 
ed in the air. It is produced by the sudden extension of 
the limbs, after they have undergone an unusual degree of 
flexion. The extent of the leap depends on the form and 
size of the body, the length and the strength of the limbs. 
‘The myriapoda are not observed to leap. Many of the spi- 
ders and insects leap with ease, both forwards, backwards, 
and laterally. In those which are remarkable for this fa- 
culty, the thighs of the hind-legs are in general of uncom- 
men size and strength. Among reptiles, the leaping frog 
is well known, in opposition to the crawling toad. Among 
quadrupeds, those are observed to leap best, which have 
the hind legs longer and thicker than the fore legs, as the 
kangaroo and the hare. These walk with difficulty, but 
leap with ease. 
The motion of leaping is not confined to animals furnish- 
ed with legs; nor, even among these, is the action confined 
to the legs. Serpents are said to leap, by folding their bo- 
dies into several undulations, which they unbend all at once, 
according as they wish to give more or less velocity to their 
motion. ‘The jumping maggot, found in cheese, erects it~ 
self upon its anus, then forms its body into a circle, by 
