PHILOSOPHY OF ZOOLOGY. 



consists in a part of their own bodies being cemented to the 

 substance on which they rest. This takes place in the com- 

 mon mussel, by means of strong cartilaginous filaments, 

 termed the lyssitif, united in the body to a secreting gland, 

 furnished with powerful muscles, and, at the other extremi- 

 ty, o-jued to the rock or other body to which it connects 

 itself. In other cases, as in the oyster, the shell itself is ce- 

 mented to the rock. This method of resisting the action 

 of the disturbing forces of the air and the water, is unknown 

 among the vertebral animals. In the Mollusca, it occurs 

 in those with shells termed fyss-iferous, and fixed. In the 

 Annulosa, among spiders and some caterpillars, protecting 

 threads are frequently employed. Among the Zoophytes, 

 the adhesion between them and the substance on which 

 they grow, is generally accomplished by means of a ce- 

 ment, connecting (as in nearly all animals fixed by cementa- 

 tion) their body for life to the spot where they first adhered. 



Let us now attend to the muscular motions of animals, 

 as displayed in the various positions assumed and actions 

 performed, in the exercise of the locomotive powers. The 

 first of these which demands our attention, is, 



1. Standing. In this position the body is raised above 

 the ground, and supported on its legs. The facility with 

 which this attitude is assumed., arid the length of time in 

 which animals can remain in it, depend on a variety of cir- 

 cumstances, connected with the figure and density of the 

 different parts of the body, and the form, position, and 

 strength of the limbs. 



In order to enable an animal to support itself firmly on 

 its limbs, it is necessary that these parts be so disposed, as 

 that the centre of gravity of the whole body fall within the 

 space which they occupy, and that the muscles have suffi- 

 cient power to counteract those movements which might 

 displace the body from that position. It is obvious, that 



