130 PHILOSOPHY OF ZOOLOGY. 
consists in a part of their own bodies bemg cemented to the 
substance on which they rest. This takes place m the com- 
mon mussel, by means of strong cartilaginous filaments, 
termed the byssus, united in the body to a secreting gland, 
furnished with powerful muscles, and, at the other extremi- 
ty, glued 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 byssiferous, and fixed. In the 
Annulosa, among spiders and some caterpillars, protecting 
threads are frequently employed. Ameng the Zoophytes, 
the adhesion between them and the substance on which 
they grow, is generally accomplished by means of a ce- 
ment, connecting (as m 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, and 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 sufh- 
cient power to counteract those movements which might 
displace the body from that position. It is obvious, that 
