454 APPLICATIONS OF MUSCULAR POWER :—SKELETON. 
form of the body in such a manner as to cause it to move, 
either altogether or in part. This is the case, for example, in 
the Leech, Earth-worm, and other Annelids; which are fur- 
nished with two sets of muscular fibres, one running along 
the body, and the other passing round it in rings. By the 
contraction of the former, the two ends are drawn together, 
so that the body is shortened ; whilst by that of the latter, 
its diameter is lessened, so that it is necessarily lengthened. 
By these two movements, which take place alternately, the 
progression of the animal is accomplished ; and by varying 
the contractions of one part or another, almost any form and 
direction can be given to the soft and flexible body. 
598. But in the higher animals we find the apparatus of 
movement to consist, not only of muscles, but also of a frame- 
work of solid pieces, which serves to augment the precision, 
the force, and the extent of the movements; whilst, at the 
same time, it determines the general form of the body, and 
protects the viscera against injury from without. This solid 
framework, or skeleton, to which the muscles are attached, 
may be, as we have seen, either internal or external. In the 
Vertebrated classes, the hard skeleton is internal; in the 
Articulated series it is external; in the Mollusks it is 
external, but does not afford fixed attachments to muscles, 
except to such as draw together its valves, or connect it with 
the soft body of the animal ; and in the Radiata its position is 
variable, being sometimes external as in the Echinodermata, 
and sometimes internal as in the stony Corals. 
599. The skeleton of Vertebrata differs from that of all the 
Invertebrated classes in the much higher character of its or- 
ganization, which enables it to grow with the growth of the 
body generally, not merely in virtue of the additions it 
receives, but by the successive removal of its previously 
formed parts as occasion may require ; so that the skeleton of 
the adult has been entirely substituted for that of the child, 
probably no part of the latter being contained in the former. 
The skeletons of the Invertebrata, where they are not formed 
of horny matter alone, are consolidated by carbonate of lime, 
which in some instances (as the shells of many Mollusks, 
and the Stony Corals) bears so large a proportion to the 
animal basis, that the latter can scarcely be detected. The — 
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growth of these skeletons takes place entirely by additions to 
