412 THE MECHANICAL FUNCTIONS. 



internal, the trunk of the body is found to be en- 

 closed on every side in a bony case, which leaves 

 openings only for the head, the tail, and the fore 

 and hind extremities. That portion of this osseous 

 expansion which covers the back is termed the 

 carapace ; and the flat plate which defends the 

 lower part of the body is termed the plastron. It 

 is a form of structure which reminds us of the de- 

 fence provided for animals very low in the scale of 

 organization, such as the Echinus, the Crustacea, 

 and the bivalve MoUusca. Yet the substance 

 which forms these strong bucklers, both above and 

 below, is a real osseous structure, developed in the 

 same manner as other bones, subject to all the 

 changes, and having all the properties of these 

 structures. The great purpose which nature seems 

 to have had in view in the formation of the Che- 

 Ionia is security ; and for the attainment of this 

 object she has constructed a vaulted and impene- 

 trable roof, capable of resisting enormous pressures 

 from without, and proof against any ordinary mea- 

 sures of assault. It is to the animal a strong castle, 

 into which he can retire on the least alarm, and defy 

 the efforts of his enemies to dislodge or annoy him. 

 These considerations supply us with a key to 

 many of those apparent anomalies, which cannot 

 fail to strike us in viewing the dispositions of the 

 parts of the skeleton (Fig. 213), and the remark- 

 able inversion they appear to have undergone, 

 when compared with the usual arrangement. We 

 find, however, on a more attentive examination, 

 that all the bones composing the skeleton in other 

 vertebrated animals exist also in the tortoise ; and 

 that the bony case which envelopes all the other 



