COMPOSITION OF THE SKELETON. 3 



But of the aqueous deposits, those laid down in the sea 

 will enclose by far the larger number of animals on ac- 

 count of the greater area which they cover. 



The structure and composition of the hard parts vary 

 considerably in different groups of animals and plants, 

 and as a result some are much more readily preserved as 

 fossils than others. Thus in some, like Argonauta, the 

 skeleton consists of a thin shell which is easily broken up; 

 then again in some sponges it is formed of needles of 

 silica which are held together by the soft parts only, and 

 consequently become easily scattered after the death of 

 the animal. But in other cases, as in most of the mollusks 

 and corals, the skeleton is very strong and not easily 

 broken up, and consequently these are abundant as fossils. 

 Perhaps even more important than the structure, is the 

 composition of the hard parts. These, in the case of 

 insects and some hydrozoans consist of a horny substance 

 known as chitin; in diatoms, in most radiolarians, and in 

 many sponges, of silica ; in the bones of vertebrates chiefly 

 of phosphate of lime ; in corals, echinoderms, mollusks and 

 many other animals -and some plants, of carbonate of lime ; 

 in most plants of woody or corky tissue. A larger or 

 smaller amount of organic matter is always combined with 

 the mineral. Of these substances, chitin is with difficulty 

 dissolved. Silica in its ordinary crystalline condition is 

 one of the most stable minerals, but when formed or- 

 ganically it is glassy and isotropic, and is dissolved with 

 comparative ease ; so that such skeletons may disappear 

 entirely, or another mineral may be deposited in the place 

 of the silica, or the isotropic silica may become crystalline. 

 In organisms with calcareous skeletons the carbonate of 

 lime is easily dissolved by water containing carbonic acid, 

 but the degree of solubility varies, according to the con- 



12 



