THE TRIGGER-FISH 



of the organism and also serves to support it. It is 

 a part of the external skeleton, forming an outer and 

 superficial layer which completes and consolidates the 

 deep inner skeleton which is not so strong, since it is 

 mainly made up of cartilaginous tissue. This is an 

 interesting dual construction which reminds us today 

 of forms which were more frequent in ages past. 

 This peculiarity is not now so marked in most verte- 

 brates, for their internal skeleton is mainly composed 

 of bone; but it is still to be remarked in the develop- 

 ment of the embryo. 



This appearance and development of the skeleton 

 of vertebrates in the embryo is very remarkable, 

 especially in the case of reptiles, birds, and mammals. 

 At first the parts of the skeleton, when they begin to 

 take shape, are not bone; many are cartilaginous. 

 Then, as they develop, they change and become 

 gradually more and more bony. The strong bony 

 tissue, impregnated with lime, is gradually substituted 

 for the softer tissue which preceded it. The child at 

 birth has a soft and mainly cartilaginous skeleton 

 which hardens with growth and gradually ossifies, 

 being enriched by additional plates, like the bones of 

 the dome of the skull, which are not osseous to begin 

 with but membranous. In the species of fish related 

 to the bichir and bony pike, we find corresponding 

 plates; they form a superficial covering, a kind of 

 outer skull, which is not even covered by the skin. 

 If we consider all these facts together, it looks as 

 though the ossification of the skeleton, a notable 

 advance, since it gives the supporting apparatus its 

 main quality, that of strength, proceeds partly from 

 the outside of the body to the inside, as if it were 

 concerned to bring inside, and there make use of, 

 pieces originally shaped outside. This brings us back 

 to a very old story, that of the armoured fishes of 

 other days. 



At the present day, the principal species of fish, 



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