THE SKELETON. 23 



the nerve-trunks seems to be accidental, depending upon the position 

 of the muscular masses to which they give attachment, and Avhieh 

 office is the essential condition of their existence. For this purpose 

 the processes of the exo-skeleton of Insecta and Crustacea must go in- 

 wards, and thus they happen to protect certain parts of the nervous 

 system. 



Only the highest of the Mollusca possess a true homologue of the 

 endo-skeleton, developed in relation to the defence of the nervous 

 centres : but it is a feeble cartilaginous rudiment in the best organised 

 Cephalopods ; and, in the cuttle-fish, is far outweighed by the calca- 

 reous dorsal plate which still represents the exo-skeleton of the tes- 

 taceous moUusks. Thus a cartilaginous cranial vertebra co-exists in 

 the highest Invertebrata with a calcareous dermal skeleton ; and 

 there is no abrupt contrast in passing thence to the consideration of 

 the skeleton in the Vertebrata. 



The exo-skeleton is by no means indeed dispensed with in the Ver- 

 tebrate series, although the endo-skeleton is constant, and here attains 

 its full development. In the lowest class, most fishes, for example, pre- 

 sent an imbricated outer covering of scales, developed like shells, 

 between the derm and epiderm : other fishes have hard osseous plates 

 or spines scattered over their exterior, or are entirely surrounded 

 by a connected armour of dense enamelled bony scales, as in the 

 Lepidosteus and the Ostracion, Avhich latter fish ofi'ers an instructive 

 example of the co-existence of an exo- with an endo-skeleton, and 

 a convincing refutation of the idea of the homology of the annular 

 segment of the crust of the lobster with a true typical vertebra. In 

 the subjoined diagram, i^fig- 7.) n is the cartilaginous neural canal ; 

 pi, the membranous pleurapophysial wall of 

 the abdomen ; h, the arterial and venous 

 trunks of the abdomen ; dn, dp, dh, the der- 

 mal ganoid plates. The ossified scutes of the 

 Crocodiles and the tesselated armour of the 

 Armadillos are examples of the exo-skeleton 

 co-existing with a well-developed and ossified 

 endo-skeleton ; and wherever the exo-skeleton 

 Segment of endo- and exo- of a Vertebrate animal is calcified, it presents 



skeletons, Ostracion. , • t i . , i 



the same organised vascular structure and 

 vital properties as the bones within. Most commonly the exo- 

 skeleton of the air-breathing Vertebrata is epidermal, as where it 

 forms the scales of the serpent or lizard, the large plates of the 

 tortoise, the imbricated pointed scales of the manis, the spines of the 

 hedgehog, the quills of the porcupine, the feathers of tlie bird, or the 

 hair of the ordinary mammal. 



c 4 



