208 



ICHTHYOLOGY. 



Introduc- phosphate of lime, which gives it its hardness. Of the and fishes the least ; bnt there is great variety in this re- Introduc 

 tion. vertebrals, birds have most earthy matter in their bones, spect even in the same class, the bones of each species tion. 



Osteology. Osteology 



Fiir. 10. 

 Skeleton of tlie Pereli. 



being adapted by their degrees of firmness and hardness for 

 the shocks they have to sustain. The ditt'erence depends 

 chiefly on the quantity of fluid matter in the cells and canals 

 of the bone. In a note the composition of the bones of a 

 cod, after the fluid portions had been evaporated, is given 

 as analvsed by Bibra.' The bones of fishes contain more 

 evaporable fluid than those of the other classes of vertebrals, 

 and also a larger portion of the salts of soda. The animal 

 portion of a Shark's skeleton has much of the properties of 

 mucus, yields no gelatine when dissolved in boiling water, 

 and is not precipitated from the solution by infusion of 

 galls, in which respect it differs fi-om the cartilaginous foun- 

 dation of the bones of beasts. The unossified skeleton of 

 the Lamprey holds only I5 per cent, of earthy salts. In 

 the vascular canals, which are permeated by tlie vessels 

 that deposit the bone and can-y on its circulation, there is 

 usually some oil, which is composed chiefly of oleine, and 

 is easily converted into soap, and then produces oleic acid, 

 glycerine, and a minute portion of margaric acid. It has 

 odour, and for the most part a yellow colom\ In the 

 Belone and Lepidosiren it imparts a deep green colour to 

 the bones. The bones of fishes continue to grow in size 

 almost throughout life. 



The limits of this treatise do not admit of a detailed ac- 

 count of the skeleton and other parts of the structure of 

 fishes, of all which, in connection with many admirable re- 

 marks on the homologies and analogies of the several parts 

 of ichthyic structure with the corresponding parts of the 

 other vertebrals, the student will find a full account in 

 Professor Owen's Lectures on the Comparative Anatomi/ 

 and Phijsiologu of the Vertebrate Animals. So complete 

 a work, so easy to be procured, renders it unnecessary to 

 introduce more anatomical details here than are absolutely 

 requisite for the understanding of the systematic re\iew of 

 the class which follows ; and these Me shall borrow from the 

 work referred to, as being the best authority existing. 



The first step in the knowledge of the skeleton of a fish. 



or of any other vertebral, is the determination of the verte- 

 bral segments, or natural grouj)s of bones which compose 

 the skeleton. A vertebra is defined by the Professor to be 

 one of tliose segments of the endo-skeleton ichich constitute 

 the axis of the body, and the protecting canals of the ner- 

 vous and vascular trunks : such a segment may also sup- 

 port diverging appendages. The elements of a vertebra 

 are — 1. The body or centrum, from which the others radi- 

 ate ; 2. Two )ieurapophi/ses, which, rising upwards, form the 

 neural canal, that incloses and protects the longitudinal 

 central chord of the nervous system ; 3. Two parapophi/ses 

 or transverse processes ; 4. Two pleurapophyses or costal 

 processes ; 5. Two licemapophi/ses, which form, with the 

 centrum, a canal for lodging the central organ and large 

 trunks of the vascular system. This canal is on the ventral, 

 that is, the opposite side of the centrum to the neural one, 

 and is less regular and more interrupted. 6. A neural 

 spine, which crowns the neurapophyses, or is interposed 

 between their tips, and is very conspicuous in the figure 

 of the skeleton of the perch, as it is in most fishes. 7. A 

 hcenial spine. 8. Two diapophyses or upper (anterior or 

 proximal in fishes) transverse processes of human anato- 

 my ; and, 9. Two zi/gapophyses or oblique processes. 



In fishes a complete typical vertebra can only be ob- 

 tained from the head : in the rest of the column the haenia- 

 pophyses are always absent or unossified ; their ha;mal canal 

 being formed by modifications of the parapophyses. The 

 nimiber of vertebrae is governed by the number of pairs 

 of nerves that unite with the spinal chord, and varies 

 greatly in different species, being very numerous in the 

 vermiform or serpentiform Apodes, and some Plagiostomi, 

 and few in the Plectognathi. In many species they are 

 extensively anchylosed. The earlier stages of vertebral 

 development are permanently represented in the lowest 

 fishes. In the Lancelet, which is at the bottom of the 

 scale, the entire vertebral column consists of a gelatino- 

 cellulai' chord and its membranous sheath. In the Lara- 



^ Chemical composition of bones of the cod after drying — 



Phosphate of lime with trace of fiuate 57'29 



Carbonate of lime 490 



Phosphate of magnesia 2-40 



Sulphate, carbonate, and chlorate of soda I-IO 



Gluten and chondrin 32-31 



Oil 200 



100 00 



Bibra, Owmt Lectures, -S:c. 



