46 Descriptions of Preparations. 



with its centrum. About twelve of the trunk vertebrae have two 

 parapophyses, developed one in front of the other^ from the lateral 

 surfaces of their centra, the more forwardly placed being- the larger 

 of the two. These two outgrowths^ besides serving- as zygapo- 

 physes, as may be seen in the dried bone_, gave in the living 

 animal protection to the air-bladder, which, though devoid of the 

 air-duct possessed by the three fresh-water orders just specified, 

 is yet largely developed in the Gadidae. There are two articular 

 facets, a larger looking outwards and a smaller one looking in- 

 wards, developed upon the anterior aspect of the base of each 

 neural arch, as it rises from the anterior edge, or close upon the 

 anterior edge of each centrum. An imperfect socket is thus 

 formed in some of the vertebrae for the posterior zygapophyses 

 of the vertebrae next in front, which zygapophyses are upgrowths 

 developed upon the centra, independently of the neural arch, and 

 have their single articular facets looking inwards. The parapo- 

 physes of the trunk vertebrae after the fifth, carry ribs, or rib-like 

 intermuscular ossifications, or both, and in the caudal region they 

 form perfect haemal arches. 



The bones of fishes are the poorest in inorganic constituents of 

 all the five classes of vertebrata, and the bone cells, the microscopic 

 morphological essential of true bone, are wanting in both Plagio- 

 stomi and Cyclostomi, in Acanthopteri, and most other fishes, with 

 the exception of the Physostomi and Ganoids. In the small pro- 

 portion of inorganic matter in their bony or 'osteoid^ tissue fish 

 resemble the Amphibia ; but as the lowest of this latter class 

 possess true bone-tissue, it may seem that the absence of bone cells 

 is correlated more or less exactly with the absence of the air-duct 

 which is possessed by the two orders, Physostomi and Ganoidei, 

 which possess true bone. The low jDi'oportion of inorganic salts in 

 the bones both of Amphibia and of Pisces, may be correlated in 

 both alike with their aquatic habits. 



Por the absence or presence of bone cells in the skeleton of fish, 

 see Kolliker, Royal Society's Proceedings, ix. p. 6^6, 1859; 

 Quekett, Histological Catalogue, ii. B. a. 133, p. 40. 



