226 The Fish World at Eome. 



tricle of its three-chambered heart. As energy of character 

 and capacity for exertion are usually proportioned to the amount 

 of heat which an animal evolves, we must expect that fishes, 

 with a respiration rendered imperfect or slow, from the nature 

 of the fluid in which they live, and with bodies whose internal 

 temperature is lower than that of mammals or birds, would be, 

 in many respects, sluggish creatures ; and to a large extent they 

 deserve this character ; though the shark tribe exhibit activities 

 very analogous to those of terrestrial beasts of prey. In Mr. 

 Couch's valuable and interesting History of the Fishes of the 

 British Islands, which follows a generally-recognized system, 

 these voracious creatures are placed at the head of all the families 

 of Fish. Together with the ray-fishes, they belong to the 

 Ghondropterygious order; and it is consolatory for those who 

 are alarmed at such an awkward, jaw-breaking term, that a 

 better-sounding and more popular word, " cartilaginous," will 

 do just as well. The characteristic of the skeleton of these 

 fishes is the absence of that quantity of mineral matter which 

 gives rigidity to ordinary bones ; but, as Professor Owen says, 

 " I know not why a flexible vascular animal substance should 

 be supposed to be raised in the histological scale because it has 

 become impregnated by the abundant intussusception of earthy 

 salts." The shark has, for a fish, a large brain, immense strength, 

 and a capacity for prolonged exertion, which Mr. Couch traces 

 to the highly-developed character of its muscles, which " bear 

 a resemblance to those of quadrupeds." The eye of this tiger 

 of the deep affords the quick vision required for its predacious 

 life, and Mr. Couch thus explains its peculiar mechanism : — 

 ' ' On examining the cavity in which the eye of the shark re- 

 volves, we find that the globe, which is the immediate seat of 

 the power of vision, is lifted from the bottom on which, in other 

 animals besides this great family, it rolls, and is placed on a 

 small table that itself forms the top of a slender pillar, the 

 bottom of which is fixed on the bony circle of the common 

 ocular cavity, or, more properly speaking, of the pillar itself, 

 which leans a little forward that it may be accommodated to 



the most usual direction in which objects are viewed 



The height of this ocular pillar has the additional advantage 

 of allowing a greater length to the muscles which move the 

 eye, and, by so doing, of providing for a more sudden as well 

 as a more extensive action of the eyes in prowling for their 

 prey." 



The general form of the jaws of the shark is well known 

 through specimens in museums, but to Mr. Couch must bo 

 assigned the merit of observations on the formation of the teeth, 

 which appear to have been simultaneous with those made known 

 by Mr. Owen, and which are thus described : — " In all fishes 



