SKATES. 79 



divisions or doublings of the membrane of the gills. Each 

 division has on each side of it one hundred and sixty sub- 

 divisions or folds of its membrane, the length of each of 

 which in a very large Skate is about one eighth of an inch, 

 and its breadth about one sixteenth of an inch; so that in 

 the whole gills there are one hundred and forty-four thousand 

 folds, the two sides of each of which are equal to the sixty- 

 fourth part of a square inch, or the surface of the whole 

 gills is equal to two thousand two hundred and fifty square 

 inches, that is, to more than fifteen square feet, which have 

 been supposed equal to the whole external surface of the 

 human body. When, after a good injection by an artificial 

 coloured fluid of the artery, a microscope is applied, the 

 whole extent of the membrane of the gills is seen covered 

 with a beautiful network of exceedingly minute vessels. — 

 (Monro, "Structure and Physiology of Fishes," p. 15.) Those 

 particulars are given the more at length because they belong 

 to the whole of this large family of Sharks and Skates, 

 although perhaps to some of them in a less degree. And, 

 indeed, they are applicable to fishes in general, although in a 

 still less degree in the class termed the bony fishes, in which 

 the water is made to travel through the gills in a more rapid 

 manner, to make amends for the less extent of surface to 

 which it is applied. 



The blood is brought;, to this purifying apparatus by a 

 vessel proceeding from a heart specially framed to be a 

 powerful instrument of propulsion; for although the general 

 cause of the circulation of the blood is formed on the same 

 principle in all fishes, the peculiarity of action and the power 

 varies considerably, as regards the present genus, compared 

 with several others. In Sharks and Skates the heart is of a 

 singularly complex structure, mingled with simplicity; the 

 organs of the great artery, or vessel which bears the blood 

 from the heart to the gills, consisting of a long muscular 

 tube, which may be regarded as an extension of the heart 

 itself, and which tends greatly to aid its powers of action; but 

 a remarkable circumstance attending it is the degree of which 

 physiologists call irritability that resides in it, and which 

 maintains its powers of life and action under circumstances 

 which to other races must speedily prove fatal. I have even 



