370 OF RESPIRATION. 



ginous Fishes, each branchial arch is inclosed in a separate cavity; 

 which communicates on the inner side with the pharynx by an orifice 

 peculiar to itself, and by another orifice with the external surface. 

 Thus there is a series of external openings, instead of a single one, on 

 each side of the neck ; and these sometimes amount to six or seven, as 

 in the Lamprey, reminding us of the spiracles of Articulated animals ; 

 whilst there is a corresponding series of internal openings into the pha- 

 rynx on either side, or into a tube that communicates with it. 



664. It is well known, that most Fishes speedily die when removed 

 from the water ; and it can be easily shown, that the deficient aeration 

 of the blood is the immediate cause of their death. But as it might 

 have been expected, that the atmosphere would exert a much more 

 energetic influence upon the blood contained in the gills, than that 

 which is exercised by the air contained in the water, the question 

 naturally arises, how this deficient aeration comes to pass. It is chiefly 

 due to the two following causes : the drying-up of the membrane of 

 the gills themselves, where it is exposed to the air, so that the aeration 

 of the blood is impeded ; and the flapping together of the filaments of 

 the gills, which no longer hang loosely and apart, but adhere in such 

 a manner as to prevent the exposure of the greater portion of their 

 surface to the air. Those fishes can live longest out of water, in which 

 the external gill-openings are very small, so that the gill-cavity may 

 be kept full of fluid ; and there are certain species which are provided, 

 like the Land-crab, with a particular apparatus for keeping the gills 

 moist, and which perform long migrations over land in search of food, 

 even (it is said) ascending trees. These are exceptions to the general 

 rule. 



665. The respiration of Fishes is much more energetic than that of 

 any of the lower aquatic animals ; and this is partly due to the great 

 extension of the surface of the gills, partly to the provision just ex- 

 plained for maintaining a constant flow of fresh water over their sur- 

 face, and partly to the position of the heart at the base of the main 

 trunk that conveys the blood to the gills ( 558), by which the regular 

 propulsion of that fluid through these organs is secured. Their blood 

 too, is furnished with red corpuscles ; which give important aid in con- 

 veying oxygen from the gills to the remote tissues of the body, and in 

 returning the carbonic acid to be excreted. The proportion of these 

 varies considerably, in the different species of the class, being very 

 small in those that approach most nearly to the Invertebrata ; and there 

 is even an entire absence of them in one remarkable fish, the Amphi- 

 oxus or Lancelot ; whilst they are present in large numbers in the blood 

 of certain Fishes, which have great muscular activity, and can maintain 

 a high independent temperature. 



666. It would seem, however, that not even this high amount of 

 respiration is always sufficient for Fishes which live in small collections 

 of water, where their temperature is liable to be greatly augmented by 

 the heat of summer ; under which condition, there is an increased prone- 

 ness to disintegration in their tissues, and a corresponding necessity for 

 the extrication of carbonic acid and for the absorption of oxygen. Many 

 fresh-water fishes, under such circumstances, may be seen to come to the 



