A DEGENERATE GLAND. PRINCE. 207 



stomach, or in other cases, the distensible gullet, with air for 

 the same end. It is possible that these fishes having swallowed 

 air for purposes of respiration, the habit has been turned to 

 this other (defensive) purpose ; but the important point to note 

 is this that the swim-bladder has been utilised in these lishes 

 neither for respiration, nor for defensive inflation and flotation. 

 It is reasonable to suppose that in the assumption of a post- 

 branchial respiratory habit the oesophagus would be utilised. 

 That is the view of all anatomists ; but, if an organ, so frequently 

 present as the swim-bladder, be really respiratory, there must 

 in addition to this very exceptional and abnormal habit 

 among flshes of swallowing air, be developed an appropriate 

 blood-circulation, i. e. venous or impure blood must be conveyed 

 to the sac for the purpose of puriflcation (oxygenation) ; but 

 this is an arrangement not existing in any flshes excepting 

 Ganoids and Dipnoi. 



The features presented by the blood supply lend no support 

 to the '■ respiratory " theory. The essential feature of respira- 

 tion is the conveyance of impure or venous blood to a special 

 organ for puriflcation (oxygenation)*, the oxygenated blood 

 passing away to the body, and leaving the carbonic acid to be 

 g'ot rid of in the readiest way ; but the swim-bladder totally 

 diflers from such an arrangement. It is in fact supplied with 

 arterial blood almost direct from the aorta or aortic arch. In 

 such a form as the haddock ( Melanogrammiis crglejinus) the two 

 epibranchial arteries unite, it is needless to say, to form the 

 dorsal aorta, and on each side anterior to the union is given oft' 

 a sub-clavian artery to the pectoral flns, and in front of the 

 sub-clavian on the right side are given oft two visceral arteries 

 (the coeliac and the mesenteric) the latter going to the swim- 

 bladder, but in the sharks it goes to the spleen, pancreas and 

 intestine. From the swim-bladder the blood oroes into the 



*As Claus (Lehrb. d. Zool.) says "The blood must nec^sdirily absorb oxygen and 

 ■exhale carbonic acid. This interchange of gases effected between the blood ana the 

 medium in which the animal lives is the essential feature in respiration either in the 

 atmosphere or in the water." 



