ZOOLOGY. 549 



3331. The lung is still subordinate to the abdomen 

 and intestine. It is therefore still separated from the 

 trachea or the branchial arches. 



3332. Fishes swallow the air and drive it into the 

 pulmonic bladder, where it is analysed or decomposed. 



3333. This lung, so soon as it stands opposite to and 

 enters into conflict with a branchia, is what serves to 

 direct or control the action of the heart. It is only when 

 the swim-bladder is allowed to hold good as lung, 

 that the circulation in Fishes admits of being understood. 

 For otherwise venous blood must flow into the heart, from 

 this into the branchiae, from these directly into the aorta, 

 and so to the different organs without again entering a 

 cardiac cavity, a structure which occurs in no other class 

 of animals, but rather in every instance its reverse. The 

 first heart is arteriose, not venous in all animals, even in 

 the Mussels and Snails and in the embryos. Now, the 

 Fish is still such an embryo, and has only an arteriose 

 heart. 



3334. Thus, the following is the " modus operandi " 

 of the parts in question ; the swim-bladder is the 

 lung, in which, seeing that it contains air, the blood is 

 oxydized ; this oxydized blood flows into the heart and 

 renders it arteriose in character, despite the concomitant 

 influx of venous blood into that organ. Upon this, the 

 blood passes out of the heart through a true aorta, which 

 is called the branchial artery. Instead then of this aorta 

 giving off only some twigs as bronchial vessels to the 

 gills, and next proceeding to pursue its course as a main 

 trunk through the body and along the back, it passes 

 wholly to the branchiae, i. e. it entirely becomes a bron- 

 chial vessel, is slightly oxydized, and then returns upon 

 itself to form the aorta, which should have taken its 

 course directly from the heart. 



Senses. 



3335. In accordance with the thorax, the corium or 

 true skin is developed. It is mucous in character and 

 muco-secernent like the intestine, from its being con- 



