90 ZOOLOGY SECT. 



upwards. Thc}^ are developed as a liollow outpiishino- from the 

 ventral wall of the embryonic forr-yut or anterior part of the 

 enteric canal ; this passes backwards and upwards, usually 

 dividing into right and left divisions, and finally coming to lie 

 in the dorsal region of the coelome. The inner surface of the 

 single or double lung thus formed is raised into a more or less 

 complex network of ridges so as to increase the surface of blood 

 exposed to the action of the air ; and, in the higher forms, the 

 ridges, increasing in number and complexity, and uniting with 

 one another across the lumen of the lung, convert it into a 

 sponge-like structure. The respiratory epithelium is, of course, 

 endodermal. Since the lungs are blind sacs, some contrivance is 

 necessary for renewing the air contained in them : this is done 

 either by a process analogous to swallowing, or by the contraction 

 and relaxation of the muscles of the trunk. 



In some Fishes there occurs, in the position occupied in air- 

 breathers by the lungs, a structure called the air-bladder, which 

 contains gas, and serves as an organ of flotation. Like the lungs, 

 it is developed as an outgrowth of the fore-gut, but, except 

 in four instances, from its dorsal instead of its ventral side. In 

 many cases the air-bladder loses its connection with the pharynx 

 and becomes a closed sac. 



The blood-vascular system attains a far higher degree of 

 complexity than in any of the groups previously studied: its 

 essential features will be best understood by a general description 

 of the circulatory organs of Fishes. 



The heart (Figs. 760 and 772) is a muscular organ contained in 

 the pericardial cavity and composed of three chambers, the sinus 

 vnwsus (s. r.), the auricle (ecu.), and the ventricle (v.), which form a 

 single longitudinal series, the hindmost, the sinus venosus, opening 

 into the auricle, and the auricle into the ventricle. They do not, 

 however, lie in a straight line, but in a zigzag fashion, so that the 

 sinus and auricle are dorsal in position, the ventricle ventral. 

 Usually a fourth chamber, the conus arteriosus (c. ai't.), is added 

 in front of the ventricle. The various chambers are separated 

 from one another by valvular aj^ertures (Fig. 773) which allow of 

 the flow of blood in one direction only, viz. from behind forwards — 

 that is, from sinus to auricle, auricle to ventricle, and ventricle to 

 conus. The heart is made of striped muscle of a special kind — the 

 only involuntary muscle in the body having this histological 

 character — which is particularly thick and strong in the ventricle. 

 It is lined internally by epithelium and covered externally by 

 the visceral layer of the pericardium. 



Springing from the ventricle, or from the conus when that 

 chamber is present, and passing directly forwards in the middle line 

 below the gills, is a large, thick-walled, elastic blood-\'essel, the 



