84 ZOOLOGY SECT. 



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 pharynx, but, except in two 

 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. 715 and 727) is a muscular organ contained in 

 the pericardial cavity and composed of three chambers, the sinus 

 venosus (s. v.'), the auricle (cm.), 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. 

 Sometimes a fourth chamber, the conus arteriosus (c. art.), is added 

 in front of the ventricle. The various chambers are separated 

 from one another by valvular apertures (Fig. 728) which allow of 

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

 or from sinus to auricle, auricle to ventricle, and ventricle to conus. 

 The heart is made of striped muscle 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-vessel, the 

 ventral aorta (Figs. 715, B, and 727, v. ao.\ formed of fibrous and 

 elastic tissue and unstriped muscle, and lined with epithelium. At 

 its origin, which may be dilated, forming a bulbus aortcv, are valves 

 so disposed as to allow of the flow of blood in one direction only, 

 viz. from the ventricle into the aorta. It gives off on each side 

 a series of half-hoop-like vessels, the afferent branchial arteries 

 (a. br. a.), one to each gill. These vessels ramify extensively, 

 and their ultimate branches open into a network of microscopic 

 tubes or capillaries (Fig. 728, G.), having walls formed of a single 

 layer of epithelial cells, which permeate the connective-tissue layer 

 of the branchial filaments, and have therefore nothing between 



