464 



COMPAEATIVE ANGIOLOGY. 



ieart is still more rudimentary, consisting of but two cavities, an airricie qjaj 

 a ventricle, but still the tissues are supplied with pure arterial blood • the 

 course of the blood being as follows : — The venous blood is conveyed by the 

 cava to the auricle, thence into the ventricle, and then by the branchial artery 

 to the BranchicB or gills, structures which are analogous to the lungs of higher 

 animals, and in which, consequently, the blood becomes arterialised. The 

 blood thus purified, is sent from the gills throughout the body to supply the 

 various tissues, and is finally returned by the cava to the heart. Hdnce the 

 circulation in these animals is single, the purified blood not being returned to 

 the heart, but driven all through the body. The impulse of the heart is there- 

 fore branchial. 



In the higher orders of the Crustacea, a still lower class of animals (Inverte- 



Fio. 172. 

 Diagram of the Circulation in 

 a, Fish, a, Branchial circulation ; 

 6, Somatic circulation ; c, Heart ; 

 d, Auricle ; e, Ventricle : /, Arte- 

 rial trunk. 



Fig. 173. 

 Diagram of the Circulation 

 in a Crustacean, a, Brau- 

 chial ; h, Somatic circulation. 

 On the right is the heart with 

 its single cavity, and on the 

 left the venous lacunae. 



"brate), we find that the cavities of the heart are reduced to one, which is 

 Tiowever contained in a kind of sac, very incorrectly called the pericardium. 

 Although these animals are supplied with gills, the impulse of the heart is 

 systemic, not branchial ; and, as in the fishes, the circulation is single. The 

 Mood is driven by the heart through the body, and then collected into certain 

 venous dilatations or lacunae, whence it is conveyed to the branchiae for 

 ■purification. The branchial vesseb then pour it into the so-called pericardium, 

 and it finally re-enters the heart by a number of valvular orifices. 



The blood thus supplied to the tissues is, however, not pure ; for it is ascer- 

 tained that certain veins, instead of leading to the lacunae, or branchiae, pass 

 to the pericardium, ia which cavity the blood thus becomes mixed. 



