384 



ANIMAL RESPIRATION 



in exchange, so to speak, the waste carbonic acid gas. The 

 purified blood is then distributed to all parts of the body (fig. 



513)- 



In cases where organs of any sort are repeated to form a 

 series, it frequently happens that increased efficiency is gained 



DORSAL AORTA . 



POST. CAROL. ■ 

 Fig. 513. — Diagram of Circulatory System in a Fish 



The direction of blood-flow is indicated by arrows; vessels containing pure blood left unshaded, those containing 

 impure blood are darkly shaded. The capillaries of head, gills {gggggJ, digestive tube (dig.), liver (lr.), trunk, 

 and fins are represented as net-works. 



The heart essentially consists of an auricle (au.) and ventricle (vent.), and pumps impure blood to the gills, where 

 it is purified, afterwards passing to the dorsal aorta for general distribution. The impure blood is returned to the 

 heart by paired anterior cardinal (ant. cardl. } and posterior cardinal (post, cardl.) veins. The liver receives pure 

 blood by a liver-artery (lr. art.) and also impure blood (from stomach, intestines, pancreas, and spleen) by an hepatic 

 portal vein (h.p.v.). Its impure blood is returned to the heart by a liver-vein (lr.vn.). The complex circulatory 

 arrangements of the liver are known as the ' hepatic portal system ". (dig. art.), Artery carrying pure blood to 

 stomach, &c. The gills and gill-vessels of one side only are indicated. 



by a reduction in number, usually accompanied by specialization 

 of the surviving members of the series. Good examples are 

 furnished by the reduction in number of the teeth of some Car- 

 nivores (see p. 7), of walking-legs in Insects as compared with 



COURSE OF FOOD 

 -BREATHING TUBE - 



Fig. 514, — Diagram showing the gill-pouches (g) of one side in a Lamprey {Peirotiryzofi) 



Scorpions or Centipedes, and of digits in the Horse, which has 

 one large efficient toe as contrasted with the five of a Man. And 

 when we examine the breathing organs of a Lamprey i^Petro- 

 myzoit) we find only seven pairs of gill-pouches, a great reduction 

 as compared with the Californian Hag- Fish. Nor do these 

 pouches open directly out of the digestive tube, but they com- 

 municate internally with a breathing tube which underlies and 

 is connected in front with this (fig. 514). 



