INTERNAL ORGANS i73 



exchange of gases takes place {cf. p. 40). Having been purified, 

 the blood, instead of returning to the heart, is again received 

 into a main artery— the dorsal aorta— giving off various 

 branches which divide again and again into smaller and 

 smaller vessels, by means of which the oxygenated blood is 

 carried to the remotest parts of the body (Fig. 70). The blood 

 gives up its oxygen to the hungry cells, receives their waste 

 products, and then, loaded with impurities, makes its way back 

 to the heart through the veins. All the large veins, with the 

 exception of those from the liver, unite into two large vessels 

 running across the body, and these meet together as they 

 open into the sinus of the heart. The details of the circulation 

 may vary somewhat in the different groups of fishes, but the 

 essential features are as described above. 



The amount of blood present in the body of a fish is a 

 good deal less than in the higher vertebrates; it is pale in 

 colour, and its flow through the arteries and veins is a sluggish 

 one. Further, except in forms such as the Tunny {Thunnus), 

 Albacore {Germo), and Sword-fish {Xiphias), remarkable for their 

 great muscular activity, in which it is abundant and com- 

 paratively warm, the temperature of the blood is but little 

 higher than that of the surrounding water. In addition to the 

 blood-vessels, there is also the fine network of tubes known as 

 the lymphatic system, widely distributed in the connective 

 tissue of different parts of the body, collecting the blood plasma 

 which oozes through the fine capillaries for the nourishment of 

 the tissues, and carrying it back to the veins. In some fishes 

 lymph hearts are present where the larger lymphatics open mto 

 the veins, and the Common Eel {Anguilla), for example, has 

 such a pulsating organ in its tail. 



Among the remaining organs occupying the mterior of the 

 body cavity are the kidneys, reproductive organs, air-bladder, 

 and such ductless glands as the thyroid, thymus, and supra- 

 renal bodies. These last glands are as yet Httle understood, 

 and need not be further considered here. The kidneys are 

 generally long, thin glands, dark red in colour, situated 

 immediately below the vertebral column. Their purpose is to 

 extract certain impurities from the blood, poisonous by-products 

 formed by the processes of combustion constantly takmg place 

 in muscles and nerves, and to convey them to the exterior by 

 means of urinary ducts. The reproductive organs are of two 

 kinds, ovaries in the female, testes or milt in the male, or as 

 they are famiUarly called, hard and soft roes (Fig. 68). These 



