. Beginners' Zoology . or below the centre of the body? Why?The air bladder makes the body of the fish about as light as waterthat it may rise and sink with little effort. When a fish dies, the gasesof decomposition distend the bladder and the abdomen, and the fishturns over. Why? Where are the kidneys? (Fig. 220.) Their ends unite close underthe spinal column. The ureters, or tubes, leading from them, unite,and after passing a small urinary bladder, lead to a tiny urinary porejust behind the opening from the ovary. (Coloured figure 4.) The Circulation. — The fish, unlike other vertebrates, ha
. Beginners' Zoology . or below the centre of the body? Why?The air bladder makes the body of the fish about as light as waterthat it may rise and sink with little effort. When a fish dies, the gasesof decomposition distend the bladder and the abdomen, and the fishturns over. Why? Where are the kidneys? (Fig. 220.) Their ends unite close underthe spinal column. The ureters, or tubes, leading from them, unite,and after passing a small urinary bladder, lead to a tiny urinary porejust behind the opening from the ovary. (Coloured figure 4.) The Circulation. — The fish, unlike other vertebrates, has itsbreathing organs and its heart in its head. The gills have alreadybeen described. The heart of an air-breathing vertebrate is near ii8 BEGINNERS ZOOLOGY its lungs. Why? The heart of a fish is near its gills for the samereason. The heart has one auricle and one ventricle. (Colouredfigure I.) Blood returning to the heart comes through several veins into asinuSy or antechamber, whence it passes down through a valve. Fig. 221.— Plan of Circulation. Ab, arteries to gills; Ba, aortic bulb; V, ventricle. into the a/zr/r/d; from the auricle it goes forward into \.\\e ventricle. The ventricle sends it into an artery, not directly, but through abull? (as, Fig. 220), which jserves to maintaina steady flow, without pulse beats, into thelarge artery (aorta) leading to the gills. Thearteries leading from the gills join to form adorsa/ aorta (Ao, Fig. 221), which passesbackward, inclosed by the lower processes ofthe spinal column. After going through thecapillai-ies of the various organs, the bloodreturns to the heart through veins. The colour of the blood is given by redcorpuscles. These are nucleated, oval, andlarger than the blood corpuscles of other ver-tebrates. The blood of the fish is slightlyabove the temperature of the water it in-habits. Notice the general shape of the brain(Fig. 222). Are its subdivisions distinct orindistinct? Are the lobes in pairs? Themiddle portion of
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