Where are the kidneys? (Fig. [220].) Their ends unite close under the spinal column. The ureters, or tubes, leading from them, unite, and after passing a small urinary bladder, lead to a tiny urinary pore just behind the opening from the ovary. (Coloured figure 4.)
The Circulation.—The fish, unlike other vertebrates, has its breathing organs and its heart in its head. The gills have already been described. The heart of an air-breathing vertebrate is near its lungs. Why? The heart of a fish is near its gills for the same reason. The heart has one auricle and one ventricle. (Coloured figure 1.)
Fig. 221.—Plan of Circulation.
Ab, arteries to gills; Ba, aortic bulb; V, ventricle.
Blood returning to the heart comes through several veins into a sinus, or antechamber, whence it passes down through a valve into the auricle; from the auricle it goes forward into the ventricle. The ventricle sends it into an artery, not directly, but through a bulb (as, Fig. [220]), which serves to maintain a steady flow, without pulse beats, into the large artery (aorta) leading to the gills. The arteries leading from the gills join to form a dorsal aorta (Ao, Fig. [221]), which passes backward, inclosed by the lower processes of the spinal column. After going through the capillaries of the various organs, the blood returns to the heart through veins.
Fig. 222.—Brain of Perch, from above.
n, end of nerve of smell; au, eye; v, z, m, fore, mid, and hind brain; h, spinal bulb; r, spinal cord.
The colour of the blood is given by red corpuscles. These are nucleated, oval, and larger than the blood corpuscles of other vertebrates. The blood of the fish is slightly above the temperature of the water it inhabits.
Notice the general shape of the brain (Fig. [222]). Are its subdivisions distinct or indistinct? Are the lobes in pairs? The middle portion of the brain is the widest, and consists of the two optic lobes. From these lobes the optic nerves pass beneath the brain to the eyes (Sn, Fig. [223]). In front of the optic lobes lie the two cerebral lobes, or the cerebrum. The small olfactory lobes are seen (Fig. [224]) in front of the cerebrum. The olfactory nerves may be traced to the nostrils. Behind the optic lobes (mid brain) is the cerebellum (hind brain) and behind it is the medulla oblongata or beginning of the spinal cord.
