FISHES 213 



artery, which, m turn, sends off branches that supply the 

 different organs of the body. These branch arteries finally 

 form minute capillaries that permeate all the tissues. The 

 capillaries unite to form veins through which the blood is 

 conveyed to the sinus venosus and from this into the thin- 

 walled auricle and then into the ventricle, thus completiag 

 the circuit (Fig. 149). 



Brain and spinal cord. — Looking at the brain from the 

 dorsal side it is seen to consist of several divisions. The 

 two hemispheres in front constitute the cerebrum. The 

 olfactory lobes project from the anterior ends of the cere- 

 brum and send the olfactory nerves forward to the nasal 

 cavities. Posterior to the cerebrum are the two large 

 optic lobes, the widest part of the brain. Behind these is 

 the cerebellum, a single, undivided portion. Underneath 

 and posterior to the cerebellum is the medulla oblongata, the 

 enlarged end of the spinal cord which extends posteriorly 

 through the bony tube formed by the dorsal projections of 

 the vertebrae. 



Plan of structure. — The fish presents an entirely new 

 plan of structure. The nervous system is on the dorsal 

 side of the body, while the nervous system of the inverte- 

 brate animals, for example, the earthworm, is on the ventral 

 side of the body. Moreover, the maui nervous system of 

 the perch is inclosed in a long cavity on the dorsal side of 

 the body. Below this cavity and separated from it is the 

 large body cavity containing the alimentary canal. There- 

 fore, in a cross section of the perch, two cavities appear, 

 while in the cross section of the body of an earthworm only 

 one cavity appears. As we pointed out earher, these dis- 

 tinctions are characteristic of vertebrates and invertebrates. 



How a fish swims. — In the dorsal part of the abdominal 



