FISHES 213 



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

 chfferent organs of the body. Tliese brancli arteries finally 

 form minute capillaries tlrat permeate all tiie tissues. The 

 capillaries unite to form veins through which the blood is 

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

 walled auricle and then into the ventricle, thus completing 

 the circuit (Fig. 149). 



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

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

 two liemispheres in front constitute the cerebrum. The 

 olfactory lobes project from tlie 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 tiie brain. Behmd these is 

 the cerebellum, a single, undivided portion. Underneath 

 and posterior to the cerebellum is the medulla oblongata, the 

 enlargeil end of the spinal cord which extends posteriorly 

 through the bony tube formed by the dorsal projections of 

 the vertebrte. 



Plan of structure. — Tlie fish presents an entirely new 

 plan of structure. The nervous system is on the dorsal 

 side of the body, while the ner\'ous system of tlie inverte- 

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

 side of the body, iloreover, the main nervous system of 

 the perch is inclosed in a long cavit}' on the dorsal side of 

 the body. Below this cavit}' and sej^arated from it is the 

 large body cavity containing the alimentary canal. There- 

 fore, in a cross section of the jiei-ch, two cavities appear, 

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

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

 tinctions are characteristic of vertebrates and invertelirates. 



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



