PISCES 367 



and similar impurities. The blood supplied direct to the kid- 

 neys from the aorta and that of the renal-portal circulation is 

 returned to the heart by way of right and left (cardinal) veins 

 which join corresponding right and left veins {jugular) from the 

 head to form the veins (ductus Cuvieri) which empty into the 

 auricle. The blood which was distributed to the stomach and 

 intestines is gathered into a vessel (hepatic portal vein) which 

 carries it to the liver, together with much of the food absorbed 

 from the intestines. The hepatic portal vein here breaks up 

 into capillaries. The blood from the liver and from the appen- 

 dages unites with that carried by the ductus Cuvieri before it 

 reaches the heart. The student should carefully follow out the 

 course of the circulation in the accompanying diagrams (Figs. 

 181, 182) and determine just what changes take place in the 

 blood in the various capillaries through which it passes. Varia- 

 tions from this typical condition are numerous, accounts of 

 which must be sought in more extended texts. 



386. Library Exercise. — Find description and figures in the reference zoologies 

 locating the unfamiliar structures in circulation of fishes in the table on page 344 

 and testing the statements found there. 



387. Habits and Food. — Fishes occur abundantly both in 

 fresh and salt water. Usually the whole life is spent under the 

 same general conditions. The salmon and shad, however, are 

 bred and partly develop in fresh water and later pass out to the 

 sea. These forms return, often with remarkable precision, to 

 the place of their own hatching to deposit their eggs. Use is 

 made of this habit in the capture of them for commercial pur- 

 poses. Unless some means are found for limiting the destruc- 

 tion of the adults during the breeding period, some of our most 

 valuable food fishes are in danger of extermination. Others, as 

 the eels, may generate in the sea and spend a part or all their 

 adult life in fresh water. Some burrow in the muddy bottoms, 

 as the eel, cat-fish, mud-fish; others lie on the bottom, as the 

 flat-fish; many quaint forms frequent the depths of the sea. 

 The most are active swimmers in open water near or away from 

 the shore. Many forms (herring, shad, salmon, etc.), are dis- 

 tinctly gregarious, moving in great shoals especially at spawning 



