CLASS FISHES. 287 



(operculum) . The gills show as red filaments when this 

 is lifted. The heart has one auricle and one ventricle, and 

 handles impure blood. It forces the blood into arteries 

 that carry it to the gills. It is gathered from these into 

 a large artery that carries it to all parts of the system, 

 where it does its work through the capillaries. There 

 are four important regions where the blood is modified 

 on this course. What happens to it in each of the follow- 

 ing : in the muscles of the body ; in the walls of the digest- 

 ive tract; in the kidneys; in the liver? It returns to the 

 heart through large veins. 



297. Food and Digestive System. The food of fishes is 

 as diverse as possible. Some, as the sharks, are fierce 

 and carnivorous, preying on forms as large or larger than 

 themselves ; other, and these are the most numerous class, 

 depend upon small animals, such as worms, Crustacea, 

 insects, and the young of their own or other species of 

 fish; some live largely on plants. Some fish are scaven- 

 gers and live upon the dead matter found in the waters. 

 The microscopic animals and plants which occur in such 

 immense numbers are important items of the food supply 

 of fishes. The digestive tract is varied in accordance 

 with the type of food used the vegetable feeders hav- 

 ing a longer intestine, as is always true. The following 

 regions are usually to be distinguished: the mouth, 

 which may have teeth on the roof as well as on the jaws; 

 the pharynx, in the side walls of which the gill-slits and 

 gills are ; a short oesophagus leading to the stomach which 

 is an enlargement of the tract, varying a good deal in 

 shape and size; from the stomach a smaller tube, known 

 as the intestine, leads, with one or more coils, to the anal 

 opening. The stomach very often has a number of hollow, 



