FISHES 325 



booty, while crustaceans, mollusks, etc., have an external 

 hard covering, if they have any hard structure at all. 

 In the animals that are below fishes in development the 

 " trunk line" of the nervous system is in the lower region 

 of the body. In the fishes and those above them there 

 is a spinal cord ; it is in the upper region of the body, and 

 it is protected by a series of bones. Each bone in the 

 series is called a vertebra, and the whole series is called 



First Dorsal F,r 



-Second Dorsal F,,, 



FIG. 266. 

 Skeleton of a Fish. 



the vertebral, or spinal, column (Fig. 266). Therefore 

 fishes and all higher animals are called vertebrates, while 

 the animals below them are called invertebrates. 



The heart of the crayfish we found to consist of one 

 chamber, set in a space for receiving the blood that came 

 back from the body. In the fish we find a heart of two 

 chambers (an auricle, which receives blood from the 

 veins) and a ventricle, which forces blood into the arteries 

 (Fig. 267). The blood of fishes is red, because of the 

 presence of red corpuscles (cf. Fig. 286, 378) . 



The respiration of the fish takes place through gills. In higher 

 fishes, such as the perch, the gills are all placed under one hard cover- 

 ing, back of the head. In fishes not so well developed, such as the dog- 

 fish and shark, there are gill slits, one for each gill. In both kinds of 



