310 GENERAL ZOOLOGY 



after union, to the urinary bladder (Fig. 153, 26). The con- 

 tents of the urinary bladder are carried to the surface of the 

 body at the urinogenital opening (Fig. 153, 37), just posterior 

 to the anal opening. 



The Skeletal System. So far in our study of the animal 

 kingdom the skeletal, or protecting and supporting, parts have 

 been found chiefly on the outside ; in the fish there is a well- 

 developed internal skeleton formed of bones composed largely 

 of phosphate of lime. Running from head to tail through the 

 body is the back-bone or vertebral column, consisting of a 

 number of separate bones called vertebrae (Fig. 153, 27), and 

 continued into a brain-case or cranium (Fig. 153, 28) at the 

 anterior end. Bones also form the foundation of the upper 

 and lower jaws, and support the gills and tongue. Attached 

 to the back-bone are a number of ribs which inclose and pro- 

 tect the organs of the body-cavity. A row of small bones 

 (interspinals, Fig. 153, 29) supports the unpaired fins. The 

 pectoral and ventral fins are each supported by a framework 

 of bones forming respectively a shoulder-girdle and a hip or 

 pelvic girdle. 



The Nervous and Muscular Systems. Four divisions are quite 

 clearly marked in the brain, the cerebrum (Fig. 153, 32); 

 the two large rounded optic lobes (Fig. 153, 33) ; the medially 

 and dorsally placed cerebellum (Fig. 153, 34) ; and the medulla 

 oblongata, the latter tapering posteriorly into the spinal cord 

 (see Fig. 153, where the medulla is shown though not num- 

 bered). Anteriorly the brain is prolonged into the olfactory 

 tracts (Fig. 153, 30), which communicate with the nostrils ; 

 nerves extend to the different sense-organs and to the various 

 parts of the body. 



The ears of. the perch consist of two closed cavities on 

 opposite sides of the cranium, containing concretions of car- 

 bonate of lime, called otoliths, or ear-stones. There has been 

 some question in the past as to whether fishes could hear, 



