316 GENERAL ZOOLOGY 



the backbone. From the kidneys two tubes, the ureters, 

 lead, after union, to the urinary bladder. The contents of 

 the urinary bladder are carried to the surface of the body 

 at the urinogenital opening, 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 com- 

 posed largely of phosphate of lime. Running from head to 

 tail through the body is the backbone, or vertebral column, 

 consisting of a number of separate bones called vertebrse 

 (Fig. 160) and continued into a brain case, or cranium, at 

 the anterior end. Bones also form the foundation of the 

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

 Attached to the backbone are a number of ribs, which in- 

 close and protect the organs of the body cavity. A row of 

 small bones (interspinals) 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 System. Four divisions are quite clearly 

 marked in the brain, — the cerebrum (Fig. 160) ; the two 

 large rounded optic lobes ; the medially and dorsally placed 

 cerebellum; and the medulla oblongata, the latter tapering 

 posteriorly into the spinal cord (see Fig. 160, where the 

 medulla is shown though not labeled). Anteriorly the brain 

 is prolonged into the olfactory tracts, 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. No one knows 

 exactly to what extent fishes hear. Some, at least, are 



