132 



FIRST LESSONS IN ZOOLOGY. 



arches of the spinous processes, the passage being called the 

 " neural canal/' and is thus preserved from injury by the 

 strong arch-work over it. But the brain must be preserved 

 from harm, and the jaws need a firm, bony support. Hence 

 we have a skull formed of numerous more or less movable 

 bones, in the upper and back part of which is the brain- 

 cavity, communicating in front with the orbits or eye-cavity 

 and the nasal cavity. 



In front are the bones of the upper and lower jaws; be- 

 hind are the four opercular bones; and below, at the base of 

 the lower jaw, the tongue-bones, while the pectoral and 



Ps 



FIG. 139. Anatomy of the Gunner, male. L, lateral line; Ht, heart; G. pharynx; 

 Ps, pseud o-branchia; .S'?>, spleen; s, all 1 - bladder; A"/, A'/', kidney; 1>I, bladder; 

 T, testis; A, aorta; B, brain; In, intestine; Li, liver; G 1 , gills. 



ventral fins are more or less connected by muscles and ten- 

 dons to the base of the skull. The bones are small and ex- 

 ceedingly numerous, a number of small bones forming the 

 skull and supporting the fins, so that we may in a single 

 fish count upwards of five hundred separate bones. 



To dissect a percb or cunner the side- wall of the mouth must be re- 

 moved, then the gill-cover; study the arrangement of the gills. Next 

 make an incision along the median ventral line from the level of the 

 pectoral fins to just before the anus, and following the upper edge of 

 the body-cavity upward and forward, cut away the body wall, taking 

 care not to injure the large swimming bladder above, nor the heart 

 in front. Now open the pericardial cavity, which lies immediately 



