CLASS PISCES 437 



The visceral skeleton, which is represented in Petromyzon by 

 the branchial basket (Fig. 354, 10), is, in the perch, composed of 

 seven arches more or less modified. The first or mandibular 

 arch, forms the jaws. The upper jaw consists principally of two 

 .pairs of bones, the premaxillcB (Fig. 372, 2) and the maxilla (o). 

 The premaxillae bear teeth. The lower jaw or mandible (u) also 

 bears teeth. The second or hyoid arch is modified as a support 

 for the gill-covers. Arches three to seven support the gills and 

 are known as gill-arches. The first four of these bear spine-like 

 ossifications, the gill-rakers, which act as a sieve to intercept 

 solid particles, and keep them away from the gills. 



The appendicular skeleton is represented in the perch by a pec- 

 toral girdle only (Fig. 372, Sch). This consists of a number of 

 bones which lie just behind the head on either side and furnish 

 a firm foundation for the attachment of the muscles that move 

 the pectoral fins. The fin-rays of the pectoral fin articulate 

 with the girdle by means of four rod-like bones, the pterygiophores 

 or radials, and a number of small cartilages. There is no pelvic 

 girdle. The ventral fins articulate with a flat bone, the 

 basepterygium (Fig. 372, Be], which is probably formed by the 

 fusion of interspinal bones (pterygiophores). 



The Muscular System. The principal muscles are those 

 used in locomotion, in respiration, and in obtaining food. The 

 movements of the body employed in swimming are produced 

 by four longitudinal bands of muscles, one heavy band on either 

 side along the back and a thinner band on either side of both 

 trunk and tail. These are arranged in zigzag myotomes. 

 Weaker muscles move the gill-arches, operculum, hyoid, and 

 jaws. 



The Digestive System. The aquatic insects, mollusks, and 

 small fishes that constitute a large part of the food of the perch 

 are captured by the jaws and held by the many conical teeth. 

 Teeth are borne on the mandibles and premaxillae, and on the 

 roof of the mouth. They are not used to masticate the food. 

 A rudimentary tongue projects from the floor of the mouth 



