THE PERCH 35 



each side, and the valve which overhangs the anterior one. The 

 nasal capsules do not open posteriorly into the mouth but are 

 wholly sensory in function. 



At the posterior end of the head is the large opercle, or gill 

 cover, and at its hinder margin the gill openings. Note the sharp 

 protective spine which projects back from each opercle. Along 

 the hinder and lower border of the opercle is the branchiostegal 

 membrane, supported by seven parallel bony rays, the branchio- 

 stegal rays, which forms a valve guarding the gill opening. Under- 

 neath the opercle on each side will be seen the four gill arches, 

 which bear the red gills, and the clefts between the arches. Note 

 the rudimentary gill, the pseudobranch, which appears as a red 

 patch on the inner surfaceof the opercle in front of the first gill arch. 



Cut off the left opercle and probe between the gills clefts into 

 the pharynx. Observe carefully the form and position of the gill 

 arches, and the double row of gill filaments on each ; also the gill 

 rakers, the row of spiny projections on the side of each arch which 

 prevent food from passing through the clefts. In the elasmobranch 

 the gill clefts are not covered by an opercle. 



The Trunk and Caudal Region. These two regions pass gradu- 

 ally into each other ; they bear the appendages. At the posterior 

 end of the trunk are the anus, and the genital and urinary pores. 

 The anus is the largest and most anterior of these three openings ; 

 the other two are minute and are situated behind it on a small 

 papilla. Behind this is often a transverse depression. 



The Appendages. Two kinds of appendages are present, the 

 paired fins and the median fins. The latter are the more primitive 

 structures, and are the only fins present in the lowest fishes ; they 

 are simply dorsal and ventral flattened expansions of the body, 

 which are stiffened by bony rays. In the perch two dorsal fins 

 and one ventral or anal fin are present, and one homocercal caudal 

 fin. Note carefully which of these fins have sharp, spiny rays, 

 and in which the ends of the rays are divided and flexible. Observe 

 that the two dorsal fins are nearly continuous with each other. 



The paired fins are also expansions of the body wall, stiffened 

 by bony rays; they are homologous to the appendages of the 

 higher vertebrates. Two pairs are present— an anterior pair, the 



