200 PHYLUM CHORDATA 



ternal openings. Note the difference in shape of these two nostrils 

 and the valve which overhangs the anterior one. The nasal cap- 

 sules do not open posteriorly into the mouth, but are wholly 

 sensory in function. 



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

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

 protective spine which projects back from each gill cover. Along 

 the hinder and lower border of the gill cover is the branchiostegal 

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

 ostegal rays, which forms a valve guarding the gill opening. Un- 

 derneath the gill cover 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, which appears as a red patch on the 

 inner surface of the gill cover in front of the first gill arch. 



Cut off the left gill cover and probe between the gill 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. 



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 alone 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 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, 



