BRANCH CHORD AT /I; CLASS PISCES: THE FISHES 265 



scales are intimately associated with a large nerve (the 

 vagifs), and probably serve an important part, not yet 

 clearly understood, in the life of the fish. 



Lift up the flap in front of one of the pectoral fins. 

 This is the opercular flap which covers the gills that lie 

 beneath. Bend this forward and find four gill-arches, 

 each with its double fringe of gills. Note the gill-rakers, 

 short and blunt, on the first gill-arch. Note also on the 

 under side of the flaps turned back, delicate red gill-like 

 structures covered by a membrane. These are t\\z false 

 gills or psendo-branchice, larger in most fishes than in the 

 sunfish. The gills in the fish subserve the same function 

 as the gills of the crayfish, that of purifying the blood 

 by eliminating carbonic-acid gas from it and taking up 

 oxygen from the air mixed with or dissolved in the water. 

 Organs subserving the same purposes in different kinds of 

 animals as, for example, the gills in fish and in crayfish, 

 are called analogous structures. But there is an important 

 morphological difference between the fish's gills and the 

 gills of the crayfish. In the latter animal they are out- 

 growths of the basal segments of the walking-legs ; in the 

 fish they are outgrowths from the alimentary canal. The 

 internal gills of the young toad (tadpole) arise in the same 

 way as those of a fish. Structures which are identical in 

 their origin, like the gills of tadpole and fish, are called 

 Jioinologous structures. 



Make a drawing of the sunfish from a lateral aspect, 

 showing the external parts named. 



Internal structure. TECHNICAL NOTE. Insert one point 

 of the scissors a little to one side of the anus and cut dorsally on the 

 left side of the body to the backbone. Now cut anteriorly from the 

 anus along the ventral wall to where the jaws unite, and cut, also 

 anteriorly, along the dorsal wall until the left side of the body can 

 be removed. Bend the opercular flap backward over the eye and 

 pin the entire fish, uncut side down, to the bottom of the dissecting- 

 pan, covering it with water. 



