166 VERTEBRATES. 



membrane, which is supported by bony rays called the 

 branchiostegal rays (or simply branchiostegals) . When 

 the operculum is lifted, the four vertical series of red 

 gills may be seen inside. They may be studied more 

 advantageously after dissection. 



Observe a longitudinal lateral line on each side of the 

 body of the fish. 



II. Structure of the Fins. Study the structure of the 

 fins. Each is made up of a double fold of membrane sup- 

 ported (except in the posterior dorsal fin) by bony rays. 

 The rays are wanting in the posterior dorsal fin, and 

 usually fat is deposited between the folds of membrane, 

 whence it is called an adipose fin. The first ray in each 

 of the pectorals and in the anterior dorsal fin is hard and 

 unjointed. It is developed as a stout, sharp-pointed spine, 

 with two serrated edges. It is articulated with the bones 

 of the pectoral arch by a beautiful joint, which secures it 

 great rigidity when erected, and makes it a formidable 

 weapon of defense. The other rays are soft and jointed. 

 Examine them carefully with a lens. Observe that they 

 are branched or split into parallel, jointed strips at their 

 outer ends. Make an enlarged drawing of one of the 

 pectoral fins, and another of the caudal fin. 



The two pairs of paired fins are homologous with the 

 two pairs of limbs of higher animals. The pectoral fins 

 are articulated upon a bony pectoral arch and the ventral 

 fins upon a smaller and more rudimentary pelvic arch, 

 which in the catfish is unconnected with other parts of 

 the skeleton. 



Dissection. Dissect the fish under water, and study 

 its internal structure. If a prepared skeleton be at hand 

 for reference in locating bony parts, it will be of great 

 assistance in the following work. With a pair of stout 

 bone snips or shears, or with a hatchet, cut off the dorsal 



