THE PERCH 53 



Observe the other structures in the cross section : the skin, with 

 the scales and the lateral line; and the spinal column, with the 

 neural arch on its dorsal side and the haemal arch on its ventral 

 side, the former containing the spinal cord and the latter the caudal 

 artery and vein. 



Exercise 25. Draw the cross section. 



The Skeletal System. The skeleton of the fish is made up of 

 two distinct portions, the exoskeleton and the endoskeleton. The 

 former is of dermal origin and consists of the ctenoid scales and 

 the teeth. The scales are embedded in the dermis, or inner layer 

 of the skin ; covering them on the outside is a thin layer of the 

 dermis and all the epidermis, which, however, are often broken 

 through by the sharp posterior tips of the scales. Of dermal origin 

 are also the membrane bones which take part in the formation of 

 the skull. These are, however, so intimately associated with the 

 other bones of the skull, which belong to the endoskeleton, that 

 they will be studied with them. 



Pull out several scales, including some from the lateral line, and 

 study them with the aid of a lens or microscope. Note the parallel 

 lines of growth. In the scales from the lateral line note the minute 

 canal on the inner side of the scale, and the pore by which it com- 

 municates with the outside. 



Exercise 26. Make a sketch of a scale from the lateral line. 



The endoskeleton forms the bony and cartilaginous framework 

 of the body. It may be divided into ( i ) the axial skeleton, which 

 includes the skull and the vertebral column, with the ribs ; and 

 (2) the appendicular skeleton, which includes the skeleton of the 

 paired and the median fins. 



To prepare the endoskeleton for study remove all the viscera ; 

 immerse the animal in hot water in order to soften the muscles, 

 and then carefully remove them from the skeleton. Separate the 

 bones from one another as little as possible at first, leaving 

 enough of the ligaments to bind them together. Be very careful 

 not to remove any of the bones of the head, some of which are 

 small and easily lost. If any of the bones are removed from the 



