214 PHYLUM CHORD ATA 



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 ven- 

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

 caudal artery and vein. 



Exercise 19. 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 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. 



Pull out several scales. Note the parallel lines of growth. 



Exercise 20. Make a sketch of a scale. 



The endoskeleton forms the bony and cartilaginous frame- 

 work 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, but leave 

 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 

 skeleton in preparation, they must be carefully preserved. It is 

 of great practical use to have a dried skeleton at hand for com- 

 parison during the dissection. It is not necessary that this skele- 

 ton be thoroughly cleaned and mounted, but any skeleton that 

 has been dried after the muscles have been removed will answer 

 the purpose. 



The Axial Skeleton. The Vertebral Column and the Ribs. The 

 vertebral column consists of a succession of bony vertebrae closely 



