26 VERTEBRATE ZOOLOGY 



in the higher fishes and the higher vertebrates generally those 

 bones called membrane bones, whose place in the skull was not 

 taken by cartilage before the skull became a bony structure, but 

 by membranes or portions of the integument, came into existence 

 in the first place by the fusion of the bony plates of placoid scales. 

 Boil a piece of the skin in a solution of caustic potash and isolate 

 the scales. Mount some of them in water and study them under 

 the microscope. 



Exercise 30. Make a drawing of several scales. 

 Exercise 31. Isolate several teeth and make a drawing of them. 



The endoskeleton of the dogfish is the inner framework of the 

 body. It is composed entirely of cartilage, which in certain places, 

 as in the vertebrae, is hardened by the presence of carbonate of lime. 

 The race of sharks is an extremely ancient one, having been already 

 in existence at the time in the world's history when cartilage and 

 not bone was the principal constituent of the vertebrate skeleton. 

 It has remained in this primitive condition down to the present 

 time, although in all the higher fishes and in other vertebrates 

 cartilage has given place to bone as the principal substance of 

 which the skeleton is formed. Notwithstanding this fact the 

 sharks are at the present time, and probably have never ceased to 

 be, the largest and most powerful fishes in the sea. 



The endoskeleton may be divided into the axial skeleton, which 

 consists of the skull (including the gill arches) and the vertebral 

 column and ribs, and the appendicular skeleton, which consists of 

 the framework of the fins. 



To prepare the endoskeleton for study, take an animal which 

 has not been dissected, open the body cavity without cutting the 

 pectoral or the pelvic girdle, and remove the viscera. Immerse the 

 animal for a short time in hot water in order to soften the tissues, 

 and then thoroughly remove the muscles from the cartilage. A 

 stiff brush is useful in doing this, but great care must be exercised 

 not to injure the more delicate cartilages of the gill arches and 

 head. The skeleton must not be allowed to get dry but must be 

 kept in water or formalin. It is not an easy matter to skeletonize 

 the dogfish and takes time and patience. A fresh animal or one 



