COMPARATIVE ANATOMY 5 



have a definite arrangement? By pressing on the skin a jelly- 

 like substance can be forced out. 



On the top of the head, between the spiracles, are the open- 

 ings of two small endolymph ducts, which connect with the ears. 

 These will be mentioned again in the study of the skull and the 

 sense organs. 



The body is covered with small placoid scales, which can best 

 be located by running a finger over the skin from the tail toward 

 the head. Examine a demonstration of them under the micro- 

 scope. They are of special interest because of their homology 

 with vertebrate teeth. 



Garpike — Class Pisces 



The body is covered with ganoid scales, a peculiar type of 

 scale found also on many fossil fishes. In fact, the ganoid fishes 

 are related to the group of fossil fishes from which the land 

 vertebrates are thought to have arisen. What fins are present.'' 

 Is the mouth ventral in position as in the shark .^ A gill-cover, 

 the operculum, covers the gills. A spiracle is absent. 



Bony Fishes — Class Pisces 



Bony fishes of several different types should be compared 

 with the garpike. Observe particularly the absence of the 

 heavily armored condition, the position of the pelvic fins, and 

 the type of scale. The ctenoid scales are bony plates derived 

 from the dermis of the skin and are set in pockets. Examine a 

 scale under the microscope. Are the small teeth on the proximal 

 or on the distal edge } The concentric rings are lines of growth. 

 With training the age of a fish may be told, with a fair degree of 

 accuracy, by the condition of the lines of growth. Are the pelvic 

 fins at the posterior end of the body in all fishes } The teleosts 

 are of more recent evolutionary origin than are the other groups 

 of fishes. 



Necturus— Class Amphibia 



Although aquatic this amphibian possesses many char- 

 acteristics of land vertebrates. The skin is scaleless, soft, and 

 slimy, and contains many glands. If you are not already 



