2 VERTEBRATE ZOOLOGY 



snout and a slitlike mouth on its ventral side. The hinder end of 

 the body is laterally compressed, and terminates with a slender 

 tail, which is the principal organ of locomotion. 



The entire body, including the fins and the tail, is covered with 

 minute placoid scales. Note in which direction their sharp points 

 project. With the aid of a hand lens examine the scales at different 

 parts of the body; they differ in size at different places, being 

 larger on the dorsal than on the ventral surface. In the neighbor- 

 hood of the mouth they have lost their sharp points. Note that 

 they pass over the lips into the mouth and are continuous with the 

 teeth ; note also that they are arranged in diagonal rows. Along 

 each side of the body will be seen the lateral line, a straight line 

 which extends the length of the body ; it is an organ of special sense. 



Note the color of the animal, and the variation of color at dif- 

 ferent parts of the body. 



The body of the dogfish may be divided into three regions,— 

 the head, the trunk, and the tail. There is no neck. 



The Head and the Trunk. The anterior end of the body is 

 flattened and bluntly pointed. On its lateral surface are the 

 elongated eyes, each with two rudimentary and immovable lids. 

 In the smooth dogfish, in addition to these, a third lid, the 

 nictitating membrane, is present; it is a translucent membrane 

 which can be drawn from the inner corner of the eye across it. 

 Back of the eyes on each side are six prominent openings, all of 

 which communicate with the pharynx ; these are the spiracle, which 

 is just back of the posterior corner of the eye, and the five gill slits, 

 just in front of the anterior fins. The spiracle is homologous to a 

 gill slit ; its function is to act as a means of ingress for the respir- 

 atory water. The gill slits contain the gills, the organs of respi- 

 ration. The respiratory water passes into the cavity of the mouth 

 and pharynx through the mouth and spiracles, and out again 

 through the gill slits, bathing the gills on its way. Notice the two 

 minute endolymphatic pores, which appear as a pair of small de- 

 pressions between the spiracles on the dorsal surface of the head ; 

 they are the external openings of the endolymphatic ducts, which 

 proceed each from the auditory vesicle within the head at the 

 side of the brain. 



