GUIDES FOR VERTEBRATE DISSECTION 



EXTERNAL ANATOMY 



Is the body compressed (flattened from side to side), depressed 

 (flattened horizontally), or cylindrical? Is it the same in all 

 parts? Can you distinguish other regions than head, trunk, and 

 tail? 



The Skin. Rub the body in various directions frith the 

 finger; what differences are noted? Take a bit of skin'' from the 

 side of the body and place for 24 hours in glycerine. Then 

 examine under a low power of the microscope, drawing the 

 scales. Each consists of a basal plate from which arises a central 

 spine. Such scales are known as placoid scales. In which 

 direction does the spine point? How are the scales arranged? 

 Isolate other scales by boiling the skin in weak caustic potash. 



Examine the side of the trunk and make oiit a lighter line 

 (lateral line) running from head to tail. 



Fins. Note that the fins may be placed in two categories: 

 median or azygos, and paired. The azygos fins are called the 

 dorsal, on the back, and the caudal, at the end of the tail. Ex- 

 amine the caudal. How many lobes does it present? Into 

 which lobe is the axis of the body continued? Caudal fins of 

 this type are called heterocercal. In some sharks there is an 

 anal fin on the ventral side behind the anus. 



The anterior paired fins are the pectorals, the posterior the 

 pelvic or ventral fins. In the female the pelvic fins are simple; 

 in the male each has a small inner lobe, the clasper, the fin thus 

 being bifid. What is the sex of your specimen? 



The head extends back to the pectorals. Are the nostrils 

 (nares) dorsal, lateral, or ventral in position? Each is provided 

 with a flap. Does this tend to divide the naris? What function 

 can you suggest for this condition? Probe the nostril; does the 

 probe enter mouth or throat? How does the mouth compare 

 in position with the mouth in other vertebrates? Are lips 

 present? Do the scales pass inside of the mouth? At either 

 angle of the mouth is an oro-nasal groove which in some sharks 

 runs to the naris. Does it do so in your specimen? 



Draw a ventral view of the head, natural size, showing mouth, 

 nostrils, etc. 



