THE DOGFISH 3 



Examine the eyes. Could the fish see any object with both 

 eyes at the same time? Are eyelids present? Make out in the 

 eye an outer ring, the iris, and in its centre an opening, the pupil. 

 (In preserved specimens the pupil is light-colored owing to the 

 opacity of the lens.) On each side of the head, a little above 

 and behind the eye, is an opening, the spiracle, while between 

 the two spiracles, and nearly in the median line of the top of 

 the head, are two small openings of the endolymph-ducts, con- 

 nected with the internal ears. 



Examine the skin of the head carefully and note the minute 

 'mucous pores,' mapping the patches you find in an outline of 

 the head. These pores are connected with the lateral-line organs 

 and with small bodies (ampullae of Savi and Lorenzini) in the 

 skin. These are probably sensory in function. 



On the sides of the 'neck' just anterior to the pectoral fins 

 are openings, the gill-slits or branchial clefts. How many on 

 each side? Do you find the vent? What relations does it bear 

 to the ventral fins? 



Make a drawing of the dogfish from the side, six or eight 

 inches long, naming the parts made out. 



THE SKELETON 



The skeleton of the dogfish is readily prepared for study by taking 

 fresh or pickled * material and placing it for a few minutes (5-10) in 

 hot water. This softens flesh, skin, and ligaments so that they are readily 

 removed by the hand. When the skeleton is roughly blocked out the 

 head, visceral arches, and the fins and their girdles should be removed 

 and cleaned separately from the trunk. Of the axial skeleton it is only 

 necessary to keep a short piece of the trunk and tail. 



In cleaning the visceral arches and fins great care must be taken not 

 to separate the jaws from the head, or the branchial rays and extrabran- 

 chials from their arches. In spite of the utmost care of the ordinary 

 student some of these will be lost, but usually enough will remain in 

 position to demonstrate the structures. Similarly, great care must be 

 exercised in cleaning the skeleton of the fins. 



The prepared skeleton may be kept in weak formalin or in alcohol. 

 In the latter case it should be removed to water shortly before study, 

 so that parts contracted may regain their original condition. 



* It is very difficult to prepare skeletons from specimens put up in formalin 

 or chromic mixtures, as these 'tan' the connective tissues so that they no 

 longer are softened by heat. 



