LABOKATOBY WORK: EMBRYO DOGFISH. 



EXTERNAL CHARACTERS. 



How do the fins compare with those of the fish already 

 studied ? Have they a supporting skeleton ? Answer by 

 pulling off a bit of the skin from the pectoral fin. Is the 

 caudal fin homocercal or heterocercal (p. 9) ? 



Place a bit of the skin in a drop of glycerine on a slide, 

 and after an hour examine it under the microscope. No- 

 tice the scales. How do they differ from those of the bony 

 fish ? This kind of scale is called placoid. 



THE HEAD. Are the eyes in the same position as in the 

 other fish ? Where are the nostrils ? Do they communi- 

 cate with the throat? Where is the mouth? Open it 

 and look for teeth. Do you find them in the same places 

 as in the bony fish ? Is there a tongue ? Behind each eye 

 is a hole (spiracle). Does it communicate with the mouth ? 

 On the sides of the "neck" occur the gill-slits. How 

 many are there ? How does this condition compare with 

 that in the bony fish ? 



Draw the fish from the side. 



INTERNAL STRUCTURE. 



Open, as in the bony fish, by cutting from the vent for- 

 ward to the pectoral fins. Make cross-cuts and pin out the 

 walls. Can you see the myotomes (p. 11) ? 



Trace the alimentary canal, In the front part of the 



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