DOGFISH. 17 



In the prepared slide of the skin, under a low power 

 of the microscope, see the scales, drawing several of them 

 in thier relative positions. Each scale consists of a basal 

 plate bearing an oblique spine (dermal tooth). This type 

 of scale is called placoid. Do the scales overlap? 



INTERNAL STRUCTURE. 



Open the fish by cutting in the mid-ventral line from 

 the vent forward to just behind the pectoral fins, and 

 then make cross-cuts at either end so that the sidewalls 

 of the body may be pinned out in the dissecting-pan. 

 Fill the pan with water so that all parts are covered. 



This lays open the body cavity, or ccelom; notice that it is 

 everywhere lined with a smooth membrane, the peritoneum. 

 Through this membrane see that the muscles of the body- 

 wall are arranged in plates (myotomes) extending from 

 dorsal to ventral surface. 



Trace the alimentary canal. In the front part of 

 the body-cavity is the liver with two large lobes and 

 dorsal to it the J -shaped stomach. The intestine begins 

 at the end of the J and extends back to the vent. In 

 the intestine recognize a large anterior portion and a 

 smaller posterior rectum, with a blind sac, the rectal 

 gland, near the junction of the two. Cut a bit of the 

 wall from the larger part of the intestine with the scissors 

 and notice the spiral valve in its interior. What function 

 can you suggest for it? 



At the bend of the stomach is the triangular spleen; 

 at the junction of stomach and intestine is the pancreas. 

 Do you find any thin membranes (mesenteries) binding the 

 alimentary tract to the dorsal wall of the ccelom? Or 

 any similar membrances (omentd) connecting the various 



