56 THE FROG 



Exercise 63. — The Gills and Viscera in Position. 



(b) Fasten the specimen ventral surface up, by pinning through 

 the tip of the head and tail. Remove the skin from the ventral half 

 of the body wall, leaving the opercular opening intact. The coils of 

 the small intestine may now be seen beneath the muscles, and just 

 anterior to them the gill region. Are there any indications of meta- 

 merism? Carefully remove the thin layer covering the gills, and find 

 the fore legs lying against the gills and within the gill chamber, or 

 opercular cavity. Probe through the opercular opening and determine 

 its relation to the gills. Find the heart between the right and left gill 

 areas. Anterior to the gills are conspicuous bands of muscle connected 

 with the jaws. Remove the muscles which cover the coils of the in- 

 testine. How are the opercular cavity and the coelom separated? 



(c) Pin out, beside the tadpole and in the same position, a small 

 fish, preferably a catfish. Examine the gills and the operculum of 

 each side, cutting anteriorly from the V where the two opercula meet. 

 What is the relation of gills and gill slits to the mouth cavity and to 

 the cavity beneath the operculum? Locate the heart by cutting along 

 the mid-line in the angle of the V. Compare these structures in the 

 tadpole and the fish. How many gills and how many gill slits in each? 

 What is the relation of the gill slits to the mouth cavity in the tadpole? 

 Using a tipped bristle, probe through the mouth cavity and out through 

 the gill slits. Recall the earlier stages of the operculum as seen in 

 Exercise 61. How does the operculum of the tadpole differ from that 

 of the fish? Is the developing fore leg on the outside or inside of the 

 body? 



(d) Draw the entire tadpole (X 2) as thus dissected, showing the 

 cut edge of the body wall, the partition between coelom and opercular 

 cavity, and the other structures observed. Show the relation between 

 opercular cavity, gill slits, and mouth cavity by arrows. Spread the 

 gills apart to show the gill slits. 



Exercise 64. — The Coelom and Its Contents. 



(e) Lift up the mass of the intestine at its posterior margin and 

 locate the large intestine against the dorsal wall on the left side of the 

 body. Cut the large intestine, leaving a short stump. Lift the mass of 

 the intestine and find the esophagus where it enters the anterior end 

 of the coelom. Follow the esophagus to the stomach and this to the 

 small intestine where the latter enters the coil. Cut the intestine at 

 this point and remove the coil after cutting its attachment to the dorsal 

 mid-line. Look for the spleen, either on the dorsal surface of the coil as 

 removed or still in the body. Uncoil the intestine and determine its 



