THE FROG 71 



Remove the air bladder and observe the dark red kid- 

 neys extending closely along the dorsal side of the ab- 

 dominal cavity, one on each side of the backbone. Just 

 above the gullet, in the pericardial cavity, the kidneys 

 unite and form a median, large, and dark colored mass, 

 the head kidney. Find this mass. 



Find the heart again and examine it. Of how many 

 parts does it seem to consist? The large vessel lying 

 across the posterior end of the pericardial cavity is the sinus 

 venosus. The irregular, darker portion lying below and 

 slightly in front of the sinus venosus is the auricle. The 

 rounded, bulblike ventricle lies below and at the side of the 

 auricle. The ventricle sends the blood forward into 

 the arterial bulb which lies just in front of the former. 

 From the latter, the aorta carries the blood directly to the 

 gills. 



D. TOPICS FOR FURTHER STUDY. If a young shark is 

 at hand, compare it with the perch as to scales, gill slits, 

 operculum, situation of the mouth, etc. Study the fish- 

 eries of the United States from reports of the Bureau of 

 Fisheries. Study the food of the fresh-water fishes. 



XXI. THE FROG 



Materials. Living and preserved specimens of frogs, tub or 

 aquaria, wire netting, flies, thin board or shingle, towel, tape, 

 tacks, thread, corn-pound microscope, cover glasses, bristles, dis- 

 secting instruments, dissecting pan, pins ; live frog's eggs, shallow 

 pans, pond scum, mud, rocks, etc. 



Directions. (The common leopard frog, the green frog, or the 

 bullfrog will serve equally well for the following outline. If desired, 

 the specimens may be preserved in a five per cent solution of formalin 

 while the dissection is going on.) 



