Branch Chordata 89 



are the gill-cpvers. The gill-arches are the four bony- 

 arches standing side by side within the gill-chambers. 

 Attached to the gill-arches are the red filamentous 

 structures, the gills. Are the gill-chambers in direct 

 communication with the exterior from both sides ; if so, 

 are the gills constantly bathed in water ? As the blood 

 passes through the many filaments of the gills, oppor- 

 tunity is given for it to exchange its carbonic acid for 

 the oxygen of the water. 



INTERNAL ANATOMY 



VI. — Body Cavities. Insert the point of the scissors just 

 in front of the anal opening, and cut through the body 

 wall to the gill-slits. Push the flaps aside, and observe 

 the main body cavity. Notice how the cavity with its 

 organs is crowded forward, leaving the posterior parts 

 for the strong muscles. Notice the shining membra- 

 nous lining, the peritoneum. Near the anterior end of 

 the body cavity you may find a membranous partial 

 partition corresponding to the diaphragm of the higher 

 vertebrates. 



VII. — Blood System. Near the middle of the smaller 

 anterior cavity a dark reddish body, the heart, will be 

 found. There are but two cavities in the heart, — the 

 rather angular, thick-walled ventricle and the dark, irreg- 

 ular auricle. You may be able to find connected with 

 the auricle a dark, thin-walled vessel, the venous sinus, 

 extending across the body cavity. The venous sinus 

 receives the blood from the veins, and passes it on to the 

 auricle. Find and trace as many vessels as you can 

 connected with the heart or venous sinus. 



VIII. — Digestive System. Pass a blunt instrument down 



