GENERAL ANATOMY OF A CRAB. 195 



its anterior edge notice a pair of circular, transparent 

 depressions, each of which is crossed by a transverse slit 

 or opening into the heart. These slit-like openings are 

 the ostia^ by which the blood passes from the cavity of the 

 pericardium into the heart, and the transparent semicircu- 

 lar flaps are valves, which allow the blood to flow into the 

 heart, but prevent it from passing back into the pericar- 

 dium. On the posterior edge of the heart notice two 

 more ostia, similar to those near the anterior border. 



1. At the anterior external angles of the pericardium 

 notice the sinuses by which the blood from the gills enters 

 it, to pass into the heart. 



2. Notice the ophthalmic artery (Fig. 100, d) which runs 

 forwards from the middle line of the anterior border of the 

 heart. 



3. On each side of this artery a hepatic artery (dis- 

 sected out on the left side of Fig. 100) passes through the 

 mandibular muscle to the ovary, the liver and anterior edge 

 of the airapace, and the antennae. 



4. A small abdominal artery, not shown in the figure, 

 runs liackwards from underneath the posterior border of 

 the heart to the al^domen. 



5. Turn the heart over and notice the larffe sternal ar- 

 tery which runs downwards and forwards from the abdomi- 

 nal artery, just as it leaves the heart. 



s. Eemove the ovary and the liver from one side of the 

 body, tracing the course of the hepatic artery, and notice 

 near the anterior edge of the floor of the body cavity, the 

 great external mandibular muscle (Fig. 100, s). 



t. TJie Respiratory Organs. Cut through the roof of 

 the branchial chamber of one side, and raising it up, notice 

 that its upper inner edge is continuous with the skeleton 

 of the flancs, while its lower external edge is continuous 



