134 ARTHROPODA. 



5. Remove the thoracic viscera as before, follow the circum- 

 esophageal connectives forward and identify the cerebral ganglia 

 in order not to destroy them. 



6. Follow one antennary artery to the green gland, antennary 

 muscle, eye muscle, etc. 



7. Follow the distribution of the ophthalmic artery. 



8. Remove the intestine and muscles of the abdomen, and find 

 and trace forward the ventral nerve chain. Notice the position 

 of the ganglia and the nerves that leave them and the connec- 

 tives. In the thorax the ventral nerve chain passes beneath a 

 system of chitinous plates (the endo-phragmal skeleton) and lies 

 in a cavity, the ventral blood sinus. Note the enlarged sub- 

 esophageal ganglion, the cross commissure just back of the esoph- 

 agus, the nerves to the mouth appendages, nerves from the 

 cerebral ganglia, and nerves from the other ganglia. What indi- 

 cation is there that the sub-esophageal ganglia represent more 

 than a single pair? 



Sketch the nervous system. 



9. The sternal artery passes through the ventral nerve chain 

 and then extends backward and forward as the ventral longitu- 

 dinal artery. Remove the nervous system and follow this ar- 

 tery. 



Draw a diagrammatic cross-section through the thorax, putting 

 in one drawing the circulation from the heart through the sternal 

 artery to the limbs and back through the gills to the heart. 



CALLINECTES HASTATUS. (Blue Crab.) 



Crabs may be found in shallow water along shore, where they 

 may be easily observed on quiet days. In what direction does 

 the animal normally move? How are the legs used? What is 

 the attitude of defense? Determine how the blue crab swims. 

 What do crabs apparently use for food? Do they conceal them- 

 selves, are they protectively colored, or do they depend entirely 

 upon their weapons for defense? 



In studying the anatomy of the crab, constant comparisons 

 should be made with the lobster. 



