A LOBSTER OR A CRAYFISH. 57 



Technical Note. As the blood vascular system is to be studied in this 

 dissection it is best to have injected specimens. Injection is accom- 

 plished, in the living specimen, by thrusting the needle of the 

 hypodermic syringe beneath the posterior border of the carapace and 

 pushing it well forward. A yellow starch injection mass will set 

 well and show up clearly. When ready to dissect, very carefully 

 remove the dorsal wall of the carapace and the abdomen piece by 

 piece, separating the hard exoskeleton from the more delicate skin 

 beneath. Clear away the latter and study the organs thus exposed. 



INTERNAL ANATOMY. Note the following organs and their relations 

 to one another: 



a) At the anterior end of the cephalothorax a large membranous sack, 



the stomach. It has an anterior cardiac chamber and a small 

 posterior portion, the pyloric chamber. 



b) A short oesophagus, almost vertical in position, connects the 



mouth and stomach. 



c) On each side of the stomach the torn ends of a mass of muscle fibers. 



These are the mandibular muscles. 



d) Back of the stomach is the shield-shaped heart, from the anterior 



end of which five arteries proceed. Three of these, the ophthal- 

 mic (central) and the two lateral antennary arteries, will be 

 easily seen. The other two, hepatic, are beneath some of the 

 other organs. 



e) The large dorsal abdominal artery with its numerous branches 



arise from the posterior angle of the heart and extends to the 

 telson. To expose this artery it will be necessary to remove the 

 superimposed layer of firm white extensor muscle. This can be 

 accomplished by simply making a shallow cut down the mid-dorsal 

 line and folding the muscle back on either side. 



f) From a point near the origin of the dorsal abdominal artery another 



artery, the sternal, takes a downward course. When it reaches 

 the ventral wall of the body it divides and is continued anteriorly 

 and posteriorly as the ventral longitudinal artery. These 

 blood-vessels can be seen later. 



