CRAYFISH 105 



already considered in the Earthworm. A further discussion of the 

 process may be left until the Vertebrate is considered. (W. f. 63.) 



2. Vascular System 



The vascular system consists of a number of muscular-walled 

 vessels, the arteries ; a pumping organ, the heart ; a series of 

 blood spaces, the sinuses, distributed through various regions of 

 the body ; and, finally, a circulating medium, the blood, which is 

 essentially similar to that of the Earthworm. 



The single heart of the Crayfish is a contractile, muscular organ 

 which develops as a dilation of a dorsal blood vessel. The latter 

 occupies a similar position to that found in the Earthworm. The 

 heart lies in the cephalothorax, in a median plane, quite close to 

 the dorsal integument. When the exoskeleton is removed from 

 this region it will be seen that the heart lies in a chamber, the peri- 

 cardial sinus. Viewed from above the heart shows a somewhat 

 irregular, pentagonal shape. It is kept in position by six strands 

 of fibrous tissue which originate from the walls of the heart and 

 extend to each side, where they are attached to the wall (peri- 

 cardium) of the pericardial sinus. 



Leading from the heart are the six main arteries, including two 

 pairs, which convey the blood from the heart to various parts of 

 the body. These may now be described. (W. f. 63.) 



Anterior Median Artery. This is an unpaired artery that leaves 

 the heart at the extreme anterior end and continues anteriorly, 

 giving off branches which supply the cardiac portion of the 

 stomach, the esophagus, and portions of the head region. 



Antennary Arteries. A pair of these arises at the anterior end 

 of the heart, one on each side of the median artery. They run 

 anteriorly for a short distance and then ventrally and laterally, 

 one to the right and one to the left. Each of these arteries gives 

 off a branch which runs to the cardiac portion of the stomach, and 

 other branches which supply various organs in the head region, 

 notably the muscles and the antennae with the green glands. 



Hepatic Arteries. A pair of these arises from the anterior end 

 of the heart, just posterior and ventral to the antennary arteries. 

 As indicated by the name, they supply the digestive glands. 



Dorsal Abdominal Artery and Branches. This large vessel 

 leaves the ventral side of the heart at the extreme posterior end. 

 It runs directly posteriorly through the abdomen, close to the 



