PHYLUM ARTHROPOD A 113 



stomach and note that it forks to supply the eyes. On each side of the ophthalmic 

 artery arises an antennary artery which curves downward over the digestive 

 gland. Trace it and note its branches to the mandibular muscle, the stomach, 

 the antennae, and the antennules. Directly below the origin of the antennary 

 arteries and from the ventral surface of the heart the paired hepatic arteries 

 extend down into the substance of the digestive gland. From the posterior end 

 of the heart a large vessel, the dorsal abdominal artery, extends backward the 

 whole length of the abdomen, forking in the sixth abdominal segment. It gives 

 branches to the intestine which is immediately beneath it and to the extensor 

 muscles of the abdomen which have been removed. Are its branches segmentally 

 arranged? From the posterior end of the heart at the same point as the origin 

 of the dorsal abdominal artery, another large artery, the sternal artery, arises 

 and proceeds directly ventrally. Push the left digestive gland carefully aside 

 to see it. Its further course will be traced later. 



Make an outline of the lobster from the side as large as your drawing page 

 and in this put the heart and its arteries. Other systems will be added to this 

 later. 



Remove the heart from the body, wash it in water, and note its peculiar 

 angled form. It is generally somewhat distorted by the injection. Find the 

 three pairs of openings, or ostia, in the wall of the heart. One pair is on the dorsal 

 surface, one pair on the ventral surface, and one pair is lateral just under the 

 lateral margins (see Hegner, Fig. 101, facing p. 195). 



The circulatory system of the lobster and other arthropods is an open system, 

 that is, the only definite vessels are the arteries, and the circulation is completed 

 through open spaces, or sinuses. In the lobster the blood passes from the arteries 

 into these sinuses and finally collects into one large sinus, the sternal sinus, in 

 the median ventral line. This will be seen later. It then passes into the gills 

 by the afferent vessels, out by the efferent vessels, already seen in the cross- 

 section of a gill, and back to the pericardial sinus by definite channels. When 

 the heart expands the ostia open and blood is sucked into the heart from the 

 pericardial sinus. The blood is colorless. 



The ophthalmic artery, heart, and dorsal abdominal artery together correspond 

 to the dorsal vessel of the earthworm. In the earthworm, it will be recalled, the 

 entire dorsal vessel is contractile, but in the lobster the power of contractility 

 has become limited to one region, which is enlarged and now designated as a 

 heart. There is also in the lobster a vessel to be seen later which corresponds 

 to the ventral vessel of the earthworm. The student should particularly notice 

 that the segmental vessels, connecting these two in each segment of the earth- 

 worm, are for the most part lacking in the lobster. 



c) The reproductive system: Lying under the heart will be found a pair of 

 slender gonads, white in the male, pinkish in the female. They extend forward 

 almost to the anterior end of the stomach and posteriorly beyond the termination 



