BLOOD VESSELS OF LUMBRICUS. 8 1 



uated at the opening of the vessel into the dorsal. In the dorso- 

 intestinal and parietal vessels the flaps are attached one to the 

 anterior and one to the posterior wall of the vessel, and the body 

 of the flap projects into the lumen of the dorsal vessel. In the 

 dorso-typhlosolars the flaps are lateral in position, are situated 

 deeper in the vessels, and do not project so far into the dorsal. 

 It is evident that pressure from the dorsal toward any of these 

 vessels would tend to close the valves. The closing of the pari- 

 etals is further secured by a thickening of the circular muscle 

 layer as in the dorsal ; and in the dorso-intestinals a thin exten- 

 sion of the muscle layer of the dorsal serves the same purpose. 

 Muscle fibers have not been observed in the dorso-typhlosolar 

 vessels. The valves in these vessels allow the blood to flow 

 from them into the dorsal only, and this accords with the results 

 obtained by the earlier experimental investigation. In the case 

 of the decapitated worms the valves in all these vessels near the 

 anterior end must have been forced. 



The hearts are better supplied with valves than are any of the 

 other vessels. In each heart are four pairs of valves (Fig. 3); 

 one situated close to the dorsal vessel, one between the first and 

 second thirds from the dorsal end, one between the second and 

 third thirds, and the fourth in the ventral end of the heart at the 

 opening into the ventral vessel. The three pairs in the body of 

 the heart are like those in the dorsal vessel but are smaller in 

 proportion to the diameter of the heart. They are inclined 

 downward and are large enough to close the heart during its 

 contraction. The presence of these valves might seem unneces- 

 sary in view of the fact that the contraction waves pass along the 

 heart from above downward. However, if from any cause the 

 contraction becomes modified or irregular or if the whole heart 

 contracts at once, the functional importance of these valves is 

 evident. It is a matter of common observation that such irregu- 

 larities in the contractions of both the hearts and the dorsal ves- 

 sel do appear in worms dissected alive under an anaesthetic, and 

 it is probable that such irregular contractions and the influence 

 of movements of the body make necessary the valves in the 

 hearts and the dorsal vessel in the normal worm. The valves in 

 the smaller ventral ends of the hearts fill the lumen and project 



