BLOOD VESSELS OF LUMBRICUS. 83 



show indistinct cell boundaries which produce an appearance of 

 striations running from base to free edge of the valve. In most 

 preparations, especially in longitudinal sections of the dorsal ves- 

 sel, which are often oblique owing to the curves of the vessel, the 

 substance of the valves appears to be sharply delimited from the 

 connective tissue layer. This would indicate that the valve is 

 formed by a thickening of the endothelial layer. It is difficult to 

 disprove this first supposition because the endothelial cells are so 

 broad that one can seldom expect to find an endothelial nucleus 

 on the surface of a valve. However, in some cases in the hearts 

 flattened nuclei similar to those of the endothelial cells are found 

 on the surface of the valves. Cross-sections of the dorsal vessel 

 through the base of the valves show a radial striation running 

 from the valves through the connective tissue and muscle layers. 

 From these facts it appears that the valves are composed of elon- 

 gated cells which run through the connective tissue layer and 

 securely anchor the valves. Since they are covered internally by 

 endothelial cells they must be regarded as belonging to the con- 

 nective tissue layer. Essentially the same structure is presented 

 by all the valves, although those in different positions differ 

 greatly in size and form in relation to the function which they 

 have to perform. The largest valves are those in the dorsal ves- 

 sel. These are thick flaps attached by broad bases to the lateral 

 walls of the vessel. When the vessel is distended the valves are 

 nearly semilunar in form. When the vessel is contracted the 

 valves become greatly compressed against one another and the 

 soft substance of the valve is forced both forward and backward 

 from its point of attachment. When the valve extends far for- 

 ward it overlaps the opening of the parietal vessel and might ap- 

 pear to function to close that vessel. Such a condition seems to 

 have been seen by Benham (loc. cit.). The valves in the dorso- 

 intestinal and parietal vessels are also paired flaps, but owing to 

 the small size of the vessels the flaps are small at their bases and 

 are longer than they are broad. Often these valves' have a 

 balloon form as they project into the dorsal vessel. The valves 

 in the dorso-typhlosolar vessels are situated somewhat farther 

 within the vessels and are more nearly simple semicircular flaps. 

 The valves at the ventral ends of the hearts are relatively large 



