CIRCULATORY CHANNELS 153 



muscular and therefore contractile for the greater part of its length, and 

 that it lacks any of the passive connective-tissue structures found so 

 often in systems that have a central pumping station or heart. This is 

 because the muscle fibers perform the function that such connective-tissue 

 cells would be required for, at the same time acting as the heart. 



The Structure of the- Blood- Vessel Walls in the Earthworm, Allolobo- 

 phora. The earthworm has a system of blood channels that are some- 

 what harder to understand than those of Cerebratulus. Owing to the 

 delicacy of many of its structures, several diverging views are held which 

 cannot be fully considered here. 



As occurs in all blood vessels, the walls are formed by layers. The 

 innermost of these layers is much questioned. Those who consider it as 

 an existent formative layer of the vessel acknowledge that it is not every- 

 where present in the blood-channel system, but only in the larger vessels. 

 It is described as composed of cells with flat, small nuclei, and the cell 

 bodies form a very thin and, at parts, incomplete lining of the vessel. 

 The cell bodies are extended in the axis of the vessel, and it is not possible 

 to define the lines of juncture by the silver method. The opponents of 

 this idea assert that these cells are blood cells that are clinging to the 

 arterial walls rather than parts of the structure of the wall. We shall 

 consider it to be an integral part of the vessel in consideration of the 

 important part it plays in the larger vessels and " hearts " where it forms 

 valves. 



Outside of this layer, and found throughout the channel system of 

 which it forms the real blood-retaining boundary, is a homogeneous 

 membrane, the " basement membrane " or intima. This is a clearly 

 defined and denser as well as a thicker structure than the intima found 

 in the blood vessels of Cerebratulus. When the vessel is contracted, this 

 intima is thrown into longitudinal folds. It stains red with Van Gieson's 

 picro-acid fuchsine. 



Outside again of the intima one finds two or three distinct types of 

 structure, which distinguish the several kinds of vessels. We shall 

 examine first the thick walls of one of the several semicircular and tubular 

 " hearts." The inner endothelial layer is very thick here (Fig. 136). 

 It is thrown up at several points into heavy masses which oppose each 

 other in pairs and act as valves, one of which is shown in the figure. 



The intima is very thick and is either striated or folded longitudinally 

 or is covered on the outside by longitudinal muscle fibers. Outside of 

 this layer come the heavy, plain, smooth muscle fibers of the circular layer. 

 They are irregularly angular in section, with a deeply staining central 

 mass (an artifact), and their cell bodies lie outside of them as large, well- 

 developed cells with large, round nuclei that are only seen in a few of the 

 cell bodies on account of the length of these latter. Outside of the muscle- 



