154 



HISTOLOGY 



us. b. 



cell bodies is a layer of body-cavity cells which surround all vessels 



that pass through that space. 



In the smaller blood vessels the place of the muscle-cell layer is taken 



by a layer of cells called the " wall cells," which partly encircle the 



vessel and are contractile in the arteries and non-contractile in the veins. 



They are furnished with 

 fibrils which show, accord- 

 ing to Schneider, a clear 

 cross striation and must 

 therefore be considered to 

 be myo-fibrils. 



The Echinoderms pos- 

 sess a Peculiar Blood and 

 a Very Weakly Developed 

 System of Channels to 

 Carry It. The histology 

 of their walls is extremely 

 simple, an endothelium 

 resting upon a loose con- 

 nective tissue. These ves- 

 sels widen into various 

 lacunae whose walls are 

 similar. 



The Mollusca all pos- 

 sess very definite blood 

 channels which are differ- 

 entiated into well-defined 

 regions. An artery (foot 

 artery) of Anodonta shows 

 the following structure in 

 its walls (Fig. 137). An 

 endothelium of irregular, 

 longitudinally extended 

 cells forms the innermost 



layer. Some of these cells are supposed to become detached and form 



the blood and lymph cells. They are absent in some of the largest 



blood vessels. 



This epithelium rests on a basement membrane of uniform thickness, 



which forms a real blood-retaining boundary. This membrane must be 



considered as a surface of the connective-tissue cells which are found 



outside of it. 



Lying in the connective tissue and almost forming a boundary 



between it and the basement membrane are a number of single muscle 



FIG. 136. Longitudinal section of part of the wall of a 

 "heart" in the wire- worm, Allolobophora. int., intima, 

 which is enlarged into a valve v. ; cu., cuticular base of 

 intima; mus.c., circular muscle cells whose fibril 

 bundles show in transection at mus.b. 



