GILLS 



329 



tissue containing two blood vessels that lie near opposite sides of the 

 filaments. This core is composed of a very delicate connective tissue, and 

 two bands of longitudinal muscle fibers lie on the edges farthest from 

 the blood vessels. 



The whole structure is covered with a tall, heavy, columnar epithe- 

 lium whose cells show but poor lateral boundaries owing to the intimate 

 way in which they are cemented together. A row of ciliated cells ex- 

 tends for the length of the filament on one side. 



.ctt. 



FIG. 293. Transverse section of a respiratory filament (tentacle) of an annelid worm, Am- 

 phitrite ornata. bl.v., blood vessels, smaller afferent vessel or artery, larger efferent vessel 

 or vein; bl.c., capillaries which conduct the blood from afferent vessel to efferent vessel 

 through the respiratory epithelium; mus.f.,one of the two bands of longitudinal muscle 

 fibers; cil., ciliated cells on one edge of filament. X 400. 



One of the blood vessels is large and the other smaller. This latter 

 is probably an artery through which the blood runs faster, while it runs 

 slower through the wider vein. The blood does not pass through the 

 artery and vein as a simple loop. Instead it passes out of the artery, 

 on its distal edge, into a great number of fine capillaries which pass 

 both ways, through the epithelium, around to the distal edge of the vein, 

 which they enter. The blood is thus brought into extensive and in- 

 timate contact with the outer surface, and is thus aerated and enabled 



