WORMS. 7 



vessel (Fig. i, dv). This runs along the middle of the 

 back above the intestine, like a very narrow red thread. 

 It becomes apparent only during the pulsations when 

 the tube is gorged with red fluid. This is regarded by 

 most authors as distinct from the white blood which 

 is found circulating in the cavities of the body, but not 

 inclosed in special vessels. The vessels are, however, 

 similar to the circulatory vessels of other animals in 

 position and function, and can be properly spoken of 

 as the circulatory system.* The pupils will observe 

 that the first segment of the body is pointed, forming 

 a sort of upper lip for the mouth which is immediately 

 below. This mouth leads into a pouch, which is some- 

 times turned out, and becomes what is called a pro- 

 boscis, for the purpose of taking food. 



If the worm is turned upon its side, or closely ob- 

 served from above on a white plate, short hairs, or 

 setae, as they are called, will be seen projecting from 

 the surface (Figs, t, 2, s). Place the animal upon 

 moist earth, and watch it closely ; it will then be seen 

 that these setce are used, upon such soft or uneven 

 surfaces, to aid in the act of crawling ; but if the earth- 

 worm be placed upon a pane of glass, or in a smooth 

 dish, it will be found that, in spite of their number, the 

 setae are not suitable for moving the worm upon such 

 hard and smooth surfaces. 



* The white fluid of the visceral cavity is generally consid- 

 ered as the nutritive fluid, and as comparable in its structure to 

 the blood of the Crustacea, which circulates in a circuit com- 

 prising vessels opening into the visceral cavity and the cavity 

 itself. The worms, therefore, have two fluids to perform the 

 two main functions of the blood in place of one combining both 

 these offices, as in other animals. 



