52 



FIRST LESSONS IN ZOOLOGY 



of the body. In burrowing it thrusts the pharynx into the 

 end of the head, causing it to swell out, and thus push the 

 earth away on all sides, while it also swallows the dirt, 

 which passes through the digestive canal. In this way it 

 may descend from three to eight feet in the soil. 



While earth-worms are in the main beneficial, from their 

 habit of boring in the soil of gardens and ploughed lands, 

 bringing the subsoil to the surface and allowing the air to 

 get to the roots of plants, they occasionally injure young 

 seedling cabbages, lettuce, beets, etc., drawing them during 

 the night into their holes, or uprooting them. 



x i 



k 



FIG. 55. Transverse section through the body of a Nereis, d, dorsal vessel or 

 heart; c'. circular blood-vessel; b, ventral vessel; n, nervous cord or ganglia; 

 /, artery to swimming appendage s"; i, intestine; s, setse or bristles. 



The leeches (Fig. 54) are allied to the earthworm, but 

 are adapted to a life in fresh-water ponds. 



Our commonest sea-worm, sometimes called the " clam- 

 worm/' is Nereis virais. It lives between tide-marks in 

 holes in the mud, and can be readily obtained. The body, 

 after the head, eyes, tentacles, and bristle-bearing feet have 

 been carefully studied, can be opened along the back by a 

 pair of fine scissors, and the dorsal and ventral red blood- 

 vessels with their connecting branches observed, as well as 

 the alimentary canal and the nervous system. 



This worm is very voracious, thrusting out its pharynx 

 and seizing ifcs prey with its two large pharyngeal teeth 



