90 LIFE BY THE SEASHORE. 



Having noticed these points, study the movements of the 

 living worm. The body is divided into three regions an 

 anterior, usually much swollen, region, with lateral tufts of 

 bristles; a median region, with the conspicuous gills and 

 less obvious bristles ; and a tail region, with neither bristles 

 nor gills. As it is thus destitute of definite locomotor 

 organs, our first query must be, How does Arenicola move 1 

 If you watch your specimens closely, you will be struck by 

 a marked and peculiar wave of motion which begins in the 

 gill region, and gradually sweeps forward to the anterior 

 end. This wave produces a very marked distension of the 

 body, and has all the appearance of being due to the passage 

 forward of fluid within the body-cavity. The distension is 

 most marked in the anterior region, and often terminates in 

 the protrusion at the extreme anterior end of a " proboscis," 

 with numerous papilla on its surface, which is obviously 

 homologous with the "introvert" of Nereis. As the wave 

 sweeps forward it will be noticed that the little tufts of 

 bristles are completely withdrawn into the body, which 

 must greatly diminish the resistance to the passage through 

 the sand. As the wave passes any particular spot, it will be 

 further observed that, immediately after its passage, the 

 bristles are protruded to their fullest extent. When the 

 worm is lying on a smooth surface the forward wave is 

 followed by a backward one, during the course of which the 

 animal slips slightly backwards. There can be little doubt, 

 however, that under ordinary conditions the protrusion of 

 the bristles must prevent this, for they will tend to grip the 

 sides of the burrow. The lob-worm thus works its way 

 through the sand as the earthworm does through the earth, 

 and in both cases the bristles are of great importance. The 

 process is a very interesting one, and can be readily watched 

 in a living Arenicola lying on wet sand. 



The lob-worm, indeed, is of interest in several respects, 

 for it seems to stand half way between the active worms 

 like Nereis, and the very passive tube-forming types like 

 Terebella and Serpula. At one time the PolychaBtes were 

 divided into two sets the sedentary tube-builders, and the 

 active free-living forms. This classification is no longer in 

 use, for it is found that many forms not nearly related have 

 independently taken to a sedentary life. Nevertheless, it 



