68 ANIMAL LIFE 



unceasingly over their food. Dustman service is the 

 lot of many. Earth and water are kept clean by their 

 unwearied efforts (fig. n). 



In order to gather their food-plants, insects and 

 Crustacea employ a similar series of mouth -part 

 diversely shaped according to the special needs of 

 each class. Their mouth is overhung by a mobile 

 upper lip, and enclosed at the sides by a pair of appen- 

 dages constructed on the same plan as the swimming 

 or walking legs, but modified to form jaws, that 



FIG. II. The Common Sand-Hopper (Gammarus locusta). A similar 

 form is common in fresh water. (After Delia Valle.) x 4. 



work from side to side and chew the food between 

 them. Behind these a lower lip is formed by a pair 

 of limbs that still show that division into inner and 

 outer lobes that betrays their alliance with swimmerets ; 

 and usually there is added to these yet another pair 

 of members of similar bifid nature. These parts of the 

 lower lip were originally sensitive vibrating paddles, 

 and their accustomed movements had to be but 

 slightly altered to fit them for chewing and tasting, 

 instead of paddling. Provided with these, insect and 



