CHAPTER II 



FEEDING, LOCOMOTION 

 AND RESPIRATION 



This chapter is really a continuation of the last, because the three 

 activities forming its subject are those which have become diversi- 

 fied and whose requirements govern the shapes assumed by various 

 crustaceans. Of the three processes, feeding has become the most 

 varied, and to a large extent the evolution of the Crustacea has been 

 the evolution of their feeding methods. 



It is now generally accepted that the primitive mode of feeding 

 in the Crustacea is by means of water currents from which small 

 particles are filtered off. The process of gathering small particles may 

 be divided into three stages: the production of feeding currents, 

 separation of particles from the currents, and transport of particles 

 to the mouth. The methods used to perform these processes are best 

 illustrated by considering two examples in some detail, and then 

 discussing the variations found in other groups. The Anostraca have 

 been chosen as one group because their feeding mechanism shows 

 certain characteristics which are present in the feeding mechanisms 

 of other Branchiopoda. The Mysidacea are chosen as the second 

 example because their feeding mechanism can be considered as the 

 primitive tvpe in the Malacostraca. 



In the Anostraca the water currents are produced by the beating 

 of the thoracic limbs. The same movements serve to propel the 

 creature through the water and to gather its food. The limbs do 

 not all move in unison; one limb is usually about one-sixth of a 

 complete beat ahead of the limb in front of it. The beating of the 

 limbs alternately enlarges and reduces the space between two suc- 

 cessive limbs, so sucking water in and blowing it out again. The 

 limbs are arranged so that the water enters the interlimb spaces 

 from the median space which lies in the midline between the two 

 rows of limbs. Water must then flow into the median space to 

 replace that which has entered the interlimb spaces. This current 

 flowing into the median space is the feeding current. Small particles 

 are swept into the median space, and are prevented from leaving 



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