22 



A BIOLOGY OF CRUSTACEA 



the base of the limb in front. By the time the particles reach the 

 front filtering limbs they are finely divided enough to be collected 

 in the normal filtering mechanism and passed forwards to the 

 mouth by spurts of water from the inter-limb spaces. The Con- 

 chostraca thus chew with their hind limbs and filter with their 

 front limbs. 



The process of taking in large particles for chewing by the hind 

 limbs leads to a danger of blocking the median space. If this 

 happens the offending particle can be removed by means of the 



INTER LIMB SPACE 



MEOIAN SPACE 



NNER\CHAMBERS OP 



f INTER LIMB 

 OUTER.) SPACE 



Fig. 17. Diagrammatic horizontal section through three 

 successive limbs of an anostracan. Note that when the 

 limbs move forwards towards the head each exite will 

 press back against the limb behind and will prevent 

 water from entering the interlimb space via the outer 

 edge, but the exites will allow the exit of water from the 

 interlimb space when the limbs are pressed together. 

 (After Cannon, 1933). 



claws on the end of the abdomen, which can be flexed forwards 

 into the median space and then kicked violently backwards. 



A similar kicking of the abdomen is seen in the Cladocera, which 

 may be thought of as small Conchostraca with only a few limbs. 

 They retain the filtering activities of the conchostracan front limbs 

 but have lost the hind chewing limbs. The actual number of 

 filtering limbs is reduced to four in Sida and only two in Daphnia. 

 Some of the Chydoridae do not filter small particles out of currents, 

 but apply their limbs closely to the surfaces of plants and scrape 

 particles off. Anchistropus feeds in the same manner on the com- 

 mon brown Hydra, and has a specially modified claw on the first 



