24 ABIOLOGYOFCRUSTACEA 



differences from other branchiopods there is a basic resemblance 

 in the fact that all the limbs play a part in the collection of food 

 and that this food is ultimately passed forwards from the hind 

 limbs towards the mouth. 



The beating of the notostracan limbs provides the swimming 

 current as well as the feeding mechanism, and is thus comparable 

 with the anostracan limb beat. The notostracan carapace does not 

 extend around the limbs and interfere with the swimming current 

 as it does in the Conchostraca and Cladocera, which have to use 

 their antennae for swimming. Respiratory exchange is assumed to 

 be through the general surface of the body, but there is some 

 evidence that the epipodites, or bracts (see fig. 16) of the Branchi- 

 opoda are more permeable than the rest of the body, and there may 

 be more exchange here than elsewhere, particularly as the bracts 

 are subjected to a constant flow of water over their surfaces. 



A rather different method of feeding which involves all the 

 thoracic limbs is that found in the barnacles. The thoracic legs 

 take the form of branched cirri, beset with fine setae, and are 

 arranged in such a way that they all curl forwards, with the fine 

 setae forming a net. This net is pushed out of the opening of the 

 mantle and then moved forwards and downwards towards the 

 mouth. During the forward movement the cirri are spread wide 

 apart and then brought closer together as they move downwards. 

 The whole movement resembles a many fingered hand grasping 

 through the water. Small particles in the water near the barnacle 

 are caught and swept towards the mouth. When a barnacle is feed- 

 ing vigorously the thorax moves up and down inside the mantle, 

 and a current of water flows in over the front edge of the shell 

 and comes out as a spurt behind the thorax as it descends. This 

 flow of water through the mantle cavity is obviously an advantage 

 from the respiratory point of view, particularly when eggs are 

 being kept in the mantle cavity prior to hatching. Sometimes a 

 barnacle will pump water through its mantle cavity without 

 extending its cirri into the water; movements of the thorax within 

 the mantle are responsible for such respiratory movements without 

 feeding. The actual rate of movement of the cirri during feeding 

 varies with a number of factors. The age of the barnacle is 

 important: as a rule older barnacles beat their cirri more slowly 

 than voting ones. Variation in rate of beating with temperature can 

 be related to the geographical distribution of allied species. Balanus 

 amphitrite lives in tropical and warm temperate regions, and has 



