FEEDING, LOCOMOTION AND RESPIRATION 25 



its maximum rate of beating at about 30 C, while B. balanus, a 

 northern species, beats most rapidly at about 20 C. Type of habitat 

 also has its effect, the highest rates of beating of Balanus balanoides 

 has been found among specimens collected from high water level 

 in a sheltered site without currents. This is perhaps what one 

 might expect, because barnacles living in such a situation will only 

 be covered by water for a short period, and they will not have the 

 advantage of food being brought towards them bv currents. Tt is 

 noticeable that barnacles high on the shore do not grow as quickly 

 as those lower down (see p. 67). 



In the remaining groups of Crustacea we find that when a filter- 

 ing method of feeding is used the filter is usually restricted to 

 particular limbs, and that those nearer the mouth are generally 

 of the greatest importance. In a filter-feeding ostracod, such as 

 Asterope, the filter is borne on the maxillule (1st maxilla) and a 

 water current is caused to flow in at the front of the shell and 

 out at the back, by the beating of a flap on the second maxilla. 

 The small particles which collect on the filter are scraped off by 

 setae which project forwards from the second maxilla, and by 

 others which project back from the base of the mandible. These 

 setae pass food to another group of setae which project from the 

 base of the maxillule towards the mouth, and finally the food is 

 pushed into the oesophagus by long processes on the bases of the 

 mandibles. Not all ostracods feed in this way. Another filter-feeder, 

 Cytherella, has a filter on the mandible, and the water current is 

 produced by the beating of a flap on the maxillule as well as the 

 second maxilla. Yet other ostracods are predators and catch cope- 

 pods, or even mysids. 



Among the copepods we find that many of the Calanoida are 

 filter-feeders. The filter is borne on the second maxilla, and trapped 

 food particles are swept towards the mouth by setae on the first 

 maxilla. The food of Calanus finmarchicus consists to a large extent 

 of diatoms, but some Crustacea are also included in the diet, and 

 it is thoueht that these must be caught by individual acts of 

 caoture. rather than bv the filtering mechanism. 



The ability to capture other creatures is probably primitive 

 among the Copepoda. The Centropagidae, which can be regarded 

 as the most primitive family of the Calanoida, feed mainlv on 

 other creatures which they capture, or on large particles of detritus. 



Calanoids swim in two distinct ways. They have a slow gliding 

 movement brought about by rotatory movement of the antennae, 



