226 THE BIOLOGY OF MARINE ANIMALS 



distributed and often abundant. Calanus feeds upon the microplankton ; 

 its food includes diatoms, flagellates and other unicellular algae, protozoa 

 and small crustaceans. The filtering apparatus is very efficient, straining 

 off minute flagellates from the nannoplankton down to a few micra in size. 



The actual food intake varies somewhat with the habits of the animal. 

 The majority of the Calanus at the surface are feeding at all hours of the 

 day. At deeper levels the proportion of individuals containing food is 

 distinctly less, in correlation with decrease in density of phytoplankton. 

 There is some evidence for a diurnal feeding rhythm with greater activity 

 at night, which may be significant in terms of daily movements to and from 

 the surface. When the animals execute vertical diurnal migrations, feeding 

 takes place mostly at the surface and during the hours of darkness (68, 69). 



Ostracods. There is much variety in the feeding habits of different ostra- 

 cods, some like Asterope and Cytherella being purely filter feeders, others 

 like Cypridina feeding on detritus and large food masses. While feeding, 

 Asterope remains buried in the mud and abstracts minute food particles 

 from the feeding-current which it passes through its burrow. In feeding, a 

 current of water is drawn through the valves of the shell by the vibratory 

 activity of the maxillae, while the first trunk limbs bear valves which allow 

 the passage of water in an antero-posterior direction only. Food particles 

 are caught in a setous filter on the maxillule, are combed off by the maxilla 

 and are transferred to the mouth by long setae on the maxillules. The 

 mechanisms of filter-feeding are essentially the same in other filter-feeding 

 ostracods, although the various processes may be carried out by different 

 limbs (12). 



Cirripedes feed largely on small marine Crustacea which they garner by 

 means of casting movements of cirri. These are really the thoracic legs 

 which terminate in a pair of rami armed with long hairs. The food collected 

 by the cirri is deposited on the mouth parts, where it is ground up by the 

 mandibles and worked into small masses to be swallowed. 



Amphipods. Several benthic amphipods are particle feeders. Ampelisca 

 lives in tubes or pockets of sand grains and mucus. When feeding, the 

 pleopods are kept in constant motion, drawing in water over the head and 

 mouth parts, and driving it outwards over the telson. Food particles brought 

 in by this current are seized by the gnathopods and mouth parts ; setae on 

 these structures probably serve for straining and selecting minute particles. 



A rather different method of feeding is used by Haustorius. This animal 

 is found on sandy beaches, where it burrows into the sand. It possesses a 

 filter-mechanism by which it feeds on small food particles suspended in 

 the water in the sand. The maxilla acts as a pump, producing an anteriorly 

 directed current, and also performs the function of a sieve plate, filtering 

 off food particles. These are removed by the maxillipeds and passed on to 

 the mouth parts (20). 



Mysids such as Hemimysis and Praunus have two methods of feeding, 

 one for dealing with large food masses and another for filtering off sus- 

 pended particles. We are here concerned with the latter process. Water 



