iv] MODES OF NUTRITION 89 



latter is closed, but because of the presence of 

 mandibular valves this water is forced out through 

 the gill-apertures and is compelled to pass through a 

 sieve formed by delicate bones on the hinder margins 

 of the gills the gill-rakers and the particles of food 

 are thus intercepted. In each case, that of the whale 

 and the pelagic fish, the food is scraped off by means 

 of the tongue and is then swallowed. Sedentary 

 shell-fish like the cockle or mussel cause a current 

 of water to flow through the shell cavity by means 

 of ciliary action. This water passes through a fine 

 filter formed by the gills and the sediment and 

 plankton contained in it are thus intercepted and are 

 carried into the mouth by means of another system 

 of cilia. Sedentary tunicates have a similar manner 

 of feeding. Sponges act in the same way, by causing 

 a current of water to flow in through the pores and 

 then out again through the osculum. In traversing 

 the system of canals which permeates the body of the 

 sponge the food substance is removed from the water 

 and is taken up directly by the cells lining these 

 cavities or canals. Barnacles open their shells at 

 regular intervals and protrude a bunch of appendages 

 known as the ' glass hand ' : these are finely fringed 

 by hairs so that when they open out they form an 

 apparatus like the ' cast-net 7 of a fisherman, and 

 this, as it sweeps through the water, captures any 

 small organisms contained therein. When the hand 



