FEEDING OF MARINE ANIMALS 



199 



rejecting the large ones which accumulate just within 

 the shell until the animal closes its shell quickly when they 

 are shot out, and only allowing the smallest ones to enter the 

 mouth and pass into the stomach. 



The sea squirts feed in a somewhat similar manner, water 

 being drawn in at the one opening and rejected by the other 

 after having been sieved through a delicate lattice work. 

 So fine is this lattice that, in a medium-sized sea squirt, 

 it has been estimated that there are almost 200,000 openings 

 and about double that number of rows of cilia, the beating 

 of which creates the current. Here again the food particles 



Fig. 42. — Oikopleura in " house." E., opening through which animal escapes from 

 house ; F., fine mesh guarding opening through which water is drawn ; H., head end 

 of animal ; O., opening through which water is expelled ; S., sieve in which food is 

 collected ; T., tail of animal. Arrows show direction of water currents in house (x 5). 



are entangled in a sticky substance and carried to the 

 mouth by special tracts of cilia. Near relatives to the 

 sessile sea squirts are the tiny floating Appendicularians, 

 which form part of the plankton. They feed in a most 

 amazing manner. They do not catch the food directly 

 themselves, but form an elaborate gelatinous " house ' 

 much larger than themselves, within which they live. 

 This " house " is really a complicated apparatus for straining 

 sea water and is illustrated in Figure 42, As a result of the 



