200 THE SEAS 



beating of the tail of the occupant, water is drawn in on the 

 upper side of the house through a pair of openings guarded 

 by a fine network and the extremely fine particles still 

 remaining in suspension are collected by a very complicated 

 sieve within the house. The food is then drawn into the 

 mouth of the animal, again by the aid of cilia, while the 

 strained water passes out. The house soon becomes 

 useless on account of accumulations of particles which 

 effectively clog it, and it is then abandoned by the animal — 

 after perhaps being in use for only a few hours — and a new 

 one in all its intricate detail constructed in half an hour 

 or so ! Other members of the plankton which feed ex- 

 clusively by means of cilia include some of the delicate 

 Pteropods or " sea butterflies." 



Many of the smaller crustaceans have feathery limbs 

 for collecting finely-divided food. The common barnacles 

 (Plate 78) are provided with six pairs of delicate limbs 

 fringed with fine bristles, which are alternatively shot out 

 from the shell and withdrawn, sweeping through the water 

 like a casting net during the latter phase. The process can 

 easily be observed by placing acorn-barnacles collected from 

 the rocks in a basin of sea water and cannot fail to be 

 admired for the grace and precision of the movements. 

 The barnacle literally ' kicks its food into its mouth." 

 Many of the smaller planktonic Crustacea, such as copepods, 

 create by the movement of their swimming legs currents 

 in the water all around them, in which particles are drawn 

 in between the limbs and then forward along the under side 

 of the body, where the water is filtered and the particles so 

 collected pushed into the mouth. Some of them feed 

 exclusively on diatoms, others on smaller copepods, and 

 others again on a mixture of the two. 



The small sea gherkins (such as Cucumaria and Thyone) 

 which live in holes and cracks in the rocks and have a ring 



