57^ THE INVERTEBRATA 



From this the animal separates its food in the form of minute plants 

 and fragments of organic debris. The current can easily be demon- 

 strated by pipetting a suspension of carmine particles in the neigh- 

 bourhood of the siphons, and the details of the process worked out 

 by observing the motion of the coloured granules over the surfaces 

 of the mantle cavity when one of the shells and its mantle lobe have 

 been removed. In this way the direction of the ciliary currents of the 

 ctenidia which transport the food particles can be demonstrated 

 (Fig. 393). On entering the wide mantle cavity the velocity of the 

 inhalant current is checked, and the heavier particles sink down and 

 are taken up by the ciliary currents of the mantle which run towards 

 the posterior region in the neighbourhood of the siphons. The main 

 ingoing current with the smaller particles of carmine is drawn over 

 the surface of the ctenidium and impinges against the individual fila- 

 ments. Their structure and the distribution of the groups of cilia 

 which all perform different functions is shown in the diagram of a 

 transverse section through a ctenidium (Fig. 392 B). That the main 

 current of water is drawn into the mantle cavity at all is the result of 

 the activity of the lateral cilia. When the current which they have 

 drawn to the ctenidium impinges on its surface the large latero-frontal 

 cilia perform their task of deflecting the particles on to the face of the 

 filaments where they come under the influence of the frontal cilia, 

 which produce a constant stream down over the surface of the 

 ctenidium towards its ventral edge. During the passage the particles 

 in the stream become entangled in mucus, and on reaching the edge 

 the string-like masses of food and mucus are directed by other cilia 

 along the edge in the direction of the mouth, travelling partly in the 

 ''food groove''. When the labial palps are reached the collected 

 material may, according to its nature, either be swept straight into 

 the mouth or come under the influence of cilia working along re- 

 jection paths which direct it away from the mouth and toward the 

 outgoing circulation on the mantle (Fig. 393 B). 



This complicated but well co-ordinated ciliary mechanism is 

 nearly always working when the lamellibranch is covered with water, 

 and the amount of water which passes through the mantle cavity of 

 a single mussel is surprisingly large. But it must be remembered that 

 this current also serves the purpose of respiration, though the ex- 

 change of gases takes place through the medium of the mantle rather 

 than the ctenidia. At low tide the animal must close its shell and COg 

 accumulates within the mantle cavity. This chemical change depresses 

 ciliary activity and finally brings the cilia to rest, so that the store of 

 oxygen in the tissues is conserved. When the tide rises, however, 

 the cilia immediately resume activity. 



Though the majority of the lamellibranchs have the power of 



