LAMELLIBRANCHIATA 50I 



which the gills are welded together to form continuous plates is the 

 distinction between the three main groups of the Lamellibranchiata, 

 the Protobranchiata {Nucula), the Filibranchiata (Mytilus) and the 

 Eulamellibranchiata {Anodonta). But even in the last-named group 

 there are left occasional holes through which water can pass into the 

 interlamellar spaces then into the epibranchial space dorsal to the gills. 



Belonging to the same system are the labial palps, two folds, one in 

 front of the mouth and one behind, which are turned backwards and 

 prolonged on each side of the visceral mass so as to form two pairs 

 of richly ciliated triangular flaps, embracing the anterior end of the 

 ctenidia, and enclosing a groove which leads to the mouth. 



In the anterior part of the mantle cavity the axis of the gill is 

 attached to the side of the animal dorsal to the foot, which here forms 

 a vertical partition dividing the cavity into a right and left half. The 

 mantle cavity continues behind the foot, however, and here the up- 

 turned ends of the inner rows of filaments of both ctenidia are united 

 so that the mantle cavity is now divided by a horizontal partition into 

 an upper or epibranchial cavity and a lower main cavity. The former 

 opens at the dorsal siphon, the latter at the ventral siphon. A con- 

 stant current of water is maintained during activity, entering by the 

 ventral siphon, passing through the gill lamellae, and leaving by the 

 dorsal. From this the animal separates its food in the form of minute 

 animals, plants and fragments of organic debris. The current can 

 easily be demonstrated by pipetting a suspension of carmine particles 

 in the neighbourhood 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. 368). 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 filaments. 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. 367 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 



