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J. H. ORTON. 



assist in producing these food-streams. The action of the mantle cilia 

 indicates fairly well the course of the food and respiratory streams 

 through the mantle cavity, and the directions in which the cilia on the 

 lower mantle lash are indicated by the arrows in Fig. 2. From the figure 

 it will be seen that the cilia lash from both lateral regions of the shell 

 towards the front middle region. Besides assisting in producing the main 

 current these cilia also reject from the mantle cavity the heavier un- 

 desirable particles brought into the shell in the main stream. On the 



Fig. 2. — View of the ventral valve of Crania, showing the ciliary currents on the mantle 

 ( X rather more than 3). 



This view serves almost equally well for the ciliary currents on the dorsal 

 mantle. 



The dotted arrows indicate the direction in which the mantle cilia lash, and 

 the large continuous arrows the regions where the greatest volume of the ingoing 

 current enters the mantle cavity. 



mantle lining the upper valve the cilia lash on the whole in similar 

 directions to those on the lower one ; there are, however, these differ- 

 ences, the cilia on either side of those in the front middle region lash more 

 and more away from the middle the nearer they are situated to the 

 anterior edge of the mantle. Thus two vortices tend to be formed 

 in the right and left upper portions of the mantle cavity. On 

 the dorsal mantle cilia also lash from between the anterior and 

 posterior occlusor muscles on each side towards the posterior sinus of 

 the mantle cavity which constitutes the space bounded by the posterior 



