MOLLUSC A. St, 



protect both the viscera and breathing organs when 

 fully developed, and, when reduced to a small size, still 

 covering the breathing organs. The shell is present in 

 nearly all instances wherever the mantle itself is found ; 

 and it is invariably found in the young, even of the 

 naked marine Mollusca, Nudibranchs. 



The foot may be said, perhaps, to be more constant 

 than the mantle. 



The general plan of the moUuscan body seems to be, 

 therefore, that of a fleshy bag or sac, containing the 

 viscera, which is normally divided or constricted, so as 

 to form two regions, — the mantle region and the foot 

 region, — though either one or the other of these may 

 be absent. The modification of the body observable in 

 the different groups can all be reduced to different 

 expressions of these two combined in the higher groups, 

 with a tendency to transform the mouth end into a 

 true cephaUc or head region. 



The Lamellibranchs are flattened bags, cleft at either 

 end, with the foot projecting from the centre. 



A cloth model can be made to show this, with the 

 foot in the centre, and the breathing organs, as folds of 

 the mantle, disposed on either side. Fig. 44 shows a 

 section of such a flattened bag, and Fig. 45 a side-view 

 of the same, the position of the ahmentary canal and 

 ganglia and nerve circle being also roughly indicated. 

 The branchiae are reduced also to their simplest form, 

 as two lateral arms bearing two rows of tubes. This 

 can be supposed to be bent together, inclosing the foot 

 and gills ; and then, if to this the shell is added, in the 

 shape of a piece of pasteboard properly drawn, and 

 the ends of the bag represented by dotted lines in 



