MOLLUSC A. 45 



oyster, as formerly supposed, but to both valves. It 

 differs principally in not being split into two valves, 

 and is built up by layers around one centre as apex 

 instead of two beaks. In order to appreciate this, 

 begin at the opening and trace the edge of the layers 

 across and around the tube. (Figs. 30-32, from 

 Morse's First Book in Zoology.) Then proceed, as in 

 the oyster, back to the beak or apex. This represents 

 the young, and has also another marked characteristic. 

 A little, smooth, bag-like shell hes here, and this was 

 the protective covering of the very young animal.* 



The opening of the shell is bordered, and, in a living 

 animal, covered in by a thick, fleshy rim, which is the 

 mantle border ; and this, as in the clam, is closely at- 

 tached to the shell, and is the part which builds the 

 layers just observed (Fig. 25). 



The foot projects out of the opening, and it is easy 

 to see that it is a curiously-formed portion of the 

 mantle itself, since they are continuous. The mantle 

 border merely folds inwards, just inside the rim of the 

 shell, and immediately comes out again to form the foot. 



There is, however, a hole on one side (Fig. 25, 30, 

 op) , the breathing orifice. This, in the living snail, may 

 be seen to open occasionally and admit air. 



A very interesting series of observations may be made 

 upon fresh-water snails, which can be collected on plants 

 and stones along the borders of ponds and quiet waters. 



* In the Mollusca, the shell begins with a disc, which par- 

 tially divides, forming two valves in the young of the Lamelli- 

 branchs, but continues single, and is built out into an egg shape, 

 and then a cone-like tube, in the young of the snails or Gastero- 

 pods. Fig. 31 is made from one in which the egg form is already 

 attained, and can be compared with the apex of Fig. 30, apx. 



