20 LESSON II. 



Teacher. Yes, and this action is performed 

 either by the serpentine movements of the foot 

 and body, or by the movement of expanded por- 

 tions of the skin or tentacula, which forces them 

 onwards in the same manner as oars propel a 

 boat. 



Child. Have all the mollusca that live in the 

 water either tentacula or expansions of the skin ? 



Teacher. No, some have no such elongated 

 parts. I think you might have discovered this 

 yourselves by observing one of the animals be- 

 fore you. 



Child. Oh yes, the oyster. Such mollusca 

 cannot move them. 



Teacher. That is not true of all such mol- 

 lusca; some are immoveably attached to the 

 spot where they first received life ; but others 

 have the power of leaping or shifting their posi- 

 tion with a sudden jerk, produced by rapidly 

 shutting the two pieces of their shells ; others 

 again transport themselves from one spot to 

 another by the force with which they draw in and 

 eject the fluid in which they live. Many species 

 are furnished with a kind of bladder, by inflating 

 or contracting which they can rise or sink in the 

 water as circumstances require. Enumerate to 

 me the various means of locomotion possessed 

 by the mollusca. 



Child. Many creep by means of a fleshy 

 elongation, which is in some a foot, in others a leg. 

 In the water they swim, making their way either 

 by the serpentine movement of their bodies, or 

 by the use of tentacula. Some can rise and sink 



