CHAPTER XXV 



SHELL-BEARERS 



IN this chapter we propose to deal briefly with certain 

 peculiarities of shell-bearing animals. To make our 

 position clearer, before we proceed on our way, let us 

 mention that our subjects fall naturally into four classes 

 univalves, of which the snail is a type ; bivalves, or 

 molluscs, with two shells, as in the mussel ; animals with 

 an internal shell, such as we find in the cuttle-fish, and 

 those without a shell, the sea-slugs. Carrying this brief 

 definition in our minds, let us learn something of the ways 

 of these animals which carry their homes about with 

 them wherever they go. Many of the most interesting 

 habits of the mollusca can only be fully understood when 

 we are conversant with their structure ; but anatomy is 

 outside our province, so we must confine our attention 

 to facts which require no such knowledge for their 

 comprehension. 



Many of the shell-bearing molluscs typify, in the popular 

 mind, a high degree of inactivity, but as a matter of fact 

 their powers of locomotion are not nearly so limited as is 

 generally supposed. If we examine a cockle or a horse- 

 mussel, we shall notice a wedge-shaped organ which, to 

 the touch, is much more rigid than any other part of the 

 animal. This wedge-shaped structure is the mollusc's 

 foot ; a queer-looking foot we must admit, nevertheless it 

 serves its purpose admirably. When a pond-mussel 

 desires to change its position, it extends its foot to 

 the full, buries it in the mud at the bottom of the 

 pond in which it is living, attaches itself to the mud 

 particles and then, by gradually contracting, drags the 

 shell along. 



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