THE SHELL. 275 



sessed by marine animals. The first of these belong to the 

 class Crustacea, consisting of crabs, lobsters, etc. These 

 differ from true molluscs, not only in the composition and 

 structure of their tests, but also in having jointed limbs ; 

 each limb being invested by a portion of the test as by a 

 sheath. The second belong to the class containing the 

 Eclmms, or Sea-urchin, of which genera and species 

 abound in many seas. The testaceous covering of Echini 

 is composed of a number of small pieces, placed edge to 

 edge, composing an outer covering to the fleshy part of the 

 animal, "which is supported within by a pyramidal arrange- 

 ment of bones. This test is fibrous in texture, and guarded 

 on the outside with movable spines, which turn on ball-and- 

 socket joints. The third class of tests to be distinguished 

 from true molluscan shells is that of the Cirnpedes. The 

 shelly pieces belonging to those interesting marine animals 

 resemble true shells in many respects, but bear a difi^erent 

 relation to the other parts of the animals, which, in their 

 true characters, are so nearly"connected with the Cnistacea, 

 that they do not enter into the science of Molluscology. 



A true molluscan shell is composed of one or more pieces ; 

 each piece being formed by a series of layers lying obliquely 

 upon each other, in such a manner that each new layer 



