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NATURAL HISTORY. 



in 



111:1 



Fig. 73. — Diagrammatic section of a 

 mollusc: b, gill; /, fool ; g, genital 



organ : I, liver; m, mantle: //, nerve: 

 r, Kidney; s, stomach; v, heart. 



which it envelops the rest of the body is known as the 'mantle,' while the 

 lower mu face of the body is produced into a 'foot/ which greatly varies 

 shape and size. The cavity between the body and the mantle is the 

 antle cavity,' and in this are usually the organs of respiration, and into 



it open the various reproductive and excretory 

 organs. Figure 73 illustrates a transverse sec- 

 tion of a typical mollusc, in which the features 

 mentioned as well as others are represented. 



As may be inferred by what has already 

 been said, shells are very characteristic of mol- 

 luscs. A shell is not to be regarded as the 

 ' home ' of the animal ; it is an integral part 

 of it ; and, except in the case of the paper- 

 sailor, no mollusc is able to leave its shell behind. The shell subserves 

 two purposes : it acts as a rigid skeleton, thus giving the muscles greater 

 efficiency, while at the same time it is protective. The molluscs are not 

 quick in their actions ; indeed, " slow as a snail " is proverbial ; but enclosed 

 in its calcareous armor, an oyster or a snail is as surely protected against 

 external foes as if it were able to dart away like a fish, at the first sign of 

 approaching danger. Still the protection thus afforded is not perfect, for 

 many of the fishes swallow the molluscs, shell and all, 

 while, as we have already seen, the starfish has its own 

 way of obtaining the meat enclosed in stony walls. Be- 

 sides, there are some of the molluscs, as we shall soon see, 

 which subsist solely upon their fellows, and are provided 

 with instruments which perforate the hardest shell. 



In some molluscs the whole outer surface of the mantle 

 has the power of secreting shell, in others the secreting 

 -urface is more restricted, and a small number have no 

 shell at all. The shell consists of a peculiar animal sub- 

 stance with which is combined a large, though varying 

 proportion of carbonate of lime. The more lime present, 

 the harder the shell ; the less, the more elastic it is. The 

 shell is not all formed at once, but during the life of the 

 -';;;";; animal constantly increases in thickness and in extent. 

 The secreting surface deposits layer after layer on the 

 inside of the previously formed shell, thus increasing its 

 thickness; but, at the same time, the mantle is contin- 

 ually growing and so each layer is larger than its pred- 

 Mwquently extends beyond it. This results in a series 

 of parallel lines on the external surface — the lines of growth. In the 



Fig. :i 

 mat 



tlir successive lay- 

 each larger 



than its predeces- 

 uv depo 



