230 MANUAL OF ZOOLOGY. 



again, as in the Chiton, the shell consists of more than two 

 pieces, and is said to be ' multivalve.' Most, however, of the 

 multivalve shells of older writers are in. reality referable to 

 the Cirripedia. 



All the testaceous Mollusca (except the Argonaut), and 

 most of the ' naked ' forms, acquire a rudimentary shell before 

 their liberation from the ovum. In. the latter this rudimentary 

 shell is cast off as the embryo grows, but in the former it be- 

 comes the ' nucleus ' of the adult shell. In the bivalves the 

 embryonic shell, or * nucleus,' is situated at the beak, or 

 'umbo,' of each valve, and is often very unlike the remainder 

 of the shell. 



In composition the shell of the Mollusca consists of carbonate 

 of lime usually having the atomic arrangement of calcite 

 with a small proportion of animal matter. In the Phola- 

 didce, however, the calcareous matter exists in the allotropic 

 condition of arragonite, which is very much harder than cal- 

 cite. As regards their texture, three principal varieties of 

 shells may be distinguished, viz. the ' porcellanous,' the 'na- 

 creous,' and the ' fibrous.' In the nacreous or pearly shells, as 

 seen in * mother-of-pearl,' the shell has a peculiar lustre, due 

 to the minute undulations of the edges of alternate layers of 

 carbonate of lime and membrane. The * fibrous ' shells are 

 composed of successive layers of prismatic cells. The ' porcel- 

 lanous ' shell has a more complicated structure, and is composed 

 of three layers, or strata, each of which is made up of very 

 numerous plates, 'like cards placed on edge.' The direction 

 in which these vertical plates are placed, is sometimes trans- 

 verse in the central layer, and lengthwise in the two others, 

 or longitudinal in the middle, and transverse in the outer and 

 inner strata. 



All living shells have an outer layer of animal matter, which 

 is known as the ' epidermis,' or ' periostracum.' This is some- 

 times of extreme tenuity, but is sometimes very thick, the 

 latter being especially the case with those shells which are 

 found in fresh water. 



In many of the spiral univalves, as the animal grows, it 

 withdraws itself from the upper portion of the shell, often par- 

 titioning off the space thus left vacant. In many instances 

 the portion thus abandoned falls off, and the shell becomes 

 * truncated,' or ' decollated ;' this being the normal condition 

 in fully grown examples of some shells. 



In the great majority of univalves the shell is coiled into a 

 spiral, the direction of which is right-handed, but in some 

 cases the spiral is left-handed, and the shell is said to be 're- 

 versed,' or * sinistral,' The reversed shell may occur as the 



