CHAPTER XIII. 



COLOURATION OF INVERTEBRATA (Continued). 



OF the Arthropoda, including the lobsters, crabs, shrimps, etc., 

 little can be said here, as we have not yet been able to study 

 them with anything like completeness. Still, we find the same laws 

 to hold good. The animals are segmented, and we find their system 

 of colouration segmental also. Thus, in the lobsters and crabs there 

 is no dorsal line, but the segments are separately and definitely 

 decorated. The various organs, such as the antennae and eyes, are 

 picked out in colour, as may be beautifully seen in some prawns. 



When we come to the Mollusca, we meet with two distinct types, 

 so far as our subject is concerned ; the naked and the shelled. In 

 the naked molluscs, like the slugs, we have decoration applied region- 

 ally, as is shown to perfection in the Nudibranchs, whose feathery 

 gills are often the seat of some of the most vivid hues in nature. 



The shell-bearing mollusca are proverbial for their beauty, but it 

 is essential to bear in mind that the shell does not bear the same 

 relation to the mollusc that the " shell " of a lobster does to that 

 animal. The lobster's shell is part of its living body ; it is a true 

 exo-skeleton, whereas the shell of a mollusc is a more extraneous 

 structure a house built by the creature. We ought, on our view, 

 to find no more relation between the decoration of a shell and the 

 structure of its occupant, than we do in the decoration of a human 

 dwelling-house to the tenant. 



The shell consists of carbonate of lime, under one or both of the 

 forms known to mineralogists as calcite and aragonite. This 

 mineral matter is secreted by an organ called the mantle, and the 

 edge, or lip, of the mantle is the part applied to this purpose. The 

 edge of the mantle is the builder's hand, which lays the calcareous 



