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 antennæ 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 regionally, 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 stones of the edifice. The shell is built up from the edge, and the action is not continuous but seasonal, hence arise the markings known as lines of growth. In some cases the mantle is expanded at times into wing-like processes, which are turned back over the shell, and deposit additional layers, thus thickening the shell.

In all the forms of life hitherto considered the colouring matter is deposited, or formed, in the substance of the organ, or epidermal covering, but in the mollusca this is not the case. The colouring matter is entirely upon the surface, and is, as it were, stencilled on to the colourless shell. This is precisely analogous to the colouring of the shells of birds' eggs. They, too, are calcareous envelopes, and the colouring matter is applied to the outside, as anyone can see by rubbing a coloured egg. In some eggs several layers of colouring matter are superimposed.

In no case does the external decoration of molluscan shells follow the structure lines of the animal, but it does follow the shape of the mantle. The secreting edge may be smooth, as in _Mactra_, regularly puckered, as in most _Pectens_, puckered at certain points, as in _Trigonia_, or thrown into long folds, as in _Spondylus_. In each of these cases the shell naturally takes the form of the mantle. It is smooth in _Mactra_, regularly ribbed in _Pecten_, tubercled in _Trigonia_, and spined in _Spondylus_. Where the inside of the shell is coloured as in some Pectens, regional decoration at once appears and the paleal lines, and muscular impressions are bounded or mapped out with colour.

It is a significant fact that smooth bivalves are not so ornate as rugose ones, and that the ridges, spines, and tubercles of the latter are the seats of the most prominent colour.

Similar remarks apply to univalve shells, which are wound on an imaginary vertical axis. They may be smooth, as in _Conus_ and _Oliva_, rugose, as in _Cerithium_, or spined, as in _Murex_. The structure of these shells being more complex than that of bivalves, we find, as a rule, they are more lavishly ornamented, and the prominent parts of the shell, and especially the borders, are the seat of strongest colour. In some cases, as in adult Cowries (_Cypræa_), the mantle is reflexed so as to meet along the median line, where we see the darkest colour.

The rule amongst spiral shells is to possess spiral and marginal decoration, and this is what we should expect. The Nautilus repeats in the red-brown markings of its shell, the shape of the septa which divide the chambers, though, as is often the case, they are generally more numerous than the septa.

The naked Cephalopoda, or cuttle-fishes, often possess a distinct dorsal stripe, and when our views were first brought before the Zoological Society, this fact was cited as an objection. To us it seems one of the strongest of favourable cases, for these animals possess a sort of backbone--the well-known cuttle-bone--and hence they have a dorsal line.

Some shells, as _Margarita catenata_, have a chain-pattern, and in this case the action of the pigment cells takes place at regular and short intervals. Others, as _Mactra stultorum_, the stencilling forms a series of lines and spots, generally enlarging into rays.

The whole subject of the decoration of shells deserves much more time than we have been able to give to it as yet.