TEGUMENTARY SysTEm.--The skin of the Cephalopods is thin and lubricous, and can be more easily detached from the subjacent muscles than in the inferior Molluscous classes. In the Poulp, Eledone, Argonaut, Cuttle-fish, and Sepiola, its texture is soft and tender, and the whole mantle is semitransparent in some species, as the Octopus hyalinus ; but in the Calamaries and Onychoteuthides it is thicker, harder, and more unyielding ; it is interesting to observe that it is in these latter genera that the epidermoid system is most developed, as is exemplified in the horny denticulations and hooks upon the acetabula.
In the Cuttle-fish the suckers are provided with simple unarmed horny rings. In the Octopods the epidermis is reflected over -the interior of the suckers without being thickened into a horny substance at that part. In the body generally the epidermis is readily de tached by maceration, and forms a thick, white, elastic, semitransparent, external layer.
The colorific stratum of the integument forms, both in its structure and vital phenomena, one of the most curious and interesting parts of the organization of this singular class of animals ; and the nature of which, when thoroughly un derstood, may be expected to elucidate the ,mysterious operations of light in producing and affecting the colours of animals.
This stratum, which is analogous to the rete mucosum, consists of a very lax and fine vascular and nervous cellular tissue, con taining an immense number of small closed vesicles, which vary in relative sizes in different species of Dibranchiata. These vesicles are of a flattened oval or circular form, and contain a fluid in which is suspended a denser colouring matter. The colour is not always the same in all the vesicles, but in general corresponds more or less closely with the tint of the secre tion of the ink-bag. This, for example, is the case in Sepiola, in which all the vesicles con tain material of the same Colour. In Sepia, be sides the vesicles which correspond to the ink in the colour of their contents, there is another series of an ochre colour. In Loligo vulgaris there are three kinds of coloured vesicles, yel, low, rose-red, and brown. In Loligo sagittata
there are four kinds, saffron, rose-red, deep blue, and light blue. In Octopus vulgaris there are also four orders of vesicles, viz. saffron, red, blackish, and blueish. The Argonauta .Argo possesses vesicles of all the colours which have been observed in other Cephalopods, and hence the variety and change of colour which the su.rface of its skin presents when exposed to the light.
These vesicles have no visible communica tion either with the vascular or the nervous systems, or with each other : yet they exhibit, during the life-time of the animal, and long after death, rapid alternating contractions and expansions.* If, when the animal is in a state of repose, and the vesicles are contracted and invisible, the skin be slightly touched, the co loured vesicles show themselves, and in an in stant, or sometimes with a more gradual mo tion the colour will be accumulated like a cloud or a blush upon the irritated surface. If a portion of the skin be removed from the body and immersed in sea-water, the lively eontractions of the vesicles continue ; when viewed in this state under the microscope by means of transmitted light, the edges of the vesicles are seen to be well defined, and to pass in their dilatations and contractions over or under one another. If the separated portion of integument be placed in the dark, and exa mined after a lapse of ten or fifteen minutes, all motion has ceased; but the vesicles, when re-exposed to a moderately strong light, soon, in obedience to that stimulus, recommence their motions. As the vibratile microscopic cilia have been recently traced through the higher classes of the animal kingdom, it is not an un reasonable conjecture that equally inexplicable motions of the colouring parts of the integu ment may also be detected in other classes than that in which we have just described them, and thus a clue may be obtained towards the explanation of the influence of geographical position on the prevailing colours of the animal kingdom.