608 LKCTURE XXIV. 



sists of numerous cells of a flattened oval or circular form, containing 

 coloured particles suspended in a fluid. The colour is rarely the 

 same in all the cells ; the most constant kind generally corresponds 

 more or less closely with the tint of the inky secretion. In the Sepia 

 there is a second series of vesicles containing a deep yellow or 

 browish pigment : in the Loligo vulgaris there are three kinds of 

 coloured vesicles, yellow, rose- red, and brown : in the Octopus vul- 

 garis there are four kinds of vesicles, red, yellow, blue, and black. 

 In the skin of the Argonauta all the colours which have been ob- 

 served in other Cephalopods are present, and contained in their ap- 

 propriate cells. These cells possess the power of rapid alternate 

 contractions and expansions, by which the pigment can be driven 

 into the deeper parts of the corium or brought into contact with 

 the semitransparent epiderm. If the skin of a living Octopus be 

 touched, the colour will be accumulated, gradually or rapidly, like a 

 cloud or a blush upon the irritated surface. If a portion of the skin 

 be removed from the body and placed in sea-water under the micro- 

 scope, the contractions of the vesicle may be watched for some time : 

 their margins are well defined, and they pass, during their dilatations 

 or contractions, over or under one another. The power which the 

 Cephalopods possess of changing their colour and of harmonising it 

 with that of the surface on which they rest, is at least as striking and 

 extensive as in the Chameleon, in which, it seems, from the latest 

 observations, to be produced by a similar property and arrangement 

 of pigmental cells. The external surface of the branchial membranes 

 in the Argonauta has many pigmental cells ; the internal surface has 

 very few ; it is covered by numerous reticulated lines, which become 

 prominent, and their interspaces deep, when the membranes contract. 

 The internal organised skeleton of the dibranchiate Cephalopod 

 is cartilaginous, as in the Nautilus, but consists of a greater number 

 of pieces, and enters into a larger proportion of the organisation of 

 the animal. The cranial cartilage is no longer limited in its position 

 to the under side of the oisophagus, but completely surrounds that 

 tube, which, together with the inferior salivary ducts and the cephalic 

 branches of the aorta, traverses a narrow canal in its centre. The 

 cartilage above forms the cavity containing the brain, while below it 

 is excavated to lodge the organ of hearing, and at the sides expands 

 into the broad and thick orbital cavities. There is also a thin and 

 long cartilage which supports the eyeball, and reminds one of the 

 ophthalmic peduncle of the Rays and Sharks. A process continued 

 from the anterior part of the cranial cartilage expands into a broad 

 transverse plate, and gives attachment to the muscles of the arms. 

 In Sepia and Loligo there is a thin semilunar cartilaginous plate. 



