272 THE CEPHALOPODA. § 284. 
delicate elastic membrane; when contracted, their form is round, but it. 
becomes dentate on dilatation. The pigment granules, which they enclose, 
are always of the same color in each cell, and produce the red, the yellow- 
ish-brown, the blue, or violet spots, whose extent and shade vary, accord~ 
ing as the cells are contracted or dilated.” Usually, adjacent cells have 
very different colors, and to their alternate contractions and dilatations in 
roups, are due those magnificent chromatic changes which have long made 
celebrated the skin of the Cephalopoda. ‘ 
These contractions, and consequently these chromatic changes, are under 
the influence of the nervous system. This is the reason of their decrease 
or disappearance, or their reappearance and increased brilliancy, in certain 
places, when the neighboring or even the distant skin is irritated. More- 
over, the fibres of the corium preserve their contractility after having been 
detached, so that the chromatic changes may be observed on portions of 
the skin that have been removed. 
§ 234, 
Behind the neck of the Cephalopoda, the skin forms a large sac-like: 
mantle, which completely envelopes the trunk, but is adherent only upon 
the back. Its anterior border is free, and can embrace, like a sphincter, the 
neck and posterior part of the head. Under the throat, the skin is pro- 
longed in the form of a funnel, the free apex of which extends in front, 
while the broad base communicates with the cavity of the mantle, and is: 
2 The movements of the chromatic cells are not 
directly due to the cell-membrane, but to the con- 
tractile fibres of the dermis which are united in 
them, and which, upon contraction, pull at their 
point of insertion, thus producing the ragged as- 
pect of these cells when expanded. They return to 
their round form when the fibres are relaxed, from 
the elasticity of the cell-walls (see Kalliker, Ent- 
wick. d. Cephalopoden, p. 71, and Harless, loc. cit.). 
When these cells are dilated, the pigment granules 
are often removed from the centre to the periphery 
of the cell, thus forming a central, colorless trans- 
parent spot, which has been regarded by Wagner 
(loc. cit.) as the nucleus of the cell. 
8 These so highly characteristic chromatic cells 
of the Cephalopoda, are found also in the skin of 
Hectocotylus, and are, therefore, one of the data 
for determining the nature of these animals, which 
have hitherto been regarded as trematode parasites. 
Delle Chiaje and Costa (loc. cit.) have repre- 
sented these cells in a colored figure of Hecto- 
cotylus argonautae. I have, also, distinctly seen 
them with individuals of Hectocotylus tremoc- 
topodis preserved in alcohol. : 
As Grube (Aktinien, Echinodermen und Wiir- 
mer des Adriat. und Mittel-Meeres, p. 49, fig. 2) 
has observed these same chromatic cells in the skin 
, 
ment, deposited, as he has so well described, in the 
chromatophuric contractile sacs. The splendid 
changeable colors of the surface appeared to be 
due, not to the pigment spots alone, hut to the in- 
tervening tissue ; and the surface color over the 
pigment spots is subject to the same variations. 
Thus, a bistre-brown spot will sometimes appear 
blue, then green, &c. These facts may be tested 
dy placing a small portion of the skin on a plate of 
of agenus of parasites, which he has called Poly- 
porus chamaeleon, it is certain that this animal, 
found on the branchiae of a marine fish, is only a 
torn off arm of one of the Loligina. The presence 
of these cells in the skin of Nautilus seems proved,, 
for Rumph (Amboinische Raritaten-Kammer von 
Schnecken und Muscheln, p. 7) expressly declares: 
of this animal which he saw living, that “its upper 
portion is reddish or bright brown with some black 
spots, which, as with the cuttle-fish, become faded.’” 
The fragment of the Mollusk, which Quoy and 
Gaimard found at the Celebes islands, and which 
they thought to belong to Nautilus pompilius 
(Ann. d. Sc. Nat. XX. 1830, p. 470, Pl. XIV. 
A. or Isis, 1834, p. 1146, Taf. XV. A. B.) 
deserves our attention in various ways. If it 
really belonged to a Cephalopod, it should have the 
chromatic cells, a point which may yet, perhaps, be 
determined from the preserved specimen at Paris. 
In the colored figure which these naturalists have 
given of it, the skin is dotted‘ with red,— a presump- 
tion in favor of the existence of these cells. 
But, indeed, is it not possible that this animal, 
from its resemblance to the Hectocotyli, is not a 
mutilated one, but the male of Nautilus pompilius, 
abortive as to its form and size ? 
glass, and introducing a little water under it, the 
evaporation of which, by changing the surface con- 
ditions, generally produces a variety of colors. 
The chromatic appearances of these animals ap- 
peared to me, therefore, as due full as much to sur- 
face phenomena as to pigment, and I have failed 
to detect different layers of pigment as described by 
Owen; see Burnett Proceed. Bost. Soc. Nat. Hist. 
IV. p. 262. — Ep. . 
