262 
contained in the ink-bag. The pigment cells are black and 
brown in the Cuttle-Fish (Sepia) ; brown, red, and yellow in 
the Squid (Loligo) ; and in the Argonaut there are said to be 
blue cells as well. According to Dr. Carpenter, there are 
very commonly different layers of these pigment cells, their 
contents having different hues in each layer ; and thus a great 
variet}^ of coloration may be given, by the alteration in the 
form of the cells of which one or another layer is made up. 
It is curious that the changes in^the hue of the skin appear 
to be influenced, as in the case of the chameleon, by the 
colour of the surface with which it may be in proximity." * 
The Arms {hrachia). — The Cephalopoda furnish two well- 
defined types of brachia. The Nautilus-f* has arms which are 
quite devoid of suckers or acetabula, such as are found on those 
of the cuttle fishes and their allies. The foot of the Nautilus, 
as described by Huxley,}: " has its margins produced externally 
into a sort of sheath, which, in front, has the form of a broad 
hood, with a tuberculated surface ; while at the sides it is 
divided into many processes of unequal lengths. Behind, the 
halves of the sheath are separated throughout the greater 
part of their length by a wide interval, but are united above 
by a thick muscular isthmus. The central portion of the 
sheath is a broad triangular hood-like plate, the apex of 
which is free. It contains two long, narrow cavities, each of 
which lodges a tentacle. The tentacle consists of a slender 
Tlie ;Microsco2ie and its Eeyelatious," Dr. TV. B. Carpenter, Chap. xiv. 
p. 658, 5 ed. Lon., 1875, 
f For Anatomy of ^Nautilus, besides the memoirs, &c., akeady cited, see: — 
" Nourelles Eecherches sur le Kautile flambe," M. A. Valenciennes, Paris, 1839. 
"Bijdragen tot de OntleekundigeKennis aangaande Nautilus Pompilius," L.VanDer 
Hoeven, Amsterdam, 1856. " Over Het Ontleedkundig Zamenstel van den 
ISTautilus Pompilius," W. Vrolik, Amsterdam, 18-18. " On the Anatomy of 
Nautilus umbilicatus compared with that of Nautilus Pompilius," J. D. Macdonald, 
London, 1855 ; and " On some Points in ^the Anatomy of Nautilus Pompilius," 
T. H. Huxley, London, 1858. 
X *' The Anatomy of Invertebrated Animals," by T. H. Huxley, London, 1877, 
p. 532. 
