480 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
darkening or blush of deep terra-cotta colour over the 
whole body. If allowed to come to rest again now, the 
colour gradually lghtens until the normal condition is 
reached. If the animal is stimulated several times in 
succession— say by poking with a glass rod, or by bringing 
a brightly coloured bottle near—it becomes exhausted, 
the pallor becomes less intense, and the consequent 
darkening less noticeable; also efforts to escape cease. 
At night the colour is like that of the resting condition. 
Structure of the Skin.—The skin consists of a 
columnar epidermis, and a subjacent and much thicker 
dermis (Text fig. IL). It may easily be detached from the 
muscular body wall, thus destroying the deeper layers of 
the dermis. The epidermal cells secrete a thin cuticular 
protective layer externally, while internally they are each 
produced into several fine processes which attach the 
epidermis closely to the dermis. 
The dermis is divisible into four layers, as follows :— 
1. External fibrous layer (Text fig. Il, H#.C.L.). 
2. Layer with chromatophores (Text fig. II, Chr.). 
3. Layer containing iridocysts (Text fig. II, Lvd.). 
4. Internal fibrous layer. This is the thickest layer, 
and connects the skin to the underlying muscles of the 
body wall. It contains the vessels and nerves of the skin, 
and also feeble muscular strands (Text fig. II, J.C.Z.). 
Chromatophores.—These are extensible pigment- 
containing vesicles, occurring in the external part of the 
dermis (Text fig. II, Chr.). Their expansion and con- 
traction cause the changes of colour so characteristic of 
all Dibranchiate Cephalopoda. The origin, structure and 
movements of the vesicles have been studied by many 
people, and much variation of opinion exists on all three 
points. The views of Rabl, Miller, Klemensiewicz, 
Frédéricq and Kélliker may be briefly summarised thus: 
