12 



darkening or blush of deep terra-cotta colour over the 

 whole body. If allowed to come to rest again now, the 

 colour gradually lightens until the normal condition is 

 reached. If the animal is stimulated several times in 

 succession say by poking with a glass rod, or bv 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. II). [i 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. II, Ex.C.L.). 



2. Layer with chromatophores (Text fig. II, Chr.). 

 '■'). Layer containing iridocysts (Text fig. II, Irid.). 

 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, I.C.L.). 



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, Miiller, Klemensiewicz, 

 Fredericq and Kolliker maybe briefly summarised thus: 



