CEPHALOPODA 59I 



tory movement of the lateral fins. But if Sepia is alarmed or excited 

 the muscles contract violently and the spasmodic ejection of water 

 through the funnel causes the animal to dart quickly backw^ards. 

 Equally by turning the funnel backward it can move quickly forward. 

 Not only is the mantle highly muscular but the dermis contains large 

 cells filled with pigment, the chromatophores ^ which can be dilated by 

 the contraction of radiating muscle fibres attached to the cell wall. 

 By alternate contraction and expansion of the chromatophores, waves 

 of colour are made to pass rapidly over the surface of the animal. The 

 colour change which is brought about in this way may be to a certain 

 extent a response to the character of the background but it is also 

 stated to be the expression of emotions. 



Sepia swims with the longest axis horizontal, the upper flattened 

 surface is that under which the shell lies and the lower the mantle- 

 cavity surface. These surfaces are dorsal and ventral respectively and 

 the mouth and tentacles are anterior. All round the mantle in the 

 horizontal plane rises a horizontal fin by which the gentler swimming 

 movements are effected. 



When the mantle cavity is opened as shown in Fig. 403, the funnel 

 is seen with its narrow external and wide internal openings, and at 

 the base of it two sockets which fit corresponding knobs on the 

 mantle. This locking arrangement ensures that the mantle fits tightly 

 on the neck and so that all water is expelled by the funnel. At the 

 anterior end of the visceral hump is seen the central anus at the end 

 of a long papilla, so placed as to discharge the faeces directly into the 

 cavity of the funnel, the shorter renal papillae immediately on each 

 side, and on the left side only the genital aperture, also at the end of a 

 long papilla. More posterior still are the large and typical ctenidia. 



On the face of the visceral hump in mature animals the accessory 

 genital glands are seen through the skin ; the chief of these are the 

 shell-forming nidamental glands of the female which occupy a con- 

 siderable area. Between these and in front of them are the accessory 

 nidamental glands. Posterior to them is the ink sac, usually seen 

 through the integument from which a narrow duct runs ventral to 

 the rectum, opening into it a short distance behind the anus. The 

 first step in dissection is to strip off the skin and then dissect out the 

 gland and its duct as carefully as possible. It usually contains a large 

 amount of the ink, which is composed of granules of melanin 

 pigment formed by the oxidation of the aminoacid tyrosin by the 

 agency of an enzyme, tyrosinase. This substance is ejected into the 

 mantle cavity and through the funnel to form a "smoke cloud" 

 when the animal is attacked. 



The next stage in dissection is the opening up of the kidneys by 

 cutting through the thin outside wall. It will at once be seen that the 



