HEAD-FOOTED MOLLUSCS 313 



round the head, and chiefly consists of the ten arms or tentacles. 

 It is from this peculiarity the name of the class is derived 

 (Gk. kephalon, a head; pous, a foot). Just above the back of the 

 head, and projecting from the opening into the mantle-cavity, 

 will be noticed a muscular conical tube, the funnel, by the 

 ejection of water through which the Cuttle- Fish is able to swim 

 rapidly backwards. It will be observed that the mantle-cavity 

 is here at the back, while in the Ormer it is in front, one result 

 of the twisting which the visceral hump of that animal has under- 

 gone. In its natural position, whether resting on the sea-bottom 

 or swimming, the front side of the head and visceral hump is 

 directed upwards, and this side is much darker than the other. 

 It may be taken as a general rule that the surface of an 

 animal habitually facing in this direction is the darkest part 

 of the body, though its actual nature varies in different animals. 

 Here it is the front side, but in a Dog- Fish it is the true upper 

 or dorsal side, and in a flat-fish either the right or left side 

 according to the species (see p. 279). One peculiarity of the 

 Cuttle- Fish's skin is especially noteworthy. If a living specimen 

 is watched, it will be seen that beautiful purplish flushes of colour 

 sweep over the body from time to time, leaving it comparatively 

 pale in the interval. The cause of this is to be sought in the 

 presence of innumerable little rounded colour-bodies (chromato- 

 phores), which, under the control of the nervous system, vary 

 in size. When reduced to their smallest dimensions the skin 

 is pale, but when fully expanded it is dark. A similar pheno- 

 menon has been described for the Frog (p. 251), where, however, 

 the colour changes are comparatively slow. 



Cutting open the mantle-cavity, we shall find similar parts 

 and openings to those described for the Ormer (p. 308). In 

 the middle line there is the projecting end of the intestine, and 

 on either side of this a kidney aperture, while a plume -like 

 gill is to be seen on either side. As is well known, the Cuttle- 

 Fish and many of its allies are able to eject an inky substance 

 into the surrounding water as a means of protection. This ink 

 is formed within a rounded ink-bag, and carried off through a 

 slender tube which has a common external opening with the 

 intestine. 



The front side of the visceral hump has imbedded in it a 

 broad " cuttle-bone ", composed of overlapping layers of cal- 



