EXTERNAL FORM AND PIGMENTATION. 739 



and by Vrolik 1 , but the latter erroneously supposed that the animal was attached to 

 the shell by means of the siphuncle, presumably because the latter became broken off 

 during the removal of his specimen from the shell. 



Whether at rest or on the alert, the dark brown and whitish mottled or piebald 

 coloration of the exposed part of the molluscous body harmonises well with the zebra- 

 like markings on the shell. This scheme of pigmentation apparently produces the same 

 effect as does the play of light on the surface ripples of the sea, and the alternating 

 light and dark bands of the shell might be appropriately described as ripple-markings. 



On one occasion when on a raft with my native servant in Sandal Bay, Lira, 

 I accidentally dropped a sprightly Nautilus overboard in four or five fathoms of clear 

 water. It instantly disappeared in a miraculous manner, baffling all our efforts at 

 recovery. Whether the gleam of colour would confer an equal degree of invisibility 

 in the phosphorescent twilight at greater depths it is of course impossible to say, 

 but when the active migratory habits of Nautilus are considered and also the fact 

 that it is subject to the attacks of voracious fishes, probably sharks and congers, as 

 is evidenced by the mutilated condition of many individuals, I think the above 

 observation may be taken to indicate that the colour-markings, being what they are, 

 exert a protective influence. 



Under certain conditions, more frequently when kept in a confined space, I noticed 

 that a great portion of the brown pigment would disappear from the outer surface of 

 the hood, so that the latter became partly, in some cases entirely, bleached. . The pigment 

 would seem to be actually discharged from the epidermal cells in which it is normally 

 lodged in an even layer near their outer ends. 



The wart-like, generally whitish, gibbosities on the upper surface of the hood, have 

 been regarded as sensory papillae, and were likened by Van der Hoeven (1848) to the 

 papillae vallatae of the human tongue. It is obvious, however, that they are not 

 sense-organs. 



In the retracted state the mouth of the shell is closed by the thick warty hood, 

 which forms the upper part of the cephalic complex and acts as an operculum. In this 

 condition the hood somewhat resembles the foot of a Gastropod, and Rumphius stated 

 that when the animal crawls the hood is directed downwards, and the side which is 

 uppermost in the attitude of swimming becomes a plantar surface when creeping. 



This mistake was repeated by Owen, who described the hood as being of a fibrous 

 texture and white colour internally, resembling dense corium, but added that it is 

 " doubtless muscular, and in creeping (the position of the animal being reversed) seems 

 calculated to act as its chief locomotor organ." It is true that the hood is traversed by 

 thin and sparse muscular fasciculi, but its structure is essentially coriaceous, and it serves 

 as a shield, not as an organ of locomotion. 



In the attitude of swimming and breathing, as shown in the photograph here 

 reproduced, the hood is raised above the margin of the shell so as to expose the 

 tentacles, the funnel and the eyes, and to allow of free communication between the 

 mantle-cavity and the external aqueous medium. In the figure the front end of the 



1 Vrolik, W., " On the Anatomy of the Pearly Nautilus." Ann. Nat. Hist. xn. 1843, pp. 173—175. 



w. vi. 97 



