CEPHALOPODA. 409 



bearing tentacles or suckers. Part of the foot forms a partial 

 or complete tube the "siphon" or "funnel" through which 

 water is forcibly expelled from the mantle cavity ', driving the 

 animal backwards. The muscular mantle flap which shelters 

 the gills is posterior in position ; the visceral hump shows no 

 trace of spiral coiling, but is elongated in a direction anatom- 

 ically dorsal and posterior, though it may point forwards 

 when the animal propels itself through the water. Except in 

 the pearly Nautilus, the shell of modern forms has been 

 enclosed by the mantle, and is, in most cases, only hinted at. 

 There is a very distinct head region, furnished with eyes and 

 other sensitive structures, and the mouth has strong beak-like 

 jaws, as also a well-developed radula. The nervous system 

 shows considerable specialisation, and the thief ganglia are 

 concentrated in the head. The true body cavity, pericardium 

 of other Molluscs, is usually well developed, and frequently 

 surrounds the chief organs. Except in the Nautilus, it com- 

 municates with the exterior by the nephridia. 



The vascular system is well developed, and, except in the 

 Nautilus, there are accessory branchial hearts. The sexes are 

 separate. Development is direct. In habit, Cephalopods are 

 predominantly active and predatory : in diet, carnivorous. 



The Cephalopods are divided into two markedly distinct 

 orders, of which the one includes Sepia and all other living 

 cuttles except Nautilus, which is the sole living type of the 

 second order. As Sepia has been already described, we 

 may briefly review some of the more striking characters of 

 the pearly Nautilus (Nautilus pompilus). 



The shells of the pearly Nautilus are common on the 

 shores of warm seas, but the animals are much less familiar. 

 The Nautilus creeps or swims gently along the bottom at no 

 great depth, and its appearance on the surface, " floating 

 like a tortoiseshell cat," is probably the result of storms. It 

 is called " pearly " on account of the appearance of the 

 innermost layer of the shell. This is exposed after the soft 

 organic stratum and the median layer which bears bands of 

 colour have been worn away, or dissolved in a dolphin's 

 stomach, or artificially treated with acid. 



The beautiful shell is a spiral in one plane, divided into a 

 set of chambers, in the last of which the animal lives, while 

 the others contain gas. The young creature inhabits a tiny 



