6o MOLLUSC A. 



unexpected interruption of the line, which is indicated 

 by the motions of the body in Gasteropods, which move 

 head-end foremost, and the LamelHbranchs, which, with 

 equal discretion and effect, hold the mouth-end fore- 

 most. The fact is, that the situation of the organs, and 

 the shape, make it more convenient for the Cephalo- 

 pod, like the Lobster, to back out of a scrape than to 

 run away from it. The so-called funnel is a tube lead- 

 ing into the mantle, which is also open at the neck {on). 

 The water is admitted or sucked in by expanding the 

 mantle. The rim of this then closes against the neck, 

 the powerful muscles in the walls lower down, contract, 

 and the confined water is driven in a powerful jet out 

 through the funnel, driving the animal back by its 

 reaction. The Nautilus, Argonauta (Fig. 43), Octopus 

 (Fig. 42), and all the Cephalopods move this way ; and 

 some, which have webs between the arms, use these 

 also, opening and shutting them like an umbrella. 



The fins (Fig. 41) in the Squid are also used in lo- 

 comotion, though they can only keep the body in posi- 

 tion, and move it slowly forwards, if necessary. 



A favorite mode of locomotion with Octopus, or 

 devil-fish, is to walk or crawl on its arms, the round 

 body above, as in Fig. 42, though it also swims like the 

 other forms. 



The ink bag in the interior is filled with an intensely 

 black secretion. This communicates with the mantle- 

 hollow by a tube opening at the base of the siphon, 

 and when frightened by sudden danger, they squirt out 

 a portion of this through the siphon, forming a dense 

 cloud, blinding their enemy, and concealing their 

 own simultaneous retreat. This excretion, when dried, 



