Glass 4. Cephalopoda. 



317 



the animal on tlie ventral surface. It is bounded externally by a 

 mantle, wbich is usually very muscular and thick, and, in many 

 cases, prolonged into a low fold behind the head on the upper 

 surface. From the opening of the mantle-cavity projects the anterior 

 end of the funnel, a tube open at both ends, and attached dorsally 

 to the dorsal wall at the boundary of the head ; the funnel is a true 

 tube in the Dibranchiata only ; in Nautilus it is a plate rolled up 

 like a paper bag ; it corresponds to the foot of other Mollusca. The 

 animal takes water into the mantle-cavity by the large aperture, 

 but expels it through the funnel by the contraction of the mantle 

 and the pressure of its edge upon the body. In the funnel there 

 is frequently a small tongue-like flap, which is attached posteriorly, 

 the anterior end being free, so that it acts as a valve, preventing 

 the entrance of water. The laminate gills lie in the mantle-cavity, 

 one pair in the Octopods and Decapods, two pairs in Nautilus. 



Fig. 263. Nautilus, the shell sawn through, o eye, t funnel, te tentacle ; s the syphon, 

 h a fold of skin which oTerlaps the shell. — After v. Martens. 



In Nautilus the body is enclosed by a shell, which, as in 

 the Gastropods, is an epidermal secretion. It is spiral, but sym- 

 metrical ; the convexity corresponds to the ventral side of the animal 

 {see Fig. 263) ; the coils touch one another. The shell is multi- 

 1 o c u 1 a r , being divided by arched transverse septa into nume- 

 rous chambers, of which the outermost and largest contains the 

 body, whilst the others contain air; each septum is perforated by 

 a liole^ through which a thiii tube, the siphunole, an extension of the 



