316 



Mollusca. 



two more long, so-called "tentacular arms" within the other 

 eight. All (the tentacular ones only towards the tips) are 

 furnished with numerous muscular suckers, ranged along the 



inner sides, i.e., towards the 

 mouth. These suckers are 

 sessile in the Octopods, usually 

 stalked in the Decapods ; in 

 the latter (but not in the 

 Octopods) there is a chitinous 

 ring at the margin of the 

 sucker, and it is usually finely 

 denticulate at its edge. In 

 certain Decapods some of the 

 suckers may be metamor- 

 .phosed into hooks by lateral 

 elongation of the ring. In 

 the Tetrabranchiata (Nau- 

 tilus) numerous thin ten- 

 tacles are arranged in 

 several circlets round the 

 mouth, instead of the one 

 circle of arms. They may be 

 withdrawn into tentacle- 

 sheaths which are partially 

 concrescent, forming hand- 

 like plates, from the edges 

 of which the tentacles arise ; 

 they have no suckers. On 

 the head there is a pair of 

 large eyes, which will be 

 described later. 



The body, whose under 

 surface corresponds with the 

 posterior side of the visceral 

 hump of Gastropods, is short 

 and thick in the Octopods 

 and Nautilus, more elongate 

 in the Decapods : in the latter 

 It is furnished with a pair of 

 horizontal fins, which arise 

 laterally and somewhat dor- 

 sally, and are usually situated 

 near the posterior end of 

 the animal. At the junction of the head and body there is a trans- 

 verse slit ventrally, leading into the spacious mantle-cavity 

 [see Fig. 268 B), which extends over the- whole length and width of 



Fig. 262. Diagram of a decapodous 

 Cephalopod viewed from below, mantle cut 

 through down one side and turned over to the 

 other, a anus in the posterior end of the funnel, 

 / fin, g genital aperture, fc gill, m mantle, n 

 urinary aperture, t anterior opening of funnel. — 

 Orig. 



