DIBRANCHIATA. 107 



Order 2. — Dibranchiata. 

 (Squids, Cuttle-fishes.) 



In this order the branchiae are two, forming a pair, each 

 with a branchial heart; the funnel is an entire tube; the 

 mantle is muscular ; there is an ink-bag ; the eyes are sessile ; 

 the beak horny ; and the shell internal (save in the females 

 of the genus Argonauta). 



Compared with the Nautilus, the cephalic organs of pre- 

 hension are much reduced in number, the external ones, 

 continued from the oral sheath, not exceeding eight, as in 

 fig. 32, c, to which in most of the genera, is added a pair of 

 internal and much longer tentacula, d. The arms are much 

 increased in size and of a more complicated structure, sup- 

 porting on their internal surface numerous suckers, and 

 sometimes connected together by a powerful muscular web. 

 The eyes are much larger and more complex, are no longer 

 pedunculated, but lodged in orbits (fig. 32, e e). The mouth 

 is armed with two piercing and trenchant horny jaws, resem- 

 bling in shape and in their vertical movements those of the 

 Nautilus. The gills are two in number, each with a ventricle, 

 expressly appropriated to the branchial circulation ; the sys- 

 temic circulation having a single muscular ventricle as in 

 the Nautilus. The infundibulum (fig. 33, /) is a complete 

 muscular tube, shaped like an inverted funnel. They possess 

 a gland and membranous receptacle for secreting and expelling 

 an inky fluid. The sexual organs are in distinct individuals, 

 as in the Tetrabranchiate order. All the species of both 

 orders of Cephalopods are aquatic and marine. 



The Dibranchiate order is subdivided into two tribes ; 

 the one provided with the eight ordinary arms (fig. 32, e) 

 and the two longer tentacles (ib. d), hence called Decapoda; 

 the other tribe without the tentacles, and called Octopoda 

 (fig. 33, i, 2, 3 , 4 ). 



