460 THE ANATOMY OF INVERTEBRATED ANIMALS. 



have their bases toward its mouth. Nothing is certainly 

 known as to the nature of the Aptychi or Anaptychi. 1 



In the Dibranchiata, the margins of the foot are pro- 

 duced into not fewer than eight, nor more than ten, arms, 

 which are provided with acetabula, or suckers. Each ace- 

 tabulum is a sessile or stalked cup, from the bottom of which 

 rises a plug, which nearly fills the cup, but can be retracted 

 by the action of muscular fibres attached to it. When the 

 margins of the acetabulum are applied to any surface, and 

 the plug is retracted, a partial vacuum is created, and the 

 acetabulum is caused to adhere to the surface by atmospheric 

 pressure. The edges of the acetabula are frequently strength- 

 ened by chitinous rings, and these may be serrated (Fig. 124, 

 B), and are sometimes produced into long, curved hooks. 



The margins of the united epipodia are not only folded 

 inward, but coalesce so as to give rise to a tubular funnel, 

 through which the water taken into the branchial sac for 

 respiratory purposes is ejected. Very often, a valve which 

 prevents the flow of water back into the mantle cavity is de- 

 veloped within the funnel. There are two branchiae, and the 

 anus terminates between them in the anterior wall of the 

 branchial sac, on which also the nidamental glands are situ- 

 ated. The apices of the horny beaks are acutely pointed, 

 and not ensheathed in calcareous matter. The liver is usual- 

 ly a compact mass. A peculiar gland, which secretes an ex- 

 tremely dark fluid — the so-called ink — and has the form of an 

 oval or pyriform sac (the ink-bag), with a long duct which 

 opens into, or close to, the rectum, is lodged sometimes in 

 the liver, sometimes further back (Fig. 126, I.). The ink is 

 ejected when the animal is alarmed, and gives rise to a dark 

 cloud in the water, by which its retreat is covered. There 

 are two branchial hearts. 



The eye is lodged in an orbit and is provided with a lens. 

 The cartilaginous endoskeleton forms a ring surrounding the 

 gullet and enveloping the principal ganglia. There is usually 

 an internal pallial shell. It may be chambered and siphun- 

 culated, but in this case the last chamber is small, and hardly 

 larger than the others. 



The Dibranchiata are divided into the Octopoda and the 

 Decapoda. The Octopoda have eight arms, and possess no 

 pallial shell. But, in the female of one genus (Argonauta, 

 the " paper Nautilus," Fig. 131), the extremities of the an- 



1 See the discussion of this question by Keferstein, in Bronn's " Thierreich." 



