THE CEPHALOPODA 



303 



appears to be an undivided ovoid or globular mass, but it is 

 traversed by the oesophagus, and its double origin is further demon- 

 strated by the hepatic ducts, which are two in number, in all . the 

 Dibranchia. In the Decapoda the hepatic ducts are long (Fig. 

 268, b.d) and traverse the kidneys ; in the Oc'topoda they are short. 

 In the former sub-order the ducts are covered by the so-called 

 " pancreatic " glandular follicles, whose structure is a little different 

 from that of the liver (Fig. 268, pri). In the Octopoda these 

 follicles are situated only on the initial part of the hepatic ducts 

 and are nearly buried in the mass of the true 

 liver. Digestion is wholly effected in the 

 muscular stomach by the action of the trypsin 

 secreted by the liver and by the diastatic 

 ferment secreted both by the liver and the 

 " pancreatic " follicles. 



The intestine is relatively short and of 

 uniform diameter. In Nautilus and the Octo- 

 poda it is slightly sinuous, but in the Decapoda 

 it is straight. The anus is situated in the 

 middle line towards the anterior part of the 

 pallial cavity (Fig. 272, a), and is often fur- 

 nished with lateral valves. With the exception 

 of Nautilus, Cirrhoteuthis, Octopus arcticus, and 

 0. piscatorum, all the Cephalopoda, including 

 the fossil Belemnites, have an ink-sac, consisting 

 of a highly-developed rectal caecum developed 

 early in embryonic life from the dorsal wall of 

 the intestine and opening into the extreme 

 terminal part of the rectum. This sac is made 

 up of a deeper part, or gland proper, the cavity 

 of which is septate, and a reservoir, into which 

 the glandular part opens by a very small , ink-bag ; 6, its opening 



a i.i ii iiJ i J i 1 ^ the rectum ; c, com- 



ormce : the reservoir specially well developed mencement of the caecum ; 

 in the Decapoda. This ink-sac occupies a some- ^Jj} ^pSU 

 what superficial position to the side of the v the ' stomach opened 



T r . , ~ . 7 ., . longitudinally ; x, probe 



visceral mass. In some species of Sepiola it is passed through the pyio- 

 trilobed, two lateral accessory organs being SfciffiS) Lankester ' 

 joined to it. It extends to the posterior ex- 

 tremity of the body in Sepia (Figs. 271, i.s -, 272, t\ and is buried 

 in the superficial part of the liver in all the Octopoda except 

 Argonauta. The Cephalopoda are able, at will, to expel the 

 secretion contained in the reservoir of this anal gland through the 

 funnel, and thus conceal themselves by producing a dense cloud in 

 the water. An oxydising diastase, called tyrosinase, is concerned in 

 the production of the secretion, the latter being known as melanin. 

 2. Circulatwy Apparatus. The Cephalopoda, or at any rate the 



Fia. 269. 

 Alimentary canal of 



