V. CEPHALOPODA 341 



chambered shells; but in other recent forms and in many extinct species 

 the shell is more or less rudimentary. In Spirilla (the animals of which 

 are extremely rare, the dead shells common) there is a similar chambered 

 shell, buried in the mantle (fig. 363). Its position (ento gastric) is the 

 reverse of that of Nautilus. 



In the Decapoda the equivalent of the shell is completely concealed in 

 the back of the animal. In the Sepias it is a lamellar calcareous structure, 

 the well-known cuttle bone; in the Loliginidae it forms a 'pen' of purely 

 organic nature (fig. 311, A). Like true shell these dorsal structures are 

 products of the external epithelium, but the epithelium, the shell gland 

 which forms them, has become folded in and the walls have united over it. 



The shell of Argonauta (fig. 363) is different. It occurs only in the female, 

 is thin as paper, spirally coiled at the tip, and is only in part a secretion of the 

 body, for a part of it is formed by two tentacles which are expanded for this 

 purpose. Internal partitions are lacking, and this shell serves as a nest for the 

 eggs. Most Octopoda also lack a shell. A word or two may be added to 

 correlate the recent and fossil shells of the Dibranchiata, which are always 

 internal and more or less rudimentary. The fossil Belemnites (fig. 354, C) had 

 a chambered shell (phragmocone) perforated for the siphuncle. In front this 

 is prolonged ventrally into a thin broad plate, the proostracum, while behind it 

 is inserted in a calcareous sheath, the guard or rostrum. From this, by compari- 

 son with the fossil Bdosepia (B), it is seen that the cuttle bone of commerce (A) 

 is the anterior part of the chambered shell, its laminae being the partitions, while 

 in the animal the rostrum and siphuncle are in part retained. On the other 

 hand, comparison with the fossil Ostracotcuthis (D) shows that in Ommastrcphes 

 (E) we have but a remnant of the phragmocone, while the bulk of the pen is 

 proostracum. In Loligo the phragmocone is entirely lacking. 



The mouth, situated in an oval buccal mass, lies between two horny 

 jaws, like the beak of a parrot (fig. 355); then follows a pharynx with a 

 radula, and in turn a long oesophagus, 

 often with a crop-like dilatation The 

 oesophagus opens into a wider stomach, 

 with which is connected a blind sac, fre- 

 quently coiled. Here the tract doubles 



on itself and goes straight, or with one or 



,.- FIG. 3"? 1 ?- Jaws of Sepia 



two convolutions, to the anus (fig. 356). officinalis. 



One or two salivary glands (upper and 



lower, the latter poisonous in Octopus) open into the oesophagus, and a 

 pair of liver sacs (frequently fused) open by two bile ducts into the 

 gastric blind sac. These ducts may bear racemose glands called the 

 pancreas. Lastly, the ink sac opens into the intestine near the anus. 

 This gland secretes a brownish or blackish pigment. When alarmed 

 the animal ejects this secretion and clouds the water so that it can 

 escape unseen. This organ is best developed in Sepia officinalis, and 



