V. CEPHALOPODA. 389 



Loliginidae it forms a < pen ' of purely organic nature (fig. 340, A). 

 Like true shell these dorsal structures are products of the external 

 epithelium, only the epithelium which forms them, the shell 

 gland, has become folded in and the walls have united over it. 



The shell of Argonauta (fig. 398) 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. 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. 388, 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 comparison with the fossil Belosepia (5), it is seen that the cuttle 

 bone as it appears in commerce (A) is the anterior part of the chambered 

 shell, its Iamina3 being the partitions, while in the animal the rostrum and 

 siphuncle are in part retained. On the other hand, comparison with 

 the fossil Ostracoteuthis (D) shows that in Ommastrephes (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. 389); then follows a pharynx 

 with a radula, and in turn a long 

 oesophagus, often with a crop-like dila- 

 tation. The oesophagus opens into a 

 wider pouch, the stomach, with which 

 is connected a blind sac, frequently Fl - 389 ~ Jaws of Se 

 coiled. Here the tract doubles on itself and goes straight to the 

 anus, or makes one or two convolutions in its course (fig. 390). 

 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, which has a duct of varying length, secretes in its interior 

 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 its secre- 

 tion forms the basis of the well-known color, sepia. Nautilus 

 has no ink sac. 



Just behind the buccal mass are the closely united chief gan- 



