MOLLUSCA : CEPHALOPODA. 353 



An ink-bag was present. The internal skeleton of a Belem- 

 nite (fig. 147) consists of a chambered cone the "phragma- 

 cone " the septa of which are pierced with a marginal tube, 

 or " siphuncle." In the last chamber of the phragmacone is 

 contained the ink-bag, often in a well-preserved condition. 

 Anteriorly the phragmacone is continued into a horny lamina 

 or " pen " (the " pro-ostracum " of Huxley), and posteriorly it 

 is lodged in a conical sheath or " alveolus/' which is excavated 

 in the substance of a nearly cylindrical, fibrous body, the 

 "guard" (fig. 147, g) which projects backwards for a longer 

 or shorter distance, and is the part most usually found in a 

 fossil condition. 



ORDER II. TETRABRANCHIATA. The members of this order 

 of the Cephalopoda are characterised by being creeping animals, 

 protected by an external, many-chambered shell, the septa 

 between the chambers of which are perforated by a membra- 

 nous or calcareous tube, termed the " siphuncle." The arms 

 are numerous, and are devoid of suckers ; the branchiae are 

 four in number, two on each side of the body ; the funnel does 

 not form a complete tube ; and there is no ink-bag. 



Though abundantly represented by many and varied extinct 

 forms, the only living member of the Tetrabranchiata is the 

 Pearly Nautilus, which has been long known by its beautiful 

 chambered shell, but the soft parts of which can hardly be said 

 to be known by more than one perfect specimen, which was 

 examined by Professor Owen. 



The soft structures in the Pearly Nautilus may be divided 

 into a posterior, soft, membranous mass (metasoma), contain- 

 ing the viscera, and an anterior muscular division, comprising 

 the head (prosoma) ; the whole being contained in the outer- 

 most, capacious chamber (the body-chamber) of the shell, from 

 which the head can be protruded at will. The shell itself (fig. 

 148) is involuted and many-chambered, the animal being con- 

 tained successively in each chamber, and retiring from it as its 

 size becomes sufficiently great to necessitate the acquisition of 

 more room. Each chamber, as the animal retires from it, is 

 walled off by a curved, nacreous septum ; the communication 

 between the chambers being still kept up by a membranous 

 tube or siphuncle, which opens at one extremity into the peri- 

 cardium, and is continued through the entire length of the 

 shell. The position of the siphuncle is in the centre of each 

 septum. 



Posteriorly the mantle of the Nautilus is very thin, but it is 

 much thicker in front, and forms a thick fold or collar sur- 

 rounding the head and its appendages. From the sides of the 



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