278 MANUAL OP ZOOLOGY. 



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,' 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 (pvosoma) ; 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 is involuted, and many- chambered, the animal being 

 contained successively in each chamber, and retiring from 

 it as its size becomes sufficiently great to necessitate the ac- 

 quisition of more room. Each chamber, as the animal re- 

 tires 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 extre- 

 mity into the pericardium, 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 

 head spring a great number of muscular prehensile processes, 

 or 'arms,' which are annulated, but' are not provided with 

 cups or suckers. In the centre of the head is the mouth, 

 surrounded by a circular fleshy lip, external to which is a 

 series of labial processes. The mouth opens into a buccal 

 cavity, armed with two horny mandibles, partially calcified 

 towards their extremities, and shaped like the ,beak of a 

 parrot, except that the under mandible is the longest. There 





