430 



MANUAL OF ZOOLOGY. 



were eight, furnished with horny hooks, with two " tentacles ; " 

 and probably the mouth was provided with horny mandibles. 

 An ink-bag was present. The inter- 

 nal skeleton of a Belemnite (fig. 232) 

 consists of a chambered cone the 

 " phragmacone " 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 con- 

 dition. Anteriorly the phragmacone 

 is continued into a horny lamina or 

 " pen " (the " pro-ostracum " of Hux- 

 ley), 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. 232, g) which projects 

 backwards for a longer or shorter dis- 

 tance, 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 be- 

 ing creeping animals, protected by an 

 external, many -chambered shell, the 

 septa between the chambers of which 

 are perforated by a membranous or 

 calcareous tube termed the " siphuncle" 

 The arms are numerous and are de- 

 void of suckers ; the branchice are four 

 in number, two on each side of the 

 body ; the funnel does not form a com- 

 plete tube ; and there is no ink-bag. 



Though abundantly represented 

 by many and varied extinct forms, 

 the only living member of the Tetra- 

 Fig. 232. Diagram of Belemnite branchiata is the Pearly Nautilus, 



(after Professor Phillips). r W 1-,j r h V, hppll InnP" known bv its 

 Horny pen or "pro-ostracum;" WHICH Ildb UCCll lOIlg KI1OYV11 uy lib 



p Chambered "phragmacone" beautiful chambered shell, but the 



?&<%>&?* " alveolus '" soft parts of which were first describ- 

 ed from a perfect specimen which 

 was examined by Professor Owen.* 



* The animal of the Pearly Nautilus is still one of the greatest rarities 

 in museums. Its anatomy was originally described from a female specimen 



