330 MANUAL OF ZOOLOGY. 



are not in contact with one another, and which is divided into 

 a series of chambers by means of partitions or septa which are 

 pierced by a ventral tube or " siphuncle." The body is provided 

 with minute terminal fins, and the arms have six rows of small 

 suckers. The sheli of the Spirula — commonly known as the 

 " post-horn " — is similar in structure to the shell of the Nauti- 

 lus, but it is lodged in the posterior part of the body of the 

 animal, and is therefore internal, whereas the shell of the 

 latter is external. It really corresponds to the " phragmacope " 

 of the Belemnite. Though the shell occurs in enormous num- 

 bers in certain localities, a single perfect specimen of the animal 

 is all that has been hitherto obtained. 



In the extinct family of the Belemnitidm, our knowledge is 

 chiefly confined to the hard parts. Certain specimens, how- 

 ever, have been discovered which show that the Belemnite had 

 essentially the structure of a Cuttle-fish, such as the recent 

 Sepia. The body was provided with lateral fins ; the arms 

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

 and probably the mouth was provided with horny mandibles. 

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

 nite (ii%. 1 1 8) 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. 1 1 8,^), 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 branchias 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. 



