MOLLUSCA ! CEPHALOPODA. 



277 



Calamary (Loligo) the fourth left arm of the male is meta- 

 morphosed towards its extremity to subserve reproduction. 



In the family of the Sepiadae, the internal 

 shell is calcareous ('cuttle-bone' or ' sepi- 

 ostaire') and is in the form of a broad plate, 

 having an imperfectly chambered apex. In 

 the living members of the family the body 

 is provided with long lateral fins, sometimes 

 as long and as wide as the body itself. 



In the singular family of the Spirulidce 

 the internal skeleton is in the form of a 

 nacreous, discoidal shell, the whorls of which 

 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. The shell of the Spirilla commonly 

 known as the ' post- horn' is similar in 

 structure to the shell of the Nautilus, 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 ' phragmacone ' of the 

 Belemnite. Though the shell occurs in enor- 

 mous numbers in certain localities, a single 

 perfect specimen of the animal is all that 

 has been hitherto obtained. 



In the extinct family of the Belemnitidoe, 

 our knowledge is chiefly confined to the hard 

 parts. Certain specimens, however, 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, fur- 

 nished with horny hooks, with two ' ten- 

 tacles ; ' and probably the mouth was provided 

 with horny mandibles. An ink-bag was 

 present. The internal skeleton of a Belemnite Fig. 85. Diagram of 

 (Jig. 85) 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 pre- 

 served condition. Anteriorly the phragmacone is continued 

 into a horny lamina, or * pen ' (the * pro-ostracum ' of Huxley), 



C-\ 



Belemnite (after Pro- 

 fessor Phillips ). a. 

 Horny pen or ' pro- 

 ostracum.' b. Cham- 

 bered ' phragmacone ' 

 in its cavity or ' al- 

 veolus.' c. ' Guard.' 



