MOLLUSCA — CEPHALOPODA. 



157 



and the phragmocone. This last is relatively much wider Gallery 



than in the belemnite and is covered by no solid guard, but tjig^^^^^^^Q 



by a thin coating of horny substance deposited by the i" 



mantle, which is still seen surrounding it (Fig. 85 /). 



The redaction of guard and phragmocone has progressed 



even further in Geoteicthis hrevipinnis. A specimen of this 



from the same rock preserves the 10 very short arms, 



mantle, and tail-fins, and shows a broad thin shell such 



as in the squids and calamaries now living is called a 



pen, with very little trace of any chambered portion 



(Fig. 85 g, h, i). A similar but narrower pen is found 



in PlesioteiUhis pnsca, of which specimens from the Solen- 



hofen Stone are exhibited, one of them with the soft parts 



also indicated (compare Fig. 86 1). These pens should 



be compared with that of the squid, Loligo vulgaris, which is 



shown beside a glass model of the complet.e animal (Fig. 78 e). 



Other models of living cephalopods with similar shells are 



exhibited, but cannot here be discussed. It is enough to realise 



that in one line of descent of these forms with ensheathed 



shell the chambered cone, long protected by a stout covering, 



retained its calcareous and septate nature, while in another 



line it became horny, and ultimately lost its septa. 



There was however another line of evolution, the origin 

 of which is difficult to trace, because one of its conspicuous 

 characters was the complete loss of the shell. Another 

 character was the absence of the long arms, reducing the 

 number to eight. These forms therefore are called Octopoda, 

 in opposition to the ten-armed squids and cuttle-fishes, 

 which have been called Decapoda. Study of the early 

 dcA^elopment of an octopus teaches us that its ancestors must 

 have had a shell, and it seems probable that the loss was due 

 to a reduction like that which took place in the squids, but 



Fig. 87. — Female Argonaut swimming from left to- right. 



greater in extent. A drawing of the oldest Octopod known 



is placed in this Case. Beside it are glass models of living 



species of Octopoda, among which ArgonaiUa deserves 



mention. As shown by the models and by the exhibited Between 



specimens, the well-known Arc^oiiaut shell is of a different Wall-cases 



^ 1 & 2. 



