Cephalopoda. 

 Order 2. DIBRANCHIA. 



49 



The members of this order have a single pair of gills and eight 

 or ten sucker-bearing arms. The shell may be coiled and many- 

 chambered, as in the living Spirula, where it is almost wholly 

 internal. In many fossil forms it is straight, and the chambered 

 shell (phragmocone) dwindles and is replaced by secondary 

 calcareous structures which lead to the calcareous ' cuttlefish- 

 bone ' of the living Sepia and the horny ' pen ' of Lolicjo and other 



Fig. 44. 



Spirula. Dorsal aspect (A), ventral aspect (B). sh. shell ; e. eye ; fu. funnel ; 

 ft. fins. 



From Lankester's Treatise on Zoology ( by permission of Messrs. A. & C. Black) 



genera. Finally, in the Octopus (Polypus) the only vestiges of 

 the shell are small chitinous stylets. 



The Dibranchia are divided into two sub-orders — Decapoda and 

 Octopoda. 



The Decapoda have ten arms, the suckers on which are 

 strengthened by horny rings which may bear hooks ; two of the 

 arms are retractile into special pouches and are known as ' tenta- 

 cular arms '. 



Some living Decapods attain a great size, e.g. Architeuthis, of 

 which a model is suspended from the ceiling of the Gallery. 

 There can be little doubt that some tales of ' Sea-serpents ' are 

 based on the appearance of such monsters. Complete examples 



I) 



