486 



PHYLUM MOLLUSCA 



Order II. Dibranchiata (see Table). 



vSub-Order Decapoda. Eight shorter and two longer arms. 

 Suckers stalked and strengthened by a chitinoid ring. 

 Large eyes with a horizontal lid. Body elongated, with 

 lateral fins. Mantle margin with a cartilaginous " hook- 

 and-eye " arrangement. Some sort of internal " shell," 



enclosed b}"- upgrowths of the mantle. 

 With calcareous internal "shell." Spirula 

 emnites ; Sepia. 



extinct Bel- 



FiG. 276. — Section of the shelJ of Spirula. 



SEPT., Calcareous partition separating chamber 

 from chamber, but perforated by the tubular 

 siphuncle (SIPH.). The initial chamber of the 

 shell, known as the protoconch (PROT.), con- 

 tains a protosiphon (PROS.), but this does not 

 lead into the siphuncle. 



With organic internal " shell." 



(a) Eyes with closed cornea, Myopsida, e.g. Loligo. 



(6) Eyes with open cornea, Oigopsida, e.g. Ommastrephes. 



Sub-Order Octopoda. Eight arms only. Suckers sessile 



without chitinoid ring. Small eyes with sphincter-like 



lid. Body short and rounded. No " hook-and-eye " 



arrangement. No " shell," except in the female 



A rgonauta. 



e.g. Octopus {Polypus), Kledone {Moschites),Argonauta, 



Cirroteuthis (with cirri on the arms and no radula). 



