432 PHYLUM MOLLUSC A. 



Order II. Dibranchiata (see Table). 



Sub Order Decapoda. Eight shorter and two longer arms. 

 Suckers stalked and strengthened by a strong 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 by upgrowths of the mantle. 



With calcareous internal "shell." Spirula; extinct Bel- 

 emnites ; Sepia. 



With organic internal "shell." 



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



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

 Sub-Order Octopoda. Eight arms only. Suckers sessile 



without horny ring. Small eyes with sphincter-like 

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

 arrangement. No "shell," except in the female 

 Argonauta. 



e.g. Octopus {Polypus}^ Eledone {Moschites}, Argo- 

 nauta, Cirroteuthis (with cirri on the arms and 

 no radula). 



The classification given above is that usually adopted, but it is not 

 certain that the Ammonites should be included in the Tetrabranchiata. 

 The Nautiloids began in the Cambrian and died out at the end of 

 the Palaeozoic period, except the Orthoceras and Nautilus-like types. 

 The genus Nautilus appeared in the Cretaceous. The Ammonite series 

 lasted from the Silurian to the early Tertiary. 



The Cephalopods are the most specialised of the 

 Molluscs, and present much diversity of type. They swim 

 freely in the sea, or creep sluggishly among the rocks. 

 They are voracious eaters, and devour very diverse kinds 

 of animals, their parrot-like jaws and powerful odontophore, 

 as well as the numerous suckers, rendering them formid- 

 able adversaries. Many live at considerable depths, and 

 their chief foes are the toothed whales, some of which, like 

 the sperm whale (Physeter), and the bottle-nose (Hyperoo- 

 dori), subsist almost entirely on cuttles. Some deep-sea 

 forms have highly evolved luminous organs. 



Shells of Cephalopods. A chambered external shell, serving as 

 a house, is present in Nautilus alone among living Cephalopods. 

 Most of the extinct forms had large and efficient shells of very 

 diverse shape, some straight like Orthoceras, or coiled, with chambers 

 separated by complex septa, as in the Ammonites. In the majority 

 of shells of the Ammonitid series, the septa between the chambers 

 are convex towards the aperture (the opposite in the Nautilus) ; the 

 siphuncle is marginal or ventral ; the septal necks of the siphuncle 

 project forwards (not backwards as in the Nautilus) ; there is 



