386 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- 



emmtes ; Sepia. 

 With organic internal "shell." 



[a) Eyes with closed cornea, e.g.^ Loligo. 



(b) Eyes with open cornea, 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 

 A?-gonaiUa. 

 e.g., Octopus, Eledone, Argoiiauta. 

 The classification given above is that usually adopted, but it may be 

 noted that the Ammonites are included in the Tetrabranchiata on 

 insufficient evidence. 



The Cephalopods are the most specialised of the Molluscs, 

 and present much variation of type. Nautihts appeared very 

 early and has persisted, apparently unchanged, until the 

 present, while the Ammonites and Belemnites, once so 

 abundant, have entirely disappeared. Among recent forms 

 we have Squid, Calamary, Octopus, Argonaut, and many 

 others. All swim freely in the sea, or lurk and creep 

 passively among the rocks. They are voracious eaters, and 

 consume very diverse kinds of animals, their parrot-like 

 jaws and powerful odontophore, as well as the numerous 

 suckers, rendering them formidable adversaries. 



A chambered external shell, serving as a house, is present 

 in Nautilus alone among living Cephalopods. In Spirula, 

 there is a spiral chambered shell, but it is very small, is 

 enclosed by the folds of the mantle, and is quite useless for 

 purposes of protection, Most of the extinct forms were 

 furnished with large and efficient shells of very variable 

 shape, some straight like Orthoceras, or coiled, with cham- 

 bers separated by complex septa, as in the Ammonites, and 

 so on. Most of the modern forms seem to be more active 

 than their ancestors, and their shells have degenerated. 

 While the fact of the degeneration is perfectly obvious, the 

 line along which it has taken place is difificult and still 



