Invertebrate Gallery of the Indian Museum. 135 



tacles ; the halves of the funnel, or siphon, are not united' 

 but are only placed in close apposition, so that the siphon 

 appears to be fissured ; and there is no ink-bag. 



The Dibranchiata include all the existing Siphonopods 

 with the exception of the Pearly Nautilus. In all of them 

 the gills are a single pair, the processes of the propodium 

 are sucker-bearing arms, the halves of the siphon are 

 united to form a complete tube, an ink-bag is present, and 

 the shell, when it exists, is " internal ", as already explained. 



The Dibranchiata form two sub-Orders, namely (i) the 

 Decapoda (Sepias and Squids) in which the '' arms " are 

 ten in number and an internal shell is present, and (2) the 

 Octopoda (Octopus and Argonaut) in which the " arms " 

 are eight in number and a true shell is absent. 



In Cases 170C-171C there are life-size models, 

 representing the chief types of both sub-Orders, along 

 with explanatory drawings, and specimens of the " internal 

 shells " of Spirula, Sepia and Loligo. The shell of 

 Argonauta (the Paper Nautilus), specimens of which 

 also are exhibited, requires a word of explantion. It 

 is not formed by the mantle, that is to say, is not a 

 true shell. It is found in the female only, and is secreted 

 by two of the " arms", which are specially dilated, and are 

 closely applied to the body to hold the shell on. 



The large drawing of the female Argonaut in Case 

 171C shows the nature of the dilated "arms", and their 

 relation to the body in life. Observe that the male of 

 Argonauta, models of which are shown, is much smaller 

 than the female, and is, like all other Octopoda, shell-less. 



4. MOLLUSCA LIPOCEPHALA. 



(Lamellibranchia : Pelecypoda). 

 [i^sk €as£5 172-179]. 



The Lipocephala, or Lamellibranchia, or Bivalves, 

 differ from all other Mollusca (i) in the non-development 

 of the head, and (2) in the absence of the odontophore. 



