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POPULAR SCIENCE REVIEW. 
Where the shell is absent or rudimentary, its protective func- 
tion is supplied either by greater activity on the part of the 
animal itself, as in the case of the “ squids,” or by organs of 
concealment, as in the case of the inky fluid possessed by the 
Poulpes and cuttle-fishes, with which they cloud the water, and 
so favour their escape. 
In the present paper I propose to point out certain peculi- 
arities in the shells of the Cephalopoda, and to inquire whether 
there is any ground for the still-prevalent idea that these 
structures are unlike any of their congeners in the Molluscan 
sub-kingdom. 
The external-shelled division of the Cephalopoda, represented 
at the present day by the “ Pearly Nautilus ’ alone, but in the 
past by the Nautilus, Ammonite, Groniatite, Orthoceratite, and 
a host of other forms, belongs to the Tetrabranchiata (four- 
gilled), and were once as extensively represented in the ancient 
seas of our globe as the naked or internal-shelled Dibranchiata 
(two-gilled) are in the seas of to-day. 
Only one other siphonated shell is now known amongst 
living Mollusca, besides the pearly Nautilus ; it is the beau- 
tiful little internal pearly shell of the Spirula (fig. 2), a small 
form of Cephalopod belonging to the Dibranchiate division of 
the class in which are placed the internal-shelled, active, free- 
swimming squids, cuttle-fishes, and the shell-less Octopus , so 
largely represented at the present day in the seas of all parts of 
the world. 
The chambered character of the shell, with its siph uncle, 
appears to be a unique molluscan structure entirely confined to 
the Cephalopoda. Nevertheless, many shells, belonging to 
widely-different families of the Mollusca, are earner ated (i.e. 
divided into chambers, or are vaulted). 
Thus, for example, certain species of Helices (as the Helix 
decollata ) always lose the apex of their spire on attaining their 
full growth. But before this takes place the animal has 
already formed a septum or septa within its shell, effectually 
shutting off the upper whorls from the inhabited portion. 
Vermetus (see Plate, figs. 8, 9), Euomphalus (fig. 5), Triton 
(fig. 6), and Turritella , all form internal septa to their shells. 
Aged Oysters (fig. 12), Gryphcea , Spondylus , JUtheria , and 
Teredo (fig. 10), also form partitions within their habitations. 
The genus Caprinella (fig. 11), one of the Hippuritidse, is 
remarkable for the symmetrical regularity of its septal parti- 
tions. These cavities in the lower, deep valve, of bivalve shells, 
generally contain water, and are called “ water-chambers .” 
The adult Magilus (fig. 7), on the contrary, fills up its 
shell, not with septal partitions or cancellated shell-structure, 
but with a solid, compact mass like arragonite. 
