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POPULAR SCIENCE REVIEW. 
a rare animal. Unlike the cuttle-fish, it has a large shell, 
which, though pallial in its origin, is external in position. 
Moreover this shell is divided transversely by a succession of 
partitions, connected by a tube traversing them, all termed the 
“ siphuncle ” (fig. 6 s). The animal itself only inhabits the last 
chamber of its shell, which serves well for its shelter and pro- 
tection. The nautilus differs from the cuttle-fish mainly in the 
presence of this peculiar external shell, in having four gills in 
the mantle cavity instead of only two ; in there being a great 
number of tentacles all devoid of acetabula, instead of not more 
than ten, and these %vith acetabula; in being destitute of 
branchial hearts ; in having the beak partly calcareous, instead of 
entirely horny ; and, lastly, in having no ink-bag, the protection 
afforded by its shell no doubt rendering the sheltering obscurily 
producible by such a secretion much less necessary. 
The characters of this animal are of interest because it is the 
type of a very large group of cephalopods, which, as living forms, 
have now passed away from the surface of this planet — that is, 
unless deep dredging should bring to light any ancient form 
still holding out, as has been lately done by Dr. Carpenter for 
Echinoderms. 
The ammonites (fig. 7) are fossil forms, essentially resembling 
the nautilus as to the hard parts, and no doubt similar also in their 
softer structures. The nautilus, the ammonites and their allies 
appear, one or other, to have existed during the whole primary 
and secondary geological periods ; but what is more singular is 
that these four-gilled cephalopods appear in ancient times to 
have represented in the economy of nature creatures of the 
w^helk class, which do not then appear to have existed in any 
number, while with the progress of time the four-gilled 
cephalopods have all but disappeared, while the whelk-like class 
of molluscs has increased more and more, and now has com- 
pletely taken the place of their more highly organised and 
complex predecessors. 
Returning once more to the cuttle-fish (and recalling to 
mind the concluding observations previously made as to the 
lobster), we may note sundry fundamental facts of structure. 
1 . The nervous system is disposed in three pairs of ganglia, 
and is not in the form of a chain either dorsal or ventral. 
2. No elongated solid structure separates the nervous centres 
from the alimentary canal. 
.3. The most anterior part of the alimentary canal passes 
between the nervous centres. 
4. The limbs are more than four in number. 
5. There is no portal system. , 
6. In development no visceral clefts appear. 
7. The jaws are not modified limbs. 
