THE CEPHALOPODA. 
173 
rows of sessile suckers. Tentacular arms elongated, 
funnel-valved. The shell, placed, vertically in the posterior 
part of the body, with the involute spire towards the 
ventral side (fig. 23 «), is delicate^, polythalamous, sub- 
internal, entirely nacreous, discoidal with separate whorls, 
and has a marginal or ventral siphon, c, concave septa, 
and round opening. The animal is retained in place by 
the tendinous filament which penetrates the siphon as 
shown in the subjoined figures, and by the retractor 
muscular masses surrounding the last chamber (fig. 23 
h, c). The dead shells are scattered profusely on the beach 
throughout the tropics. The animal is as yet imperfectly 
known, only one specimen was taken in the trawl by the ^la.^s.-Spiruiaaustraiis.L&m&rck. 
' a. Animal and shell m 
Scientific Staff of H.M.S. Challenger, durino; her explorinor ^- siieii entire showing chambers. 
" . c. Section shov/ing septa, chamber, and 
voyage round the world in 1873 — 7Q} An important siphuncie. 
addition to our knowledge of this form, however, has been recently made in the valuable 
memoir by Professor Owen, F.R.S.^ 
Order XL— TETRABRANCHIATA, Oiven, 1832. 
Tentaculifera, d'OrUgny, 1840; Cephalopodes polj^thalames, Lamarck, 1812. 
With the exception of one genus of which the Pearly Nautilus (fig. 24) may be 
regarded as a type, the whole of the Cephalopods of this order belong to fossil forms. 
They were all provided with an external polythala- 
mous or many-chambered shell, symmetrical in 
form, like the body of the animal which is lodged in 
its last or outer chamber. As we only know the 
anatomy of the living representative of this extensive 
extinct order, all our observations on the structure 
of the animal refer to the Nautilus pompilius, so 
carefully dissected and admirably figured and de- 
scribed by my old esteemed friend, Professor Owen.^ 
The head, which is closely approximated to the 
body, is provided with a great number of cylindrical, aimulated, retractile tentacles without 
acetabula, which surround the mouth, and are grouped as brachial, digital, ophthalmic, 
and labial, according to their position. The brachial, forty in number, are supported on 
1 Sir C. Wyville Thomson, 'The Voyage of the Challenger,' vol. ii, p. 350, 1877. 
2 " Supplementary Observations on the Anatomy of Spirula Australis,''' ' Ann. and Mag. of Nat. 
Hist.' for January, 1879. • 
3 'Memoir on the Pearly Nautilus ' Nautilus pompilius, Linn.), 8 plates, 4to, 1832. 
23 
Fig. 24. — Vertical section of the shell of Nautilus 
pompilius, showing tlie siphuncie and the situa- 
tion of the animal in the last chamber. 
