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 a), 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 

 b, 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 

 Scientific Staff of H.M.S. Challenger, during her exploring 

 voyage round the world in 1873 — 76.^ An important 

 addition to our knowledge of this form, however, has been recently made in the valuable 

 memoir by Professor Owen, F.R.S.^ 



Fig. 23.—Spirula australis. Lamarck. 

 a. Animal and shell in situ, 

 i. Shell entire showing chambers, 

 c. Section showing septa, chamber, and 

 siphuncle. 



Order H.— TETRABRANCHIATA, Owen, 1832. 

 Tentaculifera, d'Orhigny, 1840; Cephalopodes polythalames, LamarcJc, 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, annulated, 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 ou 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 the siphuncle and the situa- 

 tion of the animal in the last chamber. 



