70 



COMPARATIVE ANATOMY 



CHAP. 



that is, if this interpretation is correct. This shell is somewhat bent, the septa 

 are crowded together and slope downwards anteriorly. They are penetrated 

 posteriorly by an extremely thick siphon, which is enclosed throughout its whole 



length in an envelope with a very thick anterior 

 wall. The completely enclosed siphuncular 

 space is thus a wide funnel running through the 

 chambers of the shell on its posterior side (Fig. 

 67 B). The phragmocone is enclosed in a thick, 

 strongly - developed rostrum, and its anterior 

 and lateral walls are produced downwards into 

 a broad, posteriorly concave shell (pro- 

 ostracum ?). 



These arrangements seem to have culminated 

 in the extant Sepia (Figs. 67 A and 68). The 

 siphuncular space fits over the visceral dome 

 like a mould. The anterior portions of the 

 septa slope downward much more obliquely 

 from behind anteriorly, so that, in a back view 

 of the shell, the whole area of the last septum is 

 visible at the surface (Fig. 68, 1). The septa 

 are thin calcareous lamellae, closely superim- 

 posed one upon the other, with very narrow air 

 chambers between them ; and these latter are 

 traversed by perpendicular trabeculae. The shell 

 is thus very light, its specific gravity is less than 

 that of water. Behind the siphuncle, on the 

 posterior very much shortened side of the shell, 

 the short septa are closely contiguous, without 

 any intervening air spaces. 



The dorsal end of the shell is enclosed in a 

 small pointed rostrum. The whole anterior sur- 

 face is covered by a thin lamella of conchy olin, 

 which projects laterally beyond the edge of the 

 shell, and is itself covered by a calcareous layer 

 which is an anterior and ventral extension of 

 the rostrum. 



The female Argonaut is the single exception 

 to the rule stated above, that in the Octopoda 

 the shell has entirely disappeared. This animal 

 has a light, thin external shell coiled anteriorly 

 or exogastrically, which is not firmly attached 

 to the body at any point, and serves more for 

 receiving the eggs (Figs. 35, 36, pp. 24, 25) than 

 for protecting the body. This shell is sur- 

 rounded and secured by lobate processes of the 

 anterior pair of arms. It has no nacreous layer, but is porcelanous, and is 

 apparently produced from the integument of the visceral dome and the mantle. 

 The dorsal pair of arms is said only to deposit the so-called black layer on its 

 surface. 



It is usually considered that this Argonaut shell is not the homologue of the 

 shell of other Cephalopods, but is a formation peculiar to the Argonaut female. An 

 opposite view has, however, recently been very ably advanced that the Argonaut 

 shell is an Ammonite shell which has lost its septa and siphuncle and also its 



-i4 s 



r 



FIG. 68. Shell of Sepia aculeata. 

 Posterior (physiologically ventral) aspect. 

 Lettering as in Fig. 67. The last septum 

 1 is seen in its whole extent ; s, the mouth 

 of the broad, slipper-shaped siphuncular 

 cavity ; I, lateral wall of the cavity ; a-/3, 

 line of the section which in Fig. 67 A is 

 diagrammatised. The two figures should 

 be compared (principal details after 

 D'Orbigny). 



