interpretation of the shell in recent cephalopods. 287 



found in its perfect form, and the development of these rare animals 

 is not known. The shell of Argonauta exemplifies other conditions 

 which will be discussed later (p. 294). In those recent Cephalopods 

 in which the development of the shell has been investigated, it lies, 

 enclosed in the mantle (and is thus internal) on the antero-dorsal 

 surface of the body, and either develops into the so-called pen, of a 

 horny character {OmmaUrephes, Loligo, and others) or consists of 

 numerous calcareous layers built upon a horny foundation {Sepia). 

 It forms in an invagination of the ectoderm, the shell-gland. 



In a very early embryonic period, a depression appears at the 

 centre of the mantle-rudiment, this being the first indication of the 

 shell-gland (Figs. 115, 116, p. 255, Fig. 131 D, p. 280). At first, in 

 Loligo, this is a wide, shallow pit, but the margin of the pit soon grows 

 inwards and constricts its aperture, the pit finally assuming the form 

 of a sac connected with the exterior by a small opening. This sac is 

 lined with an epithelium composed of cells which, at its base, 

 are specially long, and is surrounded by mesodermal tissue. The 

 aperture of invagination completely closes at a later stage, and the 

 shell-gland then lies internally as a closed sac, surrounded by the 

 mesodermal tissue (Fig. 132 C). It extends later especially anteriorly 

 and then occupies a large part of the antero-dorsal side of the mantle 

 (Fig. 133). The secretion of the shell then takes place within it 



(USSOW, BOBRETZKY). 



The Interpretation of the Shell in Recent Cephalopods- 



There can be no doubt that we have in this case an internal shell, 

 hut the question remains, \\ hat is its relation to the large external 

 shell met with in the (living) NautilibS, in the Ammonites and other 

 extinct forms 1 This is a point of importance in studying the manner 

 of formation of the Cephalopodan shell and its relation to that of 

 other Molluscs. In solving this question it is necessary to institute 

 comparisons with the shells of various extinct forms. 



The shell of the recent THbranchia is very differently developed in the 

 various forms. It occasionally consists merely of a long, narrow, horny, 

 plate, shaped like a symmetrical bird's feather, but otherwise not specially 

 ditTerentiated (c.i/., in Loligo . In other cases, the plate is less simply formed, 

 its posterior end forming a hollow cone, the whole shell thus being slipper- 

 shaped (Ommastrephes, Fig. 140). 



The calcareous shell of Sepia (Figs. 137, 138 .4) is much more complicated 

 in structure, and is composed of many calcareous layers. Its structure is still 

 not fully understood, but it has been compared with the more highly 



