208 



CEPHALOPODA. 



and 115 shx). This pit, which has already been spoken of as the 



FIG. 115. DIAGRAM OF A VERTICAL SECTION THROUGH THE MANTLE REGION OF AN EMBRYO 

 LOLIGO. (From Lankester). 



[This figure is turned the reverse way up to fig. 114.] 



ep. epiblast; y. food-yolk; m. mesoblast; m. cellular yolk membrane ; shs. shell- 

 gland. 



shell-gland, resembles very closely the shell-gland of other Mollusca. 

 The fold around the edge of the shell-gland grows inwards so as 

 gradually to circumscribe its opening, which before long becomes 

 completely obliterated ; and the gland forms a closed sack lined by 

 epiblast which grows in an anterior direction (figs. 114 and 127 cc/0- 

 The edges of the mantle now begin to project, especially on the 

 posterior side (fig. 127), and within the cavity formed by this projecting 

 lip there are placed the anus (an], gills, etc. The projecting lip of the 

 mantle is formed both of epiblast and mesoblast. The whole of the 

 anterior side of the mantle is filled by the elongated shell-sack (cell), 

 within which the shell or pen soon becomes secreted. 



There are certain difficulties in comparing the shell-gland of the 

 Cephalopoda with that of other Mollusca which will best be rendered clear 

 by the following quotation from Lankester 1 : 



" The position and mode of development of the shell-gland of the 

 Cephalopoda exactly agree with that of the shell-gland as seen in the other 

 Molluscaii embryos figured in this paper. We are therefore fairly entitled 

 to conclude from the embryologies! evidence that the pen-sack of Cephalo- 

 poda is identical with the shell-gland of other Mollusca. 



" But here forming an interesting example of the interaction of the 

 various sources of evidence in genealogical biology palaeontology crosses the 

 path of embryology. I think it is certain that if we possessed no fossil 

 remains of Cephalopoda the conclusion that the pen-sack is a special develop 

 ment of the shell-gland would have to be accepted. 



" But the consideration of the nature of the shell of the Belemiiites and 

 its relation to the pen of living Cuttle-fish brings a new light to bear on 

 the matter. Reserving anything like a decided opinion as to the question 

 in hand, I may briefly state the hypothesis suggested by the facts ascer- 

 tained as to the Belemnitidse. The complete shell of a Belemnite is essentially 

 a straightened nautilus-shell (therefore an external shell inherited from a 



1 "Development of Pond Snail." Quart. J. of Micro. Science, 1874, pp. 371374. 



