8 W. K. BROOKS ON THE 



in a surface view. In the centre of the mantle area there is a shallow circular depression, 

 the shell area, s. The eye-stalk is much more prominent than it was at the previous 

 stage, and the layer of integument which covers it is very much thickened, and the 

 surface of the yolk at this point much further from the surface of the body than 

 it was at the stage shown in fig. 6. Near the centre of the eye-stalk, a depression 

 or invagination of the integument marks the position of the developing eye ; which 

 originates, as has been well described by Lankester and Grenacher, by the involution 

 of the ectoderm of the eye-stalk. Between the mantle and the eye-stalk on each side of 

 the body, another elevation or ridge has made its appearance, the outer siphon-fold, si. 



Fig. 8 is a view of the left side of an embryo of about the same age as the one shown in 

 fig. 7. As before, m is the mantle ; e, the eyestalk ; b, the growing edge of the blasto- 

 derm ; y, the uncovered end of the yolk ; and a, the arm-ridge. The next two figures, 

 plate 1, fig. 9, and plate 2, fig. 10, are of especial interest, since they represent two views 

 of an embryo which exhibits more clearly than any other which has been figured, the 

 relation between the Cej)halopoda and the ordinary Gasteropod Molluscs. The embryo 

 seems to be in substantially the same stage of development as the one shown in Grenacher's 

 figs. 6 and 7, but the differences between the two are very considerable, as well as very 

 instructive. 



KoUiker's figs. 17, taf 1, and 25, taf 2, as copied in Bronn's " Klassen u. Ordnungen," 

 also represent an embryo at about the same stage of development ; but the Sepia embryo 

 differs much more from the Loligo embryo than Grenacher's form does. The latter 

 and Sepia are extreme forms, with Loligo intermediate between them in most respects, 

 but with much closer resemblance to the first than to Sepia. Plate 11, fig. 10, is a 

 slightly oblique view of the anterior surface of an embryo which, like those shown in 

 figs. 6 and 7, has its dorsal surface above, and which is drawn in such a position as to 

 show rather more of the left side than of the right. Plate 1, fig. 9, is a foreshortened 

 view of the same embryo, as seen from above, in such a position as to exhibit the dorsal 

 and posterior surface. 



The mantle, m, fig. 9, is now quite sharply defined, and its posterior edge, m, fig. 10, 

 begins to overhang a little, arching over the rudimentary mantle cavity, which is 

 thus seen to be formed in Loligo, as in most Gasteropods and Lamellibranchs, by the 

 outgrowth of a flap of integument from the surface of the body. A reference to Gren- 

 acher's figures will show that -the origin of the mantle cavity was quite different from 

 this in the form which he studied. At a corresponding stage (see his figs. 6 and 7), the 

 mantle area is much larger than in Loligo, and covers almost one half of the body of 

 the embryo, and is more developed externally than it is in Loligo at the stage shown in 

 our fig. 15. The mantle cavity, however, is barely indicated, and when it makes its 

 appearance it is not formed, as it is in Lohgo, by the outgrowth of a mantle-fold, but by 

 the involution of the integument under the mantle, after the latter has spread over a 

 considerable area of the body. The resemblance between the manner in which the 

 mantle cavity is formed in most Gasteropods and Lamallibranchs, and the way it is formed 

 in Loligo, seem to indicate that the latter presents the primitive mode of development 

 among the Cephalopoda ; and the way it is formed in Grenacher's species must, therefore, 

 be regarded as a modification of the primitive mode which has been retained by Lohgo. 



