248 CEPHALOPODA. 



early completely enclosed by the blastoderm. A well developed outer yolk- 

 sack is present during early embryonic life, but is completely absorbed 

 within the body before its close. Cilia appear on the blastoderm very early, 

 but vanish again when the yolk is about two-thirds enclosed. There is, 

 during embryonic life, no trace of a shell, but the mantle and other parts of 

 the body become covered by peculiar bunches of fine setae. The shell-gland 

 develops normally in both Octopus and Argonauta, but disappears again 

 without closing up to form a sack (Lankester). 



The pelagic Decapod larva described by Grenacher, which 

 forms my second type, must be placed with reference to the de- 

 velopment of the yolk-sack at the opposite pole to Sepia. Seg- 

 mentation, as in other Cephalopods, is partial, but the blastoderm 

 almost completely envelops the yolk before any organs are de- 

 veloped ; and no external yolk-sack is present. At a stage 

 slightly before the closure of the yolk-blastopore the mantle is 

 formed as a slight prominence at the blastodermic pole of the 

 egg, and even at this early stage is marked by the presence of 

 chromatophores. The edge of the blastoderm is ciliated. At a 

 slightly later stage the embryo becomes more cylindrical, the 

 edge of the mantle becomes marked by a fold, which divides the 

 embryo transversely into two unequal parts, a smaller region 

 covered by the mantle, and a larger region beyond this. The 

 yolk is still exposed, but rudiments of the optic pit and of two 

 pairs of arms have appeared. The first-formed arms are appa- 

 rently the anterior, and not, as in Sepia, the posterior. 



At a still later stage, represented in lateral and posterior 

 views in fig. 113 A and B, considerable changes are effected. 

 The yolk-blastopore is nearly though not quite closed. The 

 mantle fold (int) is much more prominent, and on the posterior 

 side on a level with its edge may be seen the rudiments of the 

 gills (br). The funnel is formed as two independent folds on 

 each side (in/ 1 and mf*), which apparently correspond with the 

 two divisions of the funnel rudiments in Sepia. The eye has 

 undergone considerable changes. Close to each rudiment of the 

 funnel may be seen a fresh sense-organ the auditory sack (ac). 

 The ventral (upper in the figure) end of the body now forms a 

 marked protuberance, probably equivalent to the foot of other 

 Mollusca (vide p. 225), at the sides of which are seen the rudi- 

 ments of the arms (i, 2, 3). To the two previously present a 

 third one, on the posterior side, has been added. The blastopore 



