MOLLUSC A 205 



the embryo to the yolk will best be understood by reference to the 

 longitudinal section of Loligo, fig. 127. 



The arms gradually increase in length, and the second pair passes 

 in front of the first so as eventually to lie completely in front of the 

 mouth. The arms thus come to form a complete ring surrounding 

 the mouth, of which the original second pair, and not, as might be 

 anticipated, the first, completes the circle in front. The second pair 

 develops into the long arms of the adult. 



After the embryo has attained more or less completely its definite 

 form (fig. 112 C) it grows rapidly in size as compared with the yolk- 

 sack. The latter structure is at first four or five times as big as the 

 embryo, but, by the time of hatching, the embryo is two to three 

 times as big as the yolk-sack. 



Loligo mainly differs from Sepia in the early enclosure of the yolk by 

 the blastoderm, and in the embryo exhibiting the phenomena of rotation 

 within the egg-capsule so characteristic of other Mollusca. 



In Argonauta the yolk-sack is still smaller than in Loligo, and the 

 yolk is 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 sette. 

 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 Grenadier, which forms 

 my second type, must be placed with reference to the development of 

 the yolk-sack at the opposite pole to Sepia. Segmentation, as in 

 other Cephalopoda, is partial, but the blastoderm almost completely 

 envelopes the yolk before any organs are developed ; 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 cylin- 

 drical, 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 apparently 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 (mt) 

 is much more prominent, and on the posterior side on a level with 

 its edge may be seen the rudiments of the gills (6r). The funnel is 

 formed as two independent folds on each side (inf 1 and in/ 2 ), which 

 apparently correspond with the two divisions of the funnel rudi- 



