MOLLUSCA. 247 



The arms I 5 form a ring outside these parts. The whole of 

 the embryo, with the exception of the gills, the funnel, and the 

 outer border of the blastoderm, is richly ciliated. 



The embryo up to this time has had the form of a disc or 

 saucer on the surface of the yolk. After this stage it rapidly 

 assumes its permanent dome-like form, and becomes at the same 

 time folded off from the yolk. The blastoderm is very slow in 

 enveloping the yolk, and the whole yolk is not completely in- 

 vested till a considerably later stage than that represented in fig. 

 1 1 1 B. As soon as the blastoderm covers the yolk-sack cilia 

 appear upon it. The mantle grows very rapidly, and its free 

 border soon projects over the funnel and gills. After the two 

 halves of the funnel have coalesced into a tube, it comes to pro- 

 ject again beyond the edge of the mantle. 



On the completion of the above changes the resemblance of 

 the embryo to a Cuttle-fish becomes quite obvious. Three of 

 the stages in the accomplishment of these changes are represent- 

 ed in fig. 1 1 2. 



To the ventral side of the embryo is attached the enormous 

 external yolk-sack (yk\ which is continuous with an internal 

 section situated within the body of the embryo. The general 

 relations of 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 em- 

 bryo 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 



