264 CEPHALOPODA. 



occupies the place of the yolk-sac, i.e., is surrounded by the arms, a 

 position which, as is well-known, is occupied by it in the adult. 



When the yolk-sac begins to degenerate, i.e., in the last stages of 

 development, the embryo approaches ever nearer the form of the 

 adult (Fig. 121). The arms are still rather small, the eyes still 

 remind us of their former condition, but the funnel, the mantle, the 

 gills and the anal papilla have almost attained their final form. The 

 chromatophores have already formed and they also give the embryo 

 a characteristic appearance more like that of the adult. The 

 ch rowatophores appear first on the mantle on the posterior (postero- 

 dorsal) side and are only somewhat later found on the arms and on 

 the head. When the embryo leaves the egg, the external yolk-sac 

 has, to a great extent, disappeared. 



(b) Octopus. 



Although, systematically, Octopus is far removed from Loligo, the 

 course of development of the two forms is very similar. The em- 

 bryonic rudiment extends at first over a larger part of the egg and 

 later withdraws again more towards the animal pole, just as in Loligo. 

 The shell-gland appears very early, at a time when the blastoderm 

 has not yet grown round the yolk, as a depression at the animal pole. 

 This fact is of special interest, because Octopus, like most other 

 Octopoda {Cirrhotenrihis perhaps forming an exception?) has no 

 internal shell. The shell-gland in this case, therefore, has the 

 significance of a vestigial orpin. At somewhat later stages, when 

 the rudiments of the external organs appear, it can be recognised as 

 a distinct depression at the apex of the rudiment of the mantle, and 

 even later is perceptible (Fig. 122 A). According to Ray Lankester, 

 it disappears without having closed. 



At a stage somewhat earlier than that illustrated in Fig. 122 A, a 

 pair of small prominences appear on the mantle, resembling, in shape 

 and position, the fins described in Loligo. Indeed, the aspect of the 

 mantle-rudiment in Octopus in the younger stages closely resembles 

 that of the mantle-rudiment of Loligo. The prominences are retained 

 for some time; they can be recognised in Fig. 122 J and B, and in 

 later stages arc still present, but, finally, they decrease in size and 

 altogether disappear. They cannot be regarded as anything else than 

 the vestiges of a pair of fins, and must be considered as an indication 

 thai tin- Octopoda originally carried fins, like the Decapoda. This 

 fact affords further support to the view which in itself is probable 



