310 Mr. J. F. Blake on the 



which only comes in the third period of the development of 

 the latter ; and, moreover, the Spirula, which is most nearly- 

 allied to the Nautilus by its siphonated shell, has the smallest 

 ink-bag. Again, the Nautilus has its eye a simple cavity, 

 opening externally by a minute aperture ; and this is one stage 

 of the development of the eye of a Dibranchiate. In the 

 Nautilus the auditory organs are found close beneath the eyes ; 

 in the Dibranchiates they are at first found in the same posi- 

 tion, and only gradually grow closer and closer together till 

 they come into contact with each other on the ventral side. 

 Finally the tentacular and labial processes of the Nautilus are 

 flattened more or less, and lie one within the other. In the 

 development of the Dibranchiate the arms rise as broad flat 

 processes also, and one pair lies within the rest. 



This last point throws light upon another question which I 

 wish to discuss — namely, whether the six or eight processes 

 on which the tentacles of the Nautilus are found are homolo- 

 gous to the eight arms of the Octopus, each tentacle repre- 

 senting a sucker, or whether each tentacle is homologous to a 

 whole arm of an Octopus, the number having been greatly 

 reduced. The former view was propounded by Valenciennes *, 

 but has been contested by Owenf. Though to my mind 

 highly interesting and suggestive, it has scarcely been noticed 

 by other writers. Prof. Owen brings forward four reasons 

 against this homology. First, that the general order of de- 

 velopment is from the multiple to the simple, and therefore we 

 ought to expect more arms in the Nautilus. In order that 

 this might be true of the Cephalopod's arms we ought to find 

 in the development of the Dibranchiates that they arose in 

 greater numbers, and ultimately grew together into the eight. 

 But Grenadier has now shown that first three arms arise as 

 simple broad expansions on each side, and at a later period 

 the suckers and the other arms appear, the fourth pair being 

 but a process of the third, while of the first three the earlier 

 ones lie partially within the later, and the third is the largest ; 

 so that, if we accept the above homology, the Nautilus exactly 

 represents an early stage in this as well as in other respects ; 

 for it also has the fourth or anterior pair but feebly developed, 

 being represented by but one tentacle beneath the hood. The 

 third pair are the largest, and the other two are surrounded by 

 it. Thus development in this case is not from the multiple 

 to the simple in Prof. Owen's sense. It is, however, so in 

 another sense, and in one which makes for this homology. 



* Annates du MusSe, 1841. 



t Ann. & Mag. Nat. Hist. 1843, vol. xii. 



