800 MORPHOLOGY OF THE TENTACLES OF NAUTILUS. 



The siphonopodium seems to be a more highly specialised organ than the platy- 

 podium, and for this direct reason in combination with collateral considerations, it is 

 probably more useful at present to accept the dogma that the foot of the molluscan 

 archetype was a platypodium. 



17. Morphology of the Tentacles of Nautilus. 



The two theories of the special nature of the tentacles of Nautilus, considered 

 merely from a cephalopodan standpoint, have been referred to in a former chapter, and 

 may be tabulated in chronological order as follows: — 



1. Acetabular Theory of Valencieimes (1841), according to which a tentacle of Nau- 

 tilus, consisting of the sheath and the cirrus, is homologous with a sucker of a Dibranch 

 consisting of cupule and caruncle. 



2. Branchial Theory of Owen (1843), according to which a tentacle of Nautilus is 

 homologous with an arm of a Dibranch. 



Both of these theories have received authoritative support, and neither of them is to 

 be lightly rejected, since neither is capable of such final jjroof as would satisfy the higher 

 criticism. 



The independent innervation of the individual tentacles of Nautilus, the demonstration 

 of their definite suctorial ridges by which they are enabled to adhere to surfaces with great 

 tenacity, and the virtual identity of their internal structure with that of a true arm, 

 weigh so heavily, to my mind, on the side of the Branchial Theory, that I must follow 

 Owen in admitting their equivalence with the arms of the Dibranchs just as the multi- 

 tudinous legs of a Myriapod are individually equivalent to the legs of a Hexapod insect. 

 It was further suggested by Owen that the inner whorl of the cephalopodium of Nautilus 

 may be represented in the decapod Calamaries and Cuttle-fishes by the two long re- 

 tractile prehensile arms or anchors, which are provided at the base with a kind of sheath. 



It is worth repeating that the suctorial ridges of Nautilus actually tend to become 

 semilunar or half-cujJ-shaped towards the apices of the outermost tentacles, a feature 

 which is especially noticeable in the ophthalmic tentacles. Nevertheless it is quite possible 

 that a single ridge does not corresjDond with a single sucker, but rather with a trans- 

 verse row of suckers such as occurs in Gonatus, where the arms carry four series of 

 suckers'. A curious analogy is afforded by the second and third tentacles of the spadix 

 in Nautilus, the transverse costae of the former being represented by the transverse rows 

 of alveoli in the latter (cf PI. LXXIX. fig. 9). 



In support of the Acetabular Theory of Valenciennes it is of great historical interest 

 to know that the case of Girroteuthis was quoted by Johannes Miiller at a meeting of 

 the Berlin Academy on the occasion of the communication by Humboldt of an abstract 

 of the results achieved by Valenciennes in his work upon the anatomy of Nautilus^. 

 Johannes Miiller is reported to have said that the Girroteuthis of Eschricht has arms which 



1 Cf. Hoyle, W. E., op. cit. P. Zool. Soc. London, 1889. 



' Bericht Akad. Berlin, 1841. Bemerkungen des Hrn. Prof. J. Miiller, p. 58. Translated in Ann. Nat. 

 Hist. Vol. VII. 1841, pp. 241—245. 



