802 MORPHOLOGY OF THE TENTACLES OF NAUTILUS. 



graphy it is difficult to admit the probability of such a complete situs inversus, nor is it 

 necessary in order to co-ordinate the facts. 



The only author who has referred to the possibility of the epipodial origin of the 

 cephalopodium, so far as I know, is Professor Brooks 1 , and he adopts a somewhat pessi- 

 mistic attitude in the matter. Professor Brooks commences his remarks by referring to 

 what may be defined as the Velar Theory of Loven (1848) and Grenacher (1874), by 

 whom the arms of Cephalopoda were compared with the velum of other molluscan em- 

 bryos, but it cannot be said that this theory has enjoyed a great measure of success, and 

 Brooks has found that in Loligo, at the stage at which the ingrowth of the stomodoeum 

 takes place, the rudiment of a true velum appears on each side of the mouth in the 

 form of a faintly marked, undulating, ciliated band arising near the corners of the 

 mouth, and extending laterally to the eye-stalk, crossing the outer ventral edge of the 

 latter. The position of this line, its relation to the mouth and the eye-stalk, and the 

 presence of cilia upon it, are all indications that it represents a true velum. 



Professor Brooks adds that " the siphon originates as two pairs of folds 2 . . . . and 

 if we regard these four folds as homologous with the epipodial folds of a Gastropod, 

 the arms must be regarded as independently acquired structures. If we regard the 

 arms as modifications of the epipodial folds we must consider the four siphon folds 

 as independently acquired structures 3 , and as we have nothing whatever to furnish us 

 with a test, nothing seems to be gained by the uncertain homology of either the 

 arms or the siphon, with any part of the body of a typical Gastropod." 



On the other hand I think it will be no mean gain for morphology if we can 

 secure the recognition of the epipodial nature of the arms and the protopodial nature 

 of the funnel of Cephalopoda, and the longer I meditate upon the subject the safer do 

 these homologies appear to my mind. It is, as everyone knows, a difficult matter to 

 adduce a clinching argument in favour of a theoretical conclusion, and if the cumu- 

 lative weight of what has been brought forward in the preceding pages is not suffi- 

 cient to turn the scale, I fear it will be impossible to say more with advantage. 



The following is a summary of the special grounds upon which I base my Epi- 

 podial Theory : — 



1. Tentacles of Nautilus regarded as marginal appendages. 



2. Sequence of the tentacles of Nautilus and of their nerves. 



3. Topography of cephalopodium and siphonopodium, the dorsal position of the 

 former, the ventral position of the latter, and the deep longitudinal pleural raphe which 

 separates them. 



4. Comparison with Haliotis where we find in schematic disposition, mantle, epi- 



1 Brooks, W. K., "The development of the squid, Loligo pcalii (Lesueur)." Anniv. Mem. Boston Soc. 1880, 

 22 pp., 3 Plates. 



2 These folds clearly correspond with the anterior crura infundibuli and the posterior alae infundibuli of 

 the funnel of Nautilus. 



3 This is equivalent to saying, what I believe to be true, namely, that the method of development of 

 the funnel from paired primordia, is a cenogcnetic feature in the embryos of Cephalopoda which may be 

 correlated with the special adaptation of the protopodium which has resulted in the formation of the funnel 

 and with the abundance of yolk in the eggs of Cephalopoda, in which respect Nautilus surpasses all other 

 known genera. 



