DIGITAL AND OPHTHALMIC TENTACLES. 771 



The above tables seem to betray ineradicable divergencies, due merely to different 

 points of view. There is, however, one door left open for immediate criticism. It might 

 at least have been expected that there would be agreement as to the first and the 

 last, but such is not the case. Vayssiere's nineteenth tentacle is my eighth, and my last 

 is Vayssiere's seventeenth. 



It was admitted by Owen and his successors that the main bulk of the hood consists 

 of "two digitations conjoined along the mesial line 1 ." "The median antero-posterior line 

 traversing this hood exactly corresponds to the line of concrescence of the two halves 

 of the fore-foot, which primitively grew forward, one on each side of the head, and finally 

 fused together along this line 2 ." Seeing that the lateral portions of the hood are actually 

 formed by the cohesion of the sheaths of the second pair of tentacles with the main 

 body of the hood, it appears reasonable to infer that the latter arose by concrescence 

 of the sheaths of the first pair. The median portion of the hood of Nautilus is thus 

 taken to represent a dorsal symphysial tract. 



I have found no reference to the occurrence of a ventral symphysial tract 3 and yet, 

 I think, that is the nature of the bridge which forms the fundus of the concavity which 

 lodges the funnel, and at the same time completes the outer circlet (annular lobe of 

 Lankester) across the ventral median line. At the corners of this tract are placed the 

 tentacles of the nineteenth pair, in very much the same way as the tentacles of the 

 first pair occur near the outer borders of the median portion of the hood, and I think 

 it likely that this ventral tract owes its origin to the concrescence of the sheaths of the 

 last pair of tentacles, just as the dorsal tract admittedly represents the fused sheaths of 

 the first pair. An indication of the bilateral constitution of the ventral tract is furnished 

 by the mesial line which divides the laminae of the mucous gland which develops upon 

 its inner surface in the female 4 . If it is true that there is a ventral symphysial tract, then 

 it is clear that the tentacles which are most intimately associated with it and immediately 

 border it are to be reckoned as the last of the series. 



Assuming then that my identification of the nineteenth pair of tentacles of the outer 

 circlet is correct, it still remains to justify the enumeration of the tentacles which 

 intervene between the first and the last. Taking into consideration their evident character 

 as marginal appendages of an annular lobe, we may further assume that the border out 

 of which they arose was not plain but frilled like an undulating membrane, and that 



1 Owen, Memoir 1832, p. 13. Owen seems to have erred somewhat in his counting of the tentacles 

 through anxiety on behalf of even numbers. The passage runs as follows : — " Of the digital processes nineteen 

 have been enumerated on either side ; but as the hood has two perforations anteriorly from which tentacles 

 similar to those of the digitations are also protruded, we may consider this part as two digitations conjoined 

 along the mesial line, and so reckon with Eumphius the even number of twenty digitations on either side 

 of the head." 



2 Lankester, E. E., Article " Mollusca," Encycl. Brit., 9th edit., republished in Zoological Articles, 1891, see 

 p. 137. 



3 Except perhaps in the memoir by Valenciennes (1841), who describes the two inferior arms as being 

 united below by a thin plate hollowed out to receive the funnel, while on the inner surface of the plate, 

 on each side of the median line of junction, there is in the female a lamellated organ, the same which 

 has recently been shown by Mr Graham Kerr to serve as a receptacle for the spermatophore. 



4 Cf. Kerr, J. G., " On some points in the Anatomy of Nautilus pompilius." Proc. Zool. Soc. Lond. 

 1895, see PI. 39, fig. 3. I shall subsequently refer to this laminated gland as the " Organ of Valenciennes." 



w. vi. 101 



