776 DIGITAL AND OPHTHALMIC TENTACLES. 



omitted, and I have also not reckoned them numerically with the digital tentacles, but 

 their morphological valency as the first two tentacles of the outer whorl of the cephalo- 

 podium of Nautilus is I think clear, and I shall again refer to the recurrent nerve 

 of the posterior ophthalmic tentacle 1 . 



The inner whorl of the cephalopodium is incomplete dorsally, and moreover it is 

 broken up into distinct tentaculiferous and lamelligerous lobes which have become more 

 or less specialised as accessory sexual organs, and exhibit profound sexual differences 

 which are tolerably familiar to malacologists. There are two principal groups of tentacles 

 of the inner whorl which may be described as extrabuccal and infrabuccal, and correspond 

 to Owen's " external labials "and "internal labials " respectively. The objections to Owen's 

 terminology are firstly that the epithets " external " and " internal " obscure the view that 

 the tentacles belong to an inner whorl, and secondly the expression " labial " is inappropriate 

 and apt to cause confusion with the labial papillose fringe which surrounds the jaws. 



In the female the extrabuccal lobes carry twelve ordinary tentacles on each side, 

 of which eight are disposed in a descending and four in an ascending series. The lobes 

 themselves consist of the coalescent sheaths of the tentacles borne upon a fleshy basis 

 which rises up from the fundus of the fossa buccalis. The extrabuccal lobes adhere 

 ventrally to the inner surface of the outer whorl 2 by means of an intercoronal membrane 

 which proceeds on each side from the ventral border of the lobe (cf. PI. LXXX. figs. 7 

 and 8, i). 



In the male the extrabuccal tentacles are subdivided into three groups, an upper 

 series of eight tentacles with coalescent sheaths, a middle set of three, and a single 

 ventral free tentacle. The free tentacle is usually found to nestle within a groove formed 

 by a vertical flap which grows out from the sheath of the first tentacle of the middle 

 set or the ninth of the entire series, or it may follow simply at the end of the series. 

 The middle set comprising the ninth to the eleventh extrabuccal tentacles, together with 

 the free twelfth tentacle, constitute a natural group of four which are known as the 

 spadix on the one side and the antispadix on the other. The intercoronal membrane 

 arises in the male from the lower border of the upper set of coalescent tentacular sheaths 

 (1 — 8), and passes unattached in front of the tetrad group to its insertion into the 

 inner wall of the outer (annular) lobe. 



The ventral or infrabuccal lobe of the inner whorl is modified in both sexes to form 

 a special infrabuccal apparatus. In the female it consists of a median laminated organ 

 flanked on each side by a uniserial row of 12 — 14 tentacles; the lamellae are modified 

 tentacles and retain their apices (PL LXXIX. fig. 6, and PI. LXXX. fig. 11). In the 

 male there are no unmodified tentacles remaining in connection with the infrabuccal 

 apparatus, which is completely metamorphosed into a laminated organ, known as Van der 

 Hoeven's organ, retractile within a pouch and closely adherent to the ventral integument 

 of the buccal cone. 



I do not propose to enter upon a long description of these structures, partly because 

 this has already been done by others, and partly because I was unable to ascertain the 

 manner in which the various parts are brought into action. 



1 In my experience the most suitable material for the successful dissection of the tentacular nerves of 

 Nautilus consists of soft specimens and not of such as have become too hard under preservation. 



2 The outer whorl is the "oral sheath" of Owen the "annular lobe" of Laukester. 



