778 DIGITAL AND OPHTHALMIC TENTACLES. 



arrested while number 12 continues to grow, and thus appears for a time to be the 

 most important member of the confederation, until eventually its limit is reached, the 

 others forge ahead and number 12 retires behind the curtain which grows out from the 

 sheath of number 9 (cf. PL LXXX. figg. 7 and 4) 1 . 



The infrabuccal organ of the male consists of a bipartite laminated structure contained 

 in a pouch formed by the integument of the buccal membrane, which in adult specimens 

 projects far back into the peristomial haemocoel (PL LXXXII. fig. 3). It was first 

 described and figured by Van der Hoeven' 2 (1850) who regarded the lamellae as tentacles 

 modified in a manner similar to those of Owen's laminated organ in the centre of the 

 infrabuccal lobe of the female. 



Van der Hoeven's organ (PL LXXX. figg. 13 — 15) has recently been described in 

 some detail by Dr L. E. Griffin 3 . It consists in the adult of a pair of fleshy narrow 

 lobes parallel to one another, closely approximated in the middle line and bearing, upon 

 the outer surface of each about fifteen to nineteen lamellae of which only a certain 

 number are visible in external view, the others being concealed below them like the 

 leaves of a book. 



The lamelligerous lobes are united together at the base by an intermediate portion 

 which forms a sort of peduncle, and the adoral surface of lobes and peduncle is beset 

 with the openings of glands, which discharge their secretion into the cavity of the pouch 

 in which the organ is contained (PL LXXX. fig. 14). Often in preserved specimens 

 a coagulum is found at the orifice of the pouch. In sections cutting the lamellae at 

 right angles the epithelium clothing the surfaces of the lamellae appears to be richly 

 ciliated, an appearance due, as described by Griffin, to the occurrence of numerous 

 sensory cells bearing sense-hairs. 



In a very young male of iV. pompilius, the smallest specimen which I obtained, the 

 appearance of the infrabuccal organ is different from that which it presents in the adult. 

 Instead of being bent into a U -shape as it becomes at a later stage, the two halves of 

 the gland are extended in the transverse direction with a narrow median tract between 

 the ridges which represent the future lamellae (PL LXXIX. fig. 12). The appearance of 

 the infrabuccal organ at this stage suggests that its lamellae are homologous with the 

 tentacles only of the female infrabuccal organ, while the group of lamellae (Owen's 

 "olfactory" organ) between the tentaculiferous lobes is reduced to an insignificant median 

 tract in the male. If this be so, then the infrabuccal organ of the male is not simply 

 homologous with that of the female, but chiefly with the tentacular portion of the latter, 

 the median lamellae of the female organ being mostly suppressed in the male, and the 

 lateral tentacles of the former converted into lamellae in the latter. Apart, however, from 

 the interesting evidence furnished by the development of the male infrabuccal organ, its 

 composition in the adult suggests the same homologies which we may tabulate. 



1 The spadix of a young Nautilus of an age corresponding to that represented on PI. LXXX. figg. 4 — 6, 

 has been described and figured by Prof. Vayssiere op. cit. 1895, PI. 16, figg. 3 and 4, and PI. 18, fig. 14. 



2 Trans. Zool. Soc. London, Vol. iv. Part 1, 1850, PI. 8, fig. 9. 



3 Johns Hopkins Univ. Giro. xvm. November 1898, p. 12. 



