DIGITAL AND OPHTHALMIC TENTACLES. 777 



The spadix is a composite erectile organ (PI. LXXVIII. fig. 4) which occurs indifferently 

 on the right or left side of the cephalopodium. It was originally described by Van der 

 Hoeven 1 , and has since been treated in greater detail by Professors Haswell 2 and Vayssiere 3 . 

 This organ shows a gradual development from the early to the adult life of the animal, 

 only attaining its full growth at sexual maturity (cf. PI. LXXX. figg. 3 — 7 and PI. LXXIX. 

 figg. 9 and 10). The ninth extrabuccal tentacle constitutes the glans and. its sheath, the 

 prepuce. The latter gives rise to a free fold along its outer border, and at the summit 

 bears a large thick glandular cushion covered with bordered pits, which may be described 

 as the preputial gland. The tenth tentacle (the second of the spadix) is a fleshy flattened 

 appendage strongly ribbed upon its glandular surface. The third tentacle of the spadix 

 (eleventh of the series) is also a flattened foliaceous appendage, but in place of the 

 transverse costae of the preceding tentacle, it presents a marvellous system of glandular 

 crypts, producing the appearance, as Professor Haswell pointed out, of a minute honeycomb. 

 The last tentacle, the twelfth extrabuccal, is somewhat enlarged, but otherwise undergoes 

 comparatively little modification and is partially concealed, on the outer aspect of the 

 spadix, beneath the flap of the prepuce. The second tentacle of the spadix may be 

 distinguished as the costate tentacle, the third as the tessellate tentacle, and the fourth as 

 the free tentacle or cirrus. 



Like all the other accessory sexual organs of both male and female, the spadix manifests 

 a gradual development pari passu with the growth of the animal. I use the word 

 " development " in contrast with " growth " to express the fact that in the young Nautilus 

 the character of the spadix is quite different from the adult structure of the organ. 



The smallest Nautilus which I ever obtained was a male N. pompilius 1 with the 

 following dimensions : — ■ 



Length from root of siphuncle to mid-anterior point of hood (measured 



along the dorsum) 5 25 mm. 



Length of hood along middle line 10"5 „ 



Breadth of body across middle of eyes 15 „ 



The shell was perforated at the umbilicus and contained thirteen air-chambers. 



At this stage it is not possible to say upon which side of the cephalopodium the 

 definitive spadix will appear, the two groups of four tentacles — numbers 9 to 12 of the 

 inner, whorl — being almost exactly alike, notably in regard to the tardy development 

 of the 10th tentacle (PL LXXX. figg. 1 and 2). 



After this stage the growth in length of tentacles 9 — 11 of the future spadix becomes 



1 Van der Hoeven, J., "Bijdragen tot de ontleedkundige kennis aangaande Nautilus pompilius L." Verh. 

 kon. Akad., Amsterdam, 1856. 



- Haswell, W. A., " Note on certain points in the arrangement and structure of the tentaculiferous lobes 

 in Nautilus pompilius." P. Linn. Soc. N. S. Wales, (2) x. 1895, p. 544, PI. 48. 



3 Vayssiere, A., op. cit., Ann. Sci. Nat. (8) n. 1896. 



4 Willey, A., " In the Home of the Nautilus." Natural Science, Vol. vi., June 1895, p. 410. The surface 

 of the hood was white and unpigmented when the specimen was brought to me, but this may have been 

 due to previous discharge of pigment from the epidermal cells of the hood. This often happens when the 

 animals are kept under unfavourable conditions. 



6 This length in an average adult male attains to about 140 mm. I have seen much larger shells of 

 N. pompilius from the Moluccan Sea than any that I obtained in New Britain. 



