754 SIPHUNCLE AND PALLIAL VESSELS. 



In comparing Nautilus with the dibranchiate Cephalopods Professor Lankester (1883, 

 Encyc. Brit.) points out that " in the former the base of the fold forming the mantle- 

 skirt comprises in its area a part of what is unreflected visceral hump in the latter." 



The consequence of this invasion of organs into the substance of the mantle is 

 that when the ventro-lateral fold of the latter is pulled back and turned inside out in 

 order to expose the pallial organs, the normal antero-posterior relations of parts are 

 inverted and what is actually placed anteriorly in the natural position of the mantle 

 (PI. LXXV.) becomes posterior when the mantle is retroverted (PL LXXVL). This 

 simple fact has led to much confusion of terminology, actual topographical conditions 

 being confused either with mechanically retroverted positions or with inferred morpho- 

 logical relations of parts 1 . 



Of the nine apertures which open into or near the fundus of the mantle-cavity 

 only four were seen by Owen, namely the two viscero-pericardial orifices, the anus and 

 the opening of the female generative apparatus. Four more, the renal apertures, were 

 added by Valenciennes (1841), and their relations further elucidated by Vrolik 2 . The 

 full number of nine was first established in 1883 by the important discovery announced 

 by Lankester and Bourne 3 of the existence of a rudimentary antimere of the functional 

 generative orifice, represented on the left side by the external pore of the vestigial 

 structure known as the pyriform gland. 



In the male the generative pore is carried forwards and occupies a prominent 

 median position adherent to the vena cava, but the penis is divided by a septum 

 internally so that it possesses two lumina. Externally the terminal portion of the male 

 generative apparatus appears to be symmetrical, an appearance which is heightened 

 especially in the young by the existence of an arcuate radix on the left side as well 

 as that which leads to the so-called Needhamian vesicle on the right, but the left lumen 

 ends blindly behind and as shown by Lankester, Bourne, and later by Graham Kerr 4 , 

 the opening of the pyriform gland is independent and has the same relations in the 

 male as in the female. 



In the female the genital orifice is placed on the right side precisely in the angle 

 made by the insertion of the mantle into the body-wall, and is represented by a 

 transverse bilabiate vulva which is prominent in the adult but lies deep and concealed 

 in the young. 



7. SlPHXJNCLE AND PALLIAL VESSELS. 



It may seem strange to unite these structures in one heading but it is not so strange 

 as it looks, especially when we remember that the siphuncle is essentially and entirely 



1 This point will be incidentally referred to in subsequent sections of this essay. 



2 Vrolik, W., (a) " Brief aan J. J. Eochussen over het ontleedkundig zamenstel van den Nautilus 

 pompilius." Tijdschr. natuurk. Wet. (Netherlands), n. 1849, pp. 307 — 327, 2 plates. (6) " Lettre sur quelques 

 points de l'organisation de l'animal du Nautile flanibe. " Mem. Soc. Normandie, x., 1855, 16 pp., 2 plates. 



3 Lankester, E. E. and Bourne, A. G., "On the existence of Spengel's olfactory organ and of paired 

 genital ducts in the Pearly Nautilus." Quart. J. Micr. 8c, Vol. 23, 1883, pp. 340—348. 



4 Kerr, J. G., "On some points in the anatomy of Nautilus pompilius." P. Zool. Soc. London, 1895, 

 pp. 664 — 686 ; see p. 672, fig. 3 and PI. xxxix. fig. 1. 



