AFFINITIES. 805 



II. Macrobranchiate segment contains on each side : — 



1. The greater branchia. 



2. A reno-branchial vessel, to which are appended 



3. The inner pericardial gland, and 



4. The inner renal organ which lies in 



5. The inner or submedian renal chamber which opens to the exterior by 



6. Its own renal orifice. 



7. The anterior branchio-cardiac vessel. 



8. The inner (anterior) pericardial ligament. 



9. The inner or submedian osphradium. 



10. Inner viscero-branchial nerve. 



11. Inner viscero-osphradial nerve. 



12. The viscero-pericardial orifice. 



The outer visceral nerve supplies both of the gills and the interbranchial osphradium. 

 The inner visceral nerve supplies the mantle (nidamental gland in female) and the sub- 

 median osphradium. 



The heart of Nautilus has undergone a semi-rotation by which the origin of the 

 systemic aorta is carried backwards and the positions of the branchio-cardiac vessels perma- 

 nently inverted, the anterior pair draining the greater (morphologically posterior) branchia, 

 while the posterior pair is associated with the lesser (morphologically anterior) branchia. 



The diplomerism breaks down in a somewhat puzzling fashion when we come to con- 

 sider the coelom, since the pericardium and the perivisceral coelom can hardly be regarded 

 as homodynamous subdivisions of the secondary body-cavity. There is therefore no true 

 coelomic metamerism in Nautilus in spite of the existence of metameric abdominal pores, 

 namely, the generative and viscero-pericardial orifices, the latter at any rate taking the 

 place of the nephrostomes of other Cephalopoda 1 . 



An interesting, analogous, physiological interrelation between nephrostomes and ab- 

 dominal pores has been established by Bles 2 in fishes. 



19. Affinities. 



I do not propose to undertake an exhaustive discussion of the affinities of Nautilus 

 beyond what has been already said. With regard to the relationship of Nautilus and 

 Cephalopoda in general to the Amphineura, the most primitive of existing Mollusca, 

 in which the antero-posterior axis coincides with the oro-anal axis, I will refer the reader 

 to the works of Kerr, Haller, and Plate to which I have already referred 3 . 



1 Cf. Jhering, H. v., Zeitschr. wiss. Zool., Bd. 35, 1881. Grobben, C, Arb. Inst. Wien, v. 1884 and 

 vn. 1886. Kerr, J. G., P. Zool. Soc. London, 1895. Ziegler, H. E., " Uber den derzeitigen Stand der Cblomfrage." 

 Verh. Deutsch. Zool. Ges. 1898, p. 14. 



2 Bles, E. J., "Abdominal pores and nephrostomes in fishes." J. Anat. Physiol., Vol. xxxn. p. 484; also 

 P. R. Soc. London, Vol. 62, 1898, p. 232. 



3 See further Kerr, J. G., " Phylogenetic relationship between Amphineura and Cephalopoda." Zool. Anz., 

 xxiv. 1901, p. 437. 



105—2 



