804 DIPLOMERISM OF NAUTILUS. 



Should the foregoing inferences appeal to malacologists in general, an important de- 

 duction relating to the homologies of the nervous system remains for consideration. It 

 would follow that the coronal 1 ganglion and commissure of Nautilus and their outgoing 

 nerves are equivalent to the pleuro-pedal system of prosobranchiate Gastropods, and that 

 here, as in Nautilus, the central and peripheral nervous apparatus is composed of two 

 principal systems in close union with one another by means of connectives, but otherwise 

 distinct, namely, the cerebro-visceral system and the pleuro-pedal system. 



18. DlPLOMERISM OF NAUTILUS. 



There are indications of two distinct metameres in the trunk of Nautilus, and the 

 demonstration of the osphradial nature of the "post-anal papillae" adds another link to 

 the chain. 



Owing to the visceral flexure of the mantle which involves renal organs, gills, rectum, 

 etc., in other words, owing to topographical readjustments occasioned by life within a 

 shell 2 , the segments do not occur entirely in metameric succession, but the morphologically 

 anterior segment lies ectad and dorsad of the morphologically posterior segment, and even 

 behind the latter. A representation of the two segments is contained in the figure on 

 PI. LXXV., which shows the pallio-visceral region of Nautilus seen from below through 

 the transparent mantle during life, with natural colours. 



In order to avoid repetition of the terms " anterior " and " posterior " now in a mor- 

 phological, now in a topographical sense, I will call the morphologically anterior segment 

 the outer or microbranchiate segment, since it comprises the lesser gill; the morphologi- 

 cally posterior segment is the macrobranchiate or submedian segment. 



I. Microbranchiate segment contains on each side of the body : — 



1. The smaller branchia. 



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



3. The outer pericardial gland, and 



4. The outer renal organ which lies in 



5. The outer renal chamber which opens to the exterior by 



6. The outer renal orifice. 



7. The posterior branchio-cardiac vessel. 



8. Outer pericardial ligament (PL LXXXII. figg. 7 and 8). 



9. Interbranchial (outer) osphradium. 



10. Outer viscero-branchial nerve. 



11. Outer viscero-osphradial nerve. 



12. Generative orifice on right side and orifice of pyriform body on the left. 



the valve and the dorsal wall of the funnel there is a deep angle or groove and it seems possible to compare 

 this with the anterior sulcus which characterises the border of the propodium in Harpa (PI. LXXVI. fig. 3), 

 Pleurotomarta (Woodward) and other prosobranchiate Gastropods. This sulcus is the "glande pedieuse 

 anterieure" or " sillon du bord anterieur du pied" (Pelseneer, Introd. a Vetude des mollusques, 1894, p. 52). 



1 Generally called "pedal." 



2 Or accompanying it. 



