l89o.] AN ATOilT or KACTILUS POMPILIUS. 685 



Fig. C shows the condition in Nautilus, where again the same two 

 ccelomic chambers are visible. Here also a communication has 

 become formed between the two, but the two pairs of ducts to the 

 exterior still persist — the anterior nephridium here still preserving 

 its excretory portion — a more primitive condition than in Chiton, 

 and probablv to be correlated with the fact of its having become 

 shut oif from the main lumen of the duct. A few irregular 

 apertures in the wall separating the two ccelomic chambers point 

 towards the still later condition to be met with in Sepia (D), where 

 the septum has disappeared — a faint rudiment remaining in the 

 form of a transverse fold rising up from the floor of the common 

 chamber ', 



X. Summary of Conclusiotis. 



1. The perivisceral cavity in Nautilus is remarkable for the 

 almost equal participation in its formation of both coelom and 

 hsemocoel. 



2. The ccelom consists of two distinct chambers — genital and 

 pericardial — separated by a perforated septum. 



3. Each of these ccelomic chambers opens to the exterior by a 

 pair of nephridia. 



4. The genital ducts of the Cephalopoda represent portions of 

 nephridia. 



5. The ovary is remarkable for its extremely archaic character — 

 an ovigerous region of the ccelomic epithelium, roofed in by a 

 simple upgrowth of the ccelomic wall. 



6. The ova arise from syncytial masses of protoplasm. 



7. The testis is also archaic in character, and similar to the 

 ovary in its main features. Its ca^dty, however, has become sub- 

 divided into numerous delicate tubes for the provision of increased 

 area of the spermatogenic epithelium. 



8. The penis is a paired structure, its left moiety, however, 

 remaining rudimentary. 



9. An elaborate buccal nervous system is present. 



10. The " inner inferior lobe " is innervated not by a pair of 

 distinct ganglia, but by a continuous nerve-cord. 



11. Eound the base of the postanal papilla is a curious system 

 of skin-glands. 



12. A prolongation backwards of the nerve-trunk which supplies 

 the gills probably represents the postanal commissure of Amphi- 

 neura, 



13. A laminated organ lying below the mouth has a function in 

 connection with copulation — the spermatophore of the male 

 becoming attached to it, 



14. The evidence as to the " pedal " nature of the Cephalopod 

 arms appears to rest on insecure foundations, and it seems desirable 



I The view advocated by Qrobben (Morph. Stud. p. 39) that the condition 

 in Sepia ia the more primitive, and that it represents a stage in the evolution of 

 the condition met with in the other Mollusca, seems to me untenable. 



