THE MOLLUSCS OF THE GREAT AFRICAN LAKES. 185 



Fig. 4. — The same parts in a Bucciuuni, showing the enclosed spermatic 

 duct, V. d. 



Fig. 5. — Diagrammatic representation of the reproductive apparatus in 

 Pelt a, showing the similarity in position (in relation to the genital groove) 

 of the eversible penis 1o the brood-pouch of the Prosobranchs, figs. 1 and 2 

 (after Peiseneer). 



Fig. 6. — The relation of the ducts in Auricula myosotis. Spermatic 

 duct, sjt.d., is here enclosed. 



Fig. 7. — The relation of the ducts in Act eon, showing similar modifica- 

 tion of the penis to that observed in Buccinum, fig. 4. 



PLATE 17. 



Diagrams showing — I, different modifications in reproductive apparatus 

 amoug the Gastropods. II, the difFerent modifications in the nervous systems 

 of six Naticoids. Refer to pp. 163 and 175. 



PLATE 18. 

 Fig. 1. — Two views of the shell of Spekia zouatus. 

 Fig. 2. — The gills and mantle organ of Spekia zonatus. 

 Fig. 3. — Single row of teeth in the radula of Spekia zonatus. 

 Fig. 4. — The snout and tentacles of Spekia zonatus. 

 Fig. 5. — Dorsal and left lateral views of the nervous system of Spekia 

 zonatus. 



Fig. 6. — The relation of the visceral organs of Spekia zonatus. 

 Fig. 7. — Animal of Spekia zonatus removed from the shell. 

 Fig. 8. — Section through the genital groove of Spekia zonatus. 



PLATE 19. 

 Diagram to show relationships of various genera. 



