THE MOLLUSCS OF THE GREAT AFRICAN LAKES. 165 



there is absolutely no difference in the relation of the cnrious 

 external genital apparatus in Pelt a or A ply si a, and those 

 similar structures which T. rufofilosa and several Melanias 

 are now known to possess. (Compare figs., diagram I, PI. 17.) 

 It is true that the genital opening in the Opisthobranchs is 

 that of an hermaphrodite duct, but we have seen that there is 

 every reason to regard this character as secondarily acquired ; 

 and just as would be expected from Professor Pelseneer's view, 

 that the female first acquired the hermaphrodite character in 

 these forms, we find the sac, which in its original ancestral 

 prosobranchiate condition was a brood-pouch, here converted 

 with very little modification into an introvertible penis. 

 There seems, therefore, to me to be no admissible objection 

 to the conclusion that the parts of the external genital 

 apparatus which are present in T. rufofilosa and Melania 

 episcopalis are structurally homologous with the similar 

 parts of the external genital apparatus in the simpler Opistho- 

 branchs. Clearly, therefore, in the existence of these curious 

 structures which 1 found in T. rufofilosa and M. episco- 

 palis, we have still more evidence which will help to bridge 

 the rapidly diminishing gap between the two molluscan orders 

 in which they exist. 



The ordinary external genital apparatus of the Opisthobrauch 

 undergoes similar modifications to those occurring in the 

 Prosobranchs, but it exhibits these modifications in a more 

 pronounced degree. Thus the simple open grooves of Pelt a 

 and Aplysia pass through modifications such as that appear- 

 ing in Auricula myosotis (PI. 16, fig. 6), where an external 

 groove still exists, but the vas deferens has become invaginated, 

 so as to form a distinct internal tube. In Lobiga and 

 Actseon (PL 16, fig. 7) the groove has apparently disappeared, 

 and the vas deferens, as in Buccinum among the Proso- 

 branchs, appears as an enclosed tube running to the extremity 

 of a permanent external penis (PI. 16, fig. 7). 



Lastly, in Helix (see diagram, PI. 17), both male and 

 female conduits may become enclosed as separate tubes, their 

 original connection with one another only appearing in early 



