Fig. 16 Illustrating the attempted conjugation of two specimens of some- 

 what different size. Outline sketch, dorsal view (gills omitted). X f. At the 

 side, sketch showing manner in which the genital papilla is protruded. X 1. 



Fig. 17 Illustrating the copulation of two individuals of equal size; seen from 

 the ventral surface, through glass wall of aquarium; as contrasted with the pair 

 shown in Fig. 16. The animals here are somewhat contracted. X f. 



Fig. 18 A case similar to that in Fig. 17. The anterior end of individual 

 to the left is extended, the lips being passed over the surface of its partner. 

 X f. 



Fig. 19 Unsuccessful attempt of two individuals of different sizes to engage 

 in conjugation. Relative positions of reproductive appertures indicated by 

 dotted circles (2, that of individual to the left; 1, of that to the right; 2 was 

 about 1 cm. higher than 1). X i. 



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