88 MENTAL EVOLUTION IN ANIMALS. 



terminal joint works in a vertical plane, and supports the 

 sensory apparatus, which is kept in a perpetual jerking motion 

 up and down, so as to bring that apparatus into sudden con- 

 tact with any minute odoriferous particles which may be sus- 

 pended in the water — just on the same principle as we our- 

 selves smell by taking a number of small and sudden sniffs 

 of air. Any one visiting an aquarium can have no difficulty 

 in observing these movements upon any crab or lobster in a 

 healthy condition. 



The sense of taste certainly occurs at least among some 

 species of the Articulata (as, e.g., among the honey-feeding 

 insects), and the sense of touch is more or less elaborately 

 provided for in all. 



Turning now to the Mollusca, we pass in a tolerably 

 uniform series from the simple eye-spots of certain of the 

 Lamellibranchiata, through the Pteropoda, to the more com- 

 pletely organized eyes of the Gasteropoda and the Heteropoda. 

 But when we arrive at the Cephalopoda, we encounter, as it 

 were, a vast leap of development ; for the eye of an octopus, 

 in point of organization, is equal to that of a fish, which it 

 so closely resembles. And, while remembering that the 

 resemblance, striking though it be, is only superficial, we must 

 not fail to note that this enormous development in the organi- 

 zation of the molluscous eye, which brings it so strangely to 

 resemble the eye of a fish, is clearly correlated with the no 

 less enormous development of the neuro-muscular system of 

 the animal, in which respect it more resembles a fish than it 

 does the other Mollusca. This case is therefore analogous to 

 the similarly high development which has been attained by 

 the eye of the swimming worm previously described. 



If we look to the Mollusca as a class, we meet with the 

 same kind of variation in the position of the eye which we 

 have already noticed with respect to the ear in the Articulata. 

 Thus, while in the Cephalopoda and Gasteropoda the eyes 

 are situated in the head, in some of the latter group there 

 are supplementary eyes upon the back, which greatly differ 

 in structure from the eyes in the head. In the Lamelli- 

 branchiata, again, the eyes occur in large numbers on the 

 margin of the mantle. 



The sense of hearing is general to all the Mollusca, and 

 the auditory organs exhibit a progressive elaboration as we 

 ascend from the lower to the higher groups, which is analo- 



