ORIGIN OF EYE AND EAR. 



Fig. 7.— Section through the simple eye of a 

 young Dytiscus larva (after Grenadier). ?i, 

 Uypoderm ; I, lens ; o, optic nerve ; g, p, 

 modified hypodermic cells ; r, retina. 



- The tissues of tlie lowest animals have not been shown 

 to contain any special nerve-fibres, but underneath those 



parts of the surface 

 where, either in the 

 manner indicated above, 

 or in some other, the 

 effects of external stim- 

 uli are heightened by 

 any structural modifi- 

 cations, there would be 

 a tendency to the speci- 

 alization of an excep- 

 tionally sensitive tissue. 

 Moreover, such an 

 organ as that represented in Fig. 4 might serve either 

 as a rudimentary ear or an eye. It might, indeed, be 

 acted on by the waves both of light and of sound. Such 

 organs — as, for instance, in the case of marginal bodies 

 round the edge of certain jelly-fishes (Medus£e; see 

 Figs. 8 and 50) — have been regarded by some naturalists 

 as eyes, and by others as 

 ears. Haeckel suggests * that 

 some may be warmth-organs. 



Fig. 8 represents one of the 

 marginal sense-organs of a 

 Medusa (Ontochis), where we 

 have a row of brilliantly re- 

 fractive spherules, which from 

 analogy are considered to serve 

 as otoliths ; but which, under other circumstances, 

 might be, and in fact have been by some, regarded 

 as the lenses of a simply constructed organ of vision. 



* '' Report on Deep Sea Medusee," " Challenger Reports," vol. iv. 



Fig 8.— Auditory vesicle of Onto 

 chis (after Haeclcel). 



