OF SENSE-OEGANS. 5 



Nor is this a merely imaginary case. Each of the above 

 stages may be found in actual existence — that, for in- 

 stance, indicated in Fig. 2 in the limpet (Fig. 92) ; Fig. 3, 

 in Troehus (Fig. 93) ; and Fig. 4 in the snail, Helix 

 or Murex (Figs. 94, 95). Kecent researches indicate 

 that the eyes of Articulata (insects, etc.) have, in some 

 cases at least, a similar history. But more than this, 

 if the development of the eye of an individual snail be 

 watched in the egg, it will be found to pass successively 

 through stages resembling Fig. 2, then Fig. 3, and 

 then Fig. 4. 



In other cases, however, the organs of sense have a 

 different origin and history. Suppose, for instance, 



/^iiraii^piiiiii 



Fig. 5.— Diagram of origin of a sense-organ. 



that the hypodermic layer were at any spot (Fig. 5) 

 somewhat more strongly developed than elsewhere ; in 

 that case, the cuticle secreted by the hypodermic cells 

 would tend to be rather thicker than usual. This would 

 again (Fig. 6) constitute a lens, and serve to condense 

 the light. That certain eyes have actually arisen in 

 this way is indicated by Fig. 7, representing a section 



Fig. 6. — Diagram of further stage in the origin of a sense-organ. 



through the eye of the larva of a water- beetle 

 (Dytiscus). Xor, as we shall presently see, do these 

 two types of development by any means exhaust the 

 ways in which eyes may originate. In the two cases 

 given the eyes originate from the skin, but in others — 

 for instance, in ourselves — the percipient elements are 

 formed from the central nervous system. 



