THE NATURALIST S OCCUPATION. 49 



of the vertebrates, we can only expect to find out what 

 those primitive organs were by searching among inverte- 

 brate animals of the ancestral type. By common con- 

 sent we turn to the annelids, or segmented worms. 

 Here we find sense-organs of a low order, segmentally 

 arranged, and supplied by nerves bearing ganglia, which 

 correspond in position and general relations to the spinal 

 gangha of vertebrates. That the segmental nerves and 

 ganglia of the annelids are the morphological equivalents 

 of the spinal nerves and ganglia of vertebrates, is a 

 proposition that now admits of little doubt. If the ar- 

 gument holds for the nerves and ganglia, the basis is 

 given for the comparison of sense-organs. But are there 

 any sense-organs in the vertebrates that can be said to 

 agree in structure and function v/ith the segmental 

 tactile organs of annelids ? Leydig and Eisig have given 

 an affirmative answer to this question, and their views 

 have already met with general acceptation. The sense- 

 organs of the lateral line of fishes and amphibia, rudi- 

 ments of which have been found by Froriep and Beard 

 in the higher classes of vertebrates, have essentially the 

 same structure, the same or a closely allied function, 

 and, so far as known, fundamentally the same mode of 

 development. 



Allowing then that these organs are the homologues of 

 the segmental sense-organs of annelids, there arises the 

 very important question, is it possible for such organs to 

 develop into those of the special senses, taste, smell, 

 sight, and hearing .? In the vertebrates we meet with no 

 serious difficulty until we come to the eye. The sensory 

 impressions received by a visual organ differ so radically 

 from those received by a tactile organ, that it seems 



