592 WHITMAN. [VoL. Il. 
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in the caudal group has been variously estimated at six, seven, 
and eight. 
16. The importance of the segmental character of the sense- 
organs is not to be measured by its usefulness in systematic 
determinations. Leydig showed long ago, in A/zrudo, how the 
same nerve supplies successive sets of eyes. How, then, could 
the ganglia or nerves of the so-called brain be made to reveal 
the metameric character of the eyes? And how could the im- 
portant serial homology of the eyes ever have been determined 
except through the discovery of their relations to sense-organs 
known to have a metameric arrangement? It is by virtue of 
this arrangement that I have succeeded in getting such a com- 
plete and convincing picture of the origin and history of the 
leech eye. Segmental sense-organs are found in other annelids 
and in vertebrates, but nowhere is the transition from lower 
to higher sense-organs so pérfectly illustrated as in the leech. 
Branchelliopsis, Clepsine and Hirudo reveal all the intermediate 
steps, beginning with the purely tactile organ; then advancing 
to the compound organ, in which a few of the cells have been 
modified to serve the purpose of vision, while the rest have 
retained their primitive character ; and finally, culminating after 
a long series of progressive encroachments, —the visual elements 
increasing gradually at the expense of the tactile, — in an organ 
in which the original function has been entirely suppressed and 
a new one substituted for it. Here is a chapter in the evolu- 
tion of sense-organs so perfectly preserved in all its details as 
to leave no room for scepticism. 
17. Without entering into the discussion of the point, I desire 
to repeat here the suggestion made on another occasion, that 
the segmental sense-organs of the leech are identical with the 
lateral-line organs of vertebrates. I do not venture to express 
such an opinion without having duly reflected on the objections 
that might be raised. Having spent considerable time in the 
study of the lateral-line organs in the larval stages of amphibia 
and marine fishes, and having followed closely the work of my 
colleague on Amia, Lepidosteus, and various teleostei, I know, 
at first hand, where the difficulties lie, and I do not think I 
underestimate their importance. 
18. Perhaps it will not be venturing too far on speculative 
ground if, in this connection, another suggestion be offered ; 
