620 



C. O. WHITMAN, 



Structure and Serial Homology of the Eyes and Sensillae. 



Fig. E represents a seusilla that may fairly be called an eye, 

 although it falls far short of the typical Clepsine eye seen in Fig. D 

 (III). The visual cells are few, loosely arranged, and rounded in 

 form, resembling those clustered about the nerve of the anterior sense- 

 organ (Fig. D II). The sensillae V and VI, which follow this (IF), 

 also bear some resemblance to the typical eye; but as we follow the 

 series backward the visual cells are reduced to two or three, and the 

 pigment wholly disappears, or at least ceases to have any definite 



relation to the visual cells. In 

 ^ this species, then, we have 



the ordinary sensillae 

 grading into eyes by as- 

 cending steps of struc- 

 tural elaboration, and 

 thus we get conclusive 

 evidence of serial homo- 

 logy from anatomy as 

 well as from embryology. 

 It can now be seen whether 

 the following statements, made 

 in Dec. 1888, had any foun- 

 dation : 

 „The segmental sense-organs are double organs, both in 

 structure and in function. There is an axial cluster of elong- 

 ated cells, terminating at the surface in minute hairs, and repre- 

 senting most likely a tactile organ. Around and beneath the tactile 

 cells, are the large, clear visual cells, so characteristic of the eye. 

 Thus we have a visual and a tactile organ combined, both derived 

 from a common mass of indifferent epidermal cells, and both supplied 

 by fibres from a common nerve branch. 



Incredible as the double nature of these organs may at first 

 appear, there is no escape from the fact, when we once understand 

 the structure of the eye in Clepsine. A vertical section in the plane 

 of the optical axis reveals the compound nature of the eye, and the 

 identity of its structure with that of the segmental sense-organs. 

 Here stands the tactile jjart of the organ, an exact copy of every 

 feature seen in the corresponding part of a segmental sense-organ; 

 and below and behind, but in continuity with the tactile portion, lies 



Fig. E shows the median sensilla of the 

 segment foUowing the ocular segment of Clepsine 

 hollensis. 



