io6 THE APODID^ part i 



similar large cells, the smaller group placed dorsally 

 and slightly anteriorly to the other. 



As already described, a fine branch of the water 

 canal, on which this sensory body is suspended, runs 

 in towards the angle made by the dorsal and ventral 

 retinae. This fine canal is shown in Fig. 25 and in 

 sections 2 and 3, Fig. 27. 



This structure of the median eye seems to be com- 

 mon to all the specimens of the different species of 

 the Apodidae examined by us. In series of sections 

 the organ is very likely to be displaced by the tearing 

 away of the chitinous tube. On this ground it would 

 require a m.uch more extended study to ascertain 

 whether arrangements which sometimes appear to be 

 characteristic of the organ in the different species 

 are not really due to defects in the preservation of the 

 animals or in the preparation of the sections. 



Enough has here been said as to the general struc- 

 ture to bear out what we maintain — viz., that the 

 unpaired eye is no rudimentary organ, but in reality 

 a highly developed sensory body playing a most 

 important part in the life of the animal. 



Origin. — As to the probable origin of this organ, 

 everything points to its having been originally com- 

 posed of an anterior pair of eyes on the Annelidan 

 prostomium. The two posterior eyes formed the 

 paired eyes, the two anterior, which were nearer 

 together than the posterior pair, united to form the 

 unpaired eye. The evidence in favour of this origin 

 seems to us to be overwhelming. 



When the paired eyes travelled into the end of the 



