IV] SENSE ORGANS AND SENSES 65 



But a few facts must be given in illustration and 

 'explanation of the senses of touch and sight that the 

 Oligochaeta undoubtedly possess. These Annelids, 

 unlike their allies the marine Polychaeta, and even 

 their allies on two other sides, the leeches and flat- 

 worms, have no complexly organised eyes or other 

 sense organs. They have in fact no organs to which 

 a definite sense can be attached on histological 

 grounds. There is nothing comparable to an eye or 

 to the auditory sacs of other low worms. There are 

 only particular cells of the epidermis, often associated 

 into small groups, and those again into larger associa- 

 tions of rows of such groups of cells. It is to be 

 presumed that these modified groups of cells have a 

 sense function ; but no more can be said than that 

 they are doubtless tactile and also to some extent 

 receptive of the influence of light. True visual cells 

 have been asserted to exist in earthworms, consisting 

 of cells of which a part is clear and transparent and 

 has been supposed to serve as a lens for the rest of 

 the cell which represents a retina. But belief in the 

 function of these cells is by no means unanimous. 

 On the other hand many investigations have proved 

 the existence of groups of epidermic cells of an oval 

 form which are at present arranged in definite rows 

 upon the segments of both terrestrial and a<[uatic 

 forms, which are moreover comiected with nerve 

 terminations, or are at least^according to more 



B. E. 



