102 THE PINEAL ORGAN 



The Eyes of Leeches. — The general structure of the paired eye- 

 spots of leeches has already been alluded to, and it will only be necessary 

 to mention here that in Hirudo medicinalis (Fig. 65) there are five pairs 

 placed symmetrically in series on the dorsal aspect of the anterior sucker ; 

 and that their arrangement in series with the sensory papilla; which occupy 

 the same position behind them indicates that they are formed as special 

 modifications of these. The papillae are arranged in rows extending back- 

 ward from the head to the tail-end of the body. Two of these rows are 

 placed one on each side near the median plane. These are continuous 

 in front with the most anterior pair of eyes on the 1st segment. The other 

 papillae on the body are arranged serially in groups of three on the dorso- 

 lateral aspect and on the middle ring of each successive segment. The 

 innermost row of these is continuous with the eye-spots from the 2nd to 

 the 5th segment. The number of eyes is subject to considerable variation 

 in different species. They may be developed on the posterior sucker or 

 may be absent altogether. 



