SENSE ORGANS AND BEHAVIOUR 81 



have distinct eyes formed from a number of light sensitive cells 

 backed by a pigmented cup. In some leeches, such as Piscicola, 

 the eyes are quite simple structures consisting of a small number 

 of photoreceptors in a shallow cup, but we may construct a series 



Fig. 44. Light sensitive cell from Erpobdella octoculata. 

 From Grasse, 1959, based on Autrum. 



showing a progressive increase in the number of photoreceptors 

 involved and in the depth of the pigmented cup until we reach the 

 condition described in Hirudo (p. 19) where there are very many 

 photoreceptors enclosed in a deep and narrow pigmented cup. 

 Such an eye has better directional properties than the simpler 

 kinds. It is also more superficial than the eyes of the glossiphoniids 

 so that it is less affected by the light scattering properties of the 

 tissues above it. Another improvement is in the disposition of the 

 nerve fibres. In Piscicola the fibres leaving the photoreceptors 

 pass out over the rim of the pigmented cup, thus interfering with 

 the passage of light to some extent. In Erpobdella the nerve fibres 

 may pass through the side of the pigmented cup and in Hirudo 

 they pass down the centre of the eye and out at the base of the 

 pigment cup, thus affording the minimum interference with 

 incident light. In its most advanced form a leech eye should be 

 capable of giving a highly directional response to light rays and 



