82 



LEECHES 



when, as in Hirudo, there are several eyes pointing in various direc- 

 tions it should be possible to obtain a crude impression of form 

 and movement. Boehm (1947) identified a red fluorescent 

 porphyrin in the pigment layer of the eyes of Hirudo. It is known 

 that the presence of certain porphyrins renders protoplasm light 

 sensitive but as this substance is in the pigment cup its physiolo- 

 gical function is not understood. 



Fig. 45. Various leech eyes in vertical section, (a) Piscicola 

 geometra; (b) Glossiphonia complanata; (c) and (d) Erpobdella. 

 (a) after Maier 1892, (b), (c) and (d) after Hesse, 1897. 



Apart from behaviour associated with reproduction, which is 

 complex and difficult to observe, the normal behaviour ot leeches 

 seems to be relatively simple and amenable to detailed analysis. 

 Broadly speaking the animals are either in a state of hunger and 

 respond to any stimulus which might indicate the presence of a 

 suitable food organism or they have food in the crop and rest in a 



