80 



LEECHES 



(a) 



(b) 



Fig. 43. (a) A free-ending nerve fibre in the epidermis of 



Hirudo medicinalis. 



(b) Epidermal sense cells grouped in a sensilla. 



b.m, basement membrane; c, cuticle; c.t, connective tissue 



sheath surrounding a nerve cell ; e, epidermal cell ; e.c. , epidermal 



sense cell; w, nucleus of nerve cell; w./, nerve fibre; m.c, muscle 



cell ; s.n, sensory nerve. 



From Grasse, 1959, based on Apathy (a) and Autrum (b). 



shaped organs. The functions of the sensillae are presumably 

 either tactile or chemoreceptive but it has not been possible to 

 distinguish one from the other in a particular organ. There is 

 some evidence that the chemoreceptors are confined to the head 

 region (Kaiser, 1954) and if this is so, the sensillae of the general 

 body surface are touch receptors. 



Light sensitive cells (Fig. 44) are recognizable by the large 

 vacuole filled with hyaline, possibly albuminous fluid which acts 

 as a lens. Light is concentrated on to a neurofibrillar network 

 within the cytoplasm which makes contact with a sensory nerve 

 fibre. Such cells occur in small numbers in the sensillae or some- 

 times in the general epidermis but leeches, unlike earthworms. 



