CHAPTER 8 



SENSE ORGANS AND 

 BEHAVIOUR 



1. Sensory Equipment 



The basic sensory equipment of leeches corresponds almost 

 exactly with that of earthworms, but the sensory elements are 

 grouped into slightly more complex organs and the nervous 

 system appears to be capable of co-ordinating rather more complex 

 patterns of behaviour. This is not surprising when we remember 

 that many leeches rely for their nourishment on their ability to 

 make contact with vertebrate hosts capable of rapid movement. 



There are three kinds of sensory equipment : free ending nerve 

 fibres in the epidermis, epidermal sense cells and light sensitive 

 cells. The free ending nerve fibres (Fig. 43a) are the terminations 

 of nerves arising either from the main segmental nerves or, more 

 often, from the sub-epidermal or inter-muscular nerve rings 

 described on p. 72. It is probable that they respond to changes 

 in temperature or to mechanical stimuli resulting from deformation 

 of the epidermis through contact with solid objects. The epidermal 

 sense cells (Fig. 43b) are tall spindle shaped cells occurring singly 

 or in groups among the other epidermal cells. Their outer ends 

 terminate in fine sensory hairs which pass through the cuticle and 

 project about 10/x beyond it, while their inner ends lead to sensory 

 nerve fibres. Groups of such cells are called sensillae and are 

 scattered over various parts of the body, being particularly numer- 

 ous on the head. In most leeches there are also sensillae which are 

 regularly arranged on the middle annulus of each segment and are 

 known as segmental receptors. In the diflFerent species the seg- 

 mental receptors attain varying degrees of complexity and may 

 include light sensitive cells, mucous glands and special muscle cells 

 which enable them to be protruded as papillae or retracted as cup 



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