176 



HOMOLOGY 



distal 

 retinula 



cells 



of 

 .cone celt 



..pigment 



sists of more or less isolated sensory cells with their nerve 

 processes, more plentiful about the anterior end but distributed 

 nevertheless about the entire body. 



In arthropods similar sensory cells are grouped together to 



form different kinds of sen- 

 sory organs of more or less 

 complexity. In the lobsters 

 we recognize: (i) tactile 

 organs; (2) olfactory or 

 smelling organs; (3) audi- 

 tory or primitive hearing 

 organs, and (4) organs of 

 vision or eyes. 



1. Tactile Organs. The 

 organs of touch are distrib- 

 uted over the body, usually 

 on the appendages and in 

 large numbers in the cephalic 

 region, in the form of hairs. 

 Each hair contains a nerve 

 with delicate nerve endings 

 in cells forming the walls of 

 the hair, and each contains 

 a small ganglion. 



2. The olfactory organs are 

 similar to the tactile but dif- 

 fer in the position of ganglia 

 and arrangement o f t h e 

 nerve endings. They are 

 distributed mainly on the 

 antennules. 



3. The auditory organs are 

 technically termed u oto- 

 cysts" and their functions 



are incited through the action of small crystalline foreign 

 bodies termed "otoliths." The cysts or capsules are lo- 

 cated on the inner side of the basal joints of the anten- 



.distal 



retmuld cells 



cone cell 



-proximal 

 rttinuttL celh 



ij nerve fibres 



FIG. 75. Three ommatidia from the 

 compound eye of the lobster. (Modi- 

 fied after Parker.) 



