180 



HOMOLOGY 



....... cornea 



of 



...cone cell 



distal 



retinula 

 cells 



_.,..,^4 .pigment 



..distal 

 retinula cells. 



processes, more plentiful about the anterior end, but distributed 

 nevertheless about the entire body. 



In arthropods we find a great advance, over annelids, in com- 

 plexity of the peripheral, or sensory, nervous system. Here 

 similar sensory cells are grouped together to form different kinds 



of sensory organs of more or 

 less complexity. In the lob- 

 sters we recognize: (i) tactile 

 organs; (2) olfactory or smell- 

 ing organs; (3) auditory 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 hair contains 

 a small ganglion. 



2. The olfactory organs are 

 similar to the tactile, but dif- 

 fer in the position of ganglia 

 and arrangement of the 

 nerve endings. They are 

 distributed mainly on the 

 antennules. 



3. The auditory organs are 

 technically termed "oto- 



FIG. 75. Three ommatidia from the cysts," and their functions 

 compound eye of the lobster. (Modi- , -, .-, t ,1 



fied after Parker.) are incited through the ac- 



tion of small crystalline 



foreign bodies, termed "otoliths." The cysts or capsules are 

 located on the inner side of the basal joints of the antennules, 



cone cell 



proximal 

 retinuta cells 



...rhabdofne 



...nerve fibres 



