180 



HOMOLOGY 



distal 

 netinula 



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- ., -, ,1 i_ 4.1, 



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, 



+... distal . . ' 

 I retinula cells 



A,... cone cell 



Ivy ' ntlnuta: cells 



II, 



fl_ rhabdotne 



fibre 



