HANSEN: EXTERNAL SENSORY ORGANS IN ARACHNIDA. 25 I 



thing, would not accept Daiil's interpretation, and did not 

 believe it to be a sensory organ. In the same paper 

 BertkaU mentioned a very different organ found by him 

 in two genera of Aranea; in the mandibles, and he supposed 

 it to be an organ of taste. 



In the orders Pedipalpi, Palpigradi, Ricinulei and Opi- 

 liones no organ not mentioned above has been discovered. 



Before concluding this chapter it may be emphasized, that 

 a bewildering number of different external sensory organs 

 are known in the Arachnida taken together, that »lyriform 

 organs» are known in most orders, trichobothria in more than 

 half of the orders, other sensory hairs or setae in several 

 orders, while each of the remaining organs are found only 

 in a single order or at most in two orders. The occurrence, 

 distribution, or absence of the organs afford fine characters 

 for the orders, and sometimes (Pedipalpi^ Opiliones), though 

 in much lower degree, for the suborders. 



III. Myriopoda. 



Tactile setae in the two orders Pauropoda and Syiiiphyla 

 have been mentioned b}^ several authors, but I am not aware 

 of that such setœ have been found in Chilopoda or Chilo- 

 gnatha. In the Pauropoda the body has five pairs of lateral 

 tactile setae projecting beneath the lateral margins of second 

 to sixth dorsal plate. The setœ must be named tricJiobothria. 

 as each projects from a calicle; each seta has j^a bulbous 

 base connected internally with a oval mass of ganglionic 

 cells» (Kenyon, in 1895), and the seta does not fill the open- 

 ing of the calicle. In my paper on the order (1902) I 

 added various particulars on the setae, their pubescence, 

 etc., and pointed out that in the same animal they can be 

 referred to three varieties; it may be mentioned here that 

 the setai of third pair in certain forms are thickened either 

 at the middle or at the distal end. 



As is well known the Symphyla possess a single very 

 conspicuous pair of trichobothria inserted on the body near 

 the base of the cerci. The setae are very long, and their 

 basal part within the calicle is strongly bulbous. In the 



