PHYSIOLOGY OF THE INVERTEBRATA. 361 



The Arachnida. 



The palpi are the principal seat of the sense of touch, being 

 in connection with nerves arising from the cerebral ganglion. 

 The feet are also very sensitive tactile organs. 



The eyes are always simple, like the ocelli of the Insecta ; 

 and there are usually from two to twelve* in number. Audi- 

 tory organs have not been discovered in the Arachnida^ but 

 we have many proofs of the existence of this sense in these 

 animals, and it would even appear that some of them are 

 sensible " to the charms of music." The parasitic Acarina, 

 and allied groups, are entirely devoid of organs of vision. 



The Crustacea. 



The sense of touch is well developed in these animals. 

 Its principal seat is in the antennae, which also contain 

 nerves from the supra-cesophageal ganglion. Often the 

 mouth organs have one or more pairs of tactile appendages ; 

 and no doubt the limbs, especially the anterior ones, are also 

 the means of giving rise to tactile impressions. 



In the lower Crustacea, Dr. G. 0. Sars has shown that the 

 principal seat of the sensation of touch is in the antennae. 

 The antennulae have no such function. 



The olfactory organ is situated in the antennulse. In 

 'Astacus, this organ is situated in the delicate setae of the 

 endopodite of each antennule (Mg. 69, A) ; these setae are 

 provided with nerves. A similar arrangement occurs in 

 many Crustacea besides the Decapoda. If the antennulae of 

 Astacus are removed, the animal will approach a small cup 

 containing bromine placed at the bottom of the tank in 

 which the animal lives. On the other hand, if the antennulas 

 are left intact, the animal will not approach the cup. Other 

 obnoxious liquids of high density give rise to similar results. 



* Seorpionidm (Von Siebold) 



