338 PHYLUM ARTHROPODA. 



an auditory function as well. They are described on p. 350. 

 Another possible function of these organs, suggested by Balfour, 

 is the perception of the vibrations set up in the water by other 

 animals swimming in their vicinity. 



Of a wholly different structure are the chordotonal organs of 

 Insects to which an auditory function has been assigned. They 

 are groups of elements, widely scattered over the body, each of 

 which consists of a delicate rod contained in a tubular pit sunk 

 beneath, and opening at the surface. The rod is continued 

 into a ganglion cell at the base of the pit, and a mechanism exists 

 for maintaining the walls of the tube in a tense condition. The 

 tympana on the legs of insects are modified areas of cuticle in 

 relation with groups of such organs (p. 636). 



Tactile setae in relation with ganglion cells are widely found 

 on the extremities, especially the antennae of Crustacea and 

 Insects ; and many organs occur, especially in Insects, evidently 

 of a sensory nature, but on the special function of which we can 

 only vaguely surmise. Some are doubtless olfactory. 



Complementary to the organs of special sense are other organs 

 widely found among Arthropods such as the light-producing 

 organs of many Insects (fire-flies, etc.) and Crustacea (Copepoda', 

 some Ostracoda, Euphausiidae, some Decapods, etc.), the stri- 

 dulating organs of spiders, grasshoppers, cicadas and decapod 

 Crustacea, and organs for diffusing special odours which are 

 widely found among Insects. 



The respiration of Arthropods is generally effected by bran- 

 chiae or by tracheae. The Copepoda, most Ostracoda and some 

 Cladocera have no specialized respiratory organs, and in 

 Birgus latro and, in less degree, in some other terrestrial 

 Decapods the branchial chamber is, as pointed out by 

 Semper, modified into a sort of lung (p. 540). The branchiae 

 are originally (with the exception of the tracheal gills of some 

 larval Insects and the gills of sessile Cirripedes, and some 

 Ostracods) appendages of the limbs. The lung-books of 

 the Arachnida, although modified for breathing air, are, as the 

 interesting researches of Simmons on Spiders, and Brauer on 

 Scorpio have shown, to be regarded as modifications of such 

 branchiae as are borne on the abdominal appendages of Limulus. 



Tracheae are tubular involutions of the outer layer of the 

 body by means of which air is carried to the tissues. In members 



