NATURAL HISTORY OF ARTHROPODS. 



tennse ; in the closely allied Mysis^ on the posterior pair of appendages. In the Acri- 

 didse (grasshoppers) the ears are found on the base of the abdomen, while in the Locus- 

 tidre (locusts) the auditory organs are on the first pair of legs. In the Crustacea the 

 ear, when found, consists of a sac, more or less completely closed by a membrane, and 

 containing small sand-like particles (otoliths) suspended in a mucous fluid. The inner 

 wall of the sac bears a row of numerous fine hairs, each connected Avith the extremity 

 of a nerve fibrile. A sound causes the membrane of the sac to vibrate, this in turn sets 

 the granular contents in motion, and these, touching the hairs, convey the impression 

 to the nerves and thence to the brain. In the common grasshopper the ear is of a 

 different type. On either side of the first segment of the abdomen is found a large 



rl 



FIG. 2. Section of part of eye of tipula. c. Crystalline 

 cells, ct. Cuticula. 1. Corueal lenses, n. Nerves. 

 p. Pigment cells, r. Kods. rl. Retinula. 



FIG. 3. A. Section of eye of cray-fish. B. A single 

 corneal lens, with corresponding cone, rod, and retin- 

 ula, greatly enlarged. 



membrane, roughly corresponding to the tympanic membrane of the Vertebrates. At- 

 tached to the inner surface of this are two horny processes. A large tracheal vessel is 

 distributed over the inner side of the membrane, and between its walls and the latter a 

 nerve passes to the region occupied by the processes above referred to, and there 

 enlarges into a ganglion, the outer face of which, beset with numerous glassy rods, 

 arranged side by side, is in contact with the tympanic membrane. In the crickets and 

 locusts the ears have an essentially similar structure. In other insects great uncer- 

 tainty as yet exists regarding the auditory organs. Professor Mayer thinks that the 

 fine hairs on the antennas of the mosquito have acoustic functions, but this is far from 

 proved. 



Regarding the other organs of sense in the Arthropoda, our knowledge is very 

 slight, and all statements are largely a matter of speculation. The hairs which are 

 found disti'ibuted over the surface of the body, are frequently concerned in the sense 

 of touch, and possibly sometimes in smell, taste, and hearing as well. The difficulty 

 which attends any attempt at experiment in this direction is the chief cause of this 

 uncertainty. 



All Arthropods further agree in the fact that in the ripening of the egg (except in 

 a very few forms) no polar globules are known to be formed. They have also a com- 

 mon mode of egg segmentation ; but neither of these points are to be regarded as of 



