1 86 ARTHROPODA [CH. 



flexible. Armed with this long antenna the crayfish explores the 

 width and extent of the various nooks and crannies amongst the 

 stones in the river bed in which he hides himself. 



The first antenna or antennule is a much smaller organ. The 

 protopodite consists of three stout joints, not two, as is the case with 

 the thoracic protopodites, and hence we cannot label any of them 

 basipodite or coxopodite. On the upper surface of the most proximal 

 is an open pit the entrance to which is guarded by a fringe of 

 bran ched setae. This is the otocystor rudimentary ear, the structure 

 and function of which will be explained later. Both exopodite and 

 endopodite have the form of flexible filaments made up of a multi- 

 tude of joints, the exopodite being by a little the longer and attaining 

 about half the length of the head. On the under surface of each of 

 its joints are two tufts of fine setae which are flattened and the tips 

 of which are obtuse. These are believed to be olfactory in function 

 so that the senses of smell and hearing are situated in the antennule. 

 If a crayfish be watched when at rest it will be noticed that the 

 antennules are in constant movement; the animal twitches them 

 first in one direction and then in another. If now a piece of some- 

 what stale fish be dropped into the water near the animal, in a little 

 while as soon as the odour therefrom has had time to diffuse, the 

 twitching becomes much more violent and is now confined to the 

 vertical plane joining the animal and the food and soon the crayfish 

 creeps forward and secures the tasty morsel. It appears therefore 

 that the animal not only perceives odours by means of the antennule, 

 but by moving it about is able to discover the direction from which 

 the odour has emanated. 



In front of the first antennae are situated the eyes which 

 form the summits of two short unjointed stalks freely movable 

 on the carapace. Whether the eye-stalks are to be regarded as 

 appendages comparable with the others is a disputed question. In 

 the time of Huxley it was regarded as self-evident that they were 

 appendages. Later when due weight was given to the facts that 

 in many of the lower Crustacea the eyes are mere convex areas of 

 the carapace, and that in none of them are the eye-stalks movable, 

 it was considered more probable that the eye-stalks were simply 

 portions of the carapace which had become movable to enable the 

 eyes to have a wider range of vision. The study of development 

 has however shown that the absence of an eye-stalk is a secondary 

 condition of affairs and the remarkable fact that in some Shrimps 

 when the eye-stalk with its contained optic ganglion is torn out, an 



