236 ZOOLOGY FOR MEDICAL STUDENTS chap. 



bristles — fine hair-like projections of the cuticle ensheathing a prolonga- 

 tion of one or more underlying sensory cells. The function of these 

 sensory bristles appears in many cases to resemble the sense of touch ; 

 in other cases, more especially those of the antennules of Crustacea and 

 the antennae of insects, they would appear to have to do with a sense 

 which detects differences in chemical composition of the surrounding 

 medium, like our senses of smell and taste ; but in all probability the 

 actual kinds of sensation with which they are concerned are such as we 

 can form no conception of, as they are not included within the limits of 

 human sensory experience. 



Just as is the case with the sensory cells of Coelenterates, so we find 

 also in the Arthropoda the occasional occurrence of aggregations of 

 these sensory bristles in special, more or less closed-in, depressions of 

 the surface forming otocysts. An excellent example of the arthropodan 

 otocyst is to be seen in the Decapoda (Lobsters, Crayfish, Prawns) where 

 it is situated in the base of the first antenna, and forms a chamber 

 which remains open to the exterior by a narrow slit guarded by bristles. 

 From the floor of the chamber there project into its cavity a group of 

 specially developed ~1 -shaped sensory bristles, on the top of which rest 

 a number of otoliths. In cases where the otocysts become completely 

 closed the otoliths are secreted by the otocyst wall, but in the animals 

 now under consideration where the otocyst remains open to the exterior 

 the otoliths are grains of sand which the animal itself inserts into the 

 cavity. 



At each ecdysis the entire lining of the otocyst with the contained 

 sand-grains is shed and consequently a new supply of otoliths has to be 

 inserted before the organ can function. This has rendered possible a 

 fascinating experiment to demonstrate the function of the otocyst. 

 After ecdysis had taken place a prawn was provided not with ordinary 

 sand but with fine iron filings — to which it duly helped itself. In 

 ordinary circumstances the iron " otoliths," under the pull of gravity, 

 served exactly as the ordinary grains of sand. When, however, a 

 powerful electro-magnet in the immediate neighbourhood of the animal 

 was brought into action, the result was the same as if the direction of 

 gravity had been suddenly altered and the animal immediately heeled 

 over, away from the direction of the magnet. 



Of the sense organs of the Arthropoda the most interesting are the 

 eyes which are normally present in the head region. An eye consists 

 primarily, like any other sense organ, of an aggregation of sensory cells. 

 These differ from the ordinary type of sensory cell in the fact that they 

 are not provided with a sensory hair but have a portion of their cyto- 



