208 AN INTRODUCTION TO ZOOLOGY 



toward the basal end of the ommatidia (Fig. 103, B). When this 

 occurs the ommatidia no longer act separately, but a combined 

 image is thrown on the retinular layer. " In this manner an 

 erect ' superposition image ' is formed, the rays refracted by a large 

 number of crystalline cones being superposed at the focus on the 

 retina, and a stimulus far stronger in proportion to the intensity 

 of the illumination than that of the apposition image, though 

 probably much less distinct in detail, is given to the retinulae " 



(183, p. 333). 



STATOCYSTS. The statocysts of Cam jams are chitinous- walled 

 sacs situated one in the basal segment of each antennule. In the 

 base of the statocyst is a ridge, called the sensory cushion, and 

 three sets of hairs, over two hundred in all, each innervated by a 

 single nerve fiber. Among these hairs are a number of large 

 grains of sand, the statoliths, which are placed there by the cray- 

 fish. Beneath the sensory cushion are glands which secrete a sub- 

 stance for the attachment of the statoliths to the hairs (181). 



The statocyst for many years was considered an auditory 

 organ, and it may possibly function as such, though recent 

 investigations have proven that it is primarily an organ of 

 equilibration. The contact of the statoliths with the statocyst 

 hairs determines the orientation of the body while swimming, 

 since any change in the position of the animal causes a change 

 in the position of the statoliths under the influence of gravity. 

 When the crayfish changes its exoskeleton in the process of molt- 

 ing, the statocyst is also shed. Individuals that have just molted, 

 or have had their statocysts removed, lose much of their powers 

 of orientation. Perhaps the most convincing proof of the func- 

 tion of equilibration is that furnished by the experiments of 

 Kreidl (172). This investigator placed shrimps, which had just 

 molted and were therefore without statoliths, in filtered water. 

 When supplied with iron filings, the animals filled their statocysts 

 with them.' A strong electro-magnet was then held near the 

 statocyst, and the shrimp took up a position corresponding to the 

 resultant of the two pulls, that of gravity and of the magnet. 



