198 ARTHROPODA fcH. 



stimulated by the colour and intensity of the light proceeding from 

 a small area of the outside world directly in front of it. Since the 

 eye consists of a great number of pits, it follows that an image is 

 formed as a mosaic or pattern of light and shade, depending on the 

 extent to which each individual retinula is stimulated. The fine- 

 ness and exactness of the image will depend on the number of 

 eye-pits and retinulae, The principle of the formation of this 

 image is exactly similar to that on which the image is formed in 

 the human eye viz. the concentration of the light from a particular 

 area of the outside world on a particular element of the eye, only 

 that the form of the human lens enables a much larger beam of 

 light to be concentrated on each element, and the number of 

 elements is large, so that the image in the crayfish's eye is dimmer 

 and coarser than the image in our own. The sensations from all 

 the various eye-pits are transmitted by the basal fibres of the 

 retinula cells to the optic ganglion situated in the centre of the 

 eye-stalk, by which they are combined and the resultant image is 

 transmitted to the brain. 



The optic ganglion is developed by a budding of the ectoderm 

 cells along the side of the stalk. In shrimps, as already mentioned, 

 when this ganglion is destroyed an antenna-like organ is developed 

 in place of the eye, but if it remains intact the stump of the eye- 

 stalk will produce a new eye. 



The ear is a very much simpler organ than the eye. It ib 

 merely an open ectodermic pit, lined with cuticle, situated on the 

 upper surface of the basal segment of the antennule. The entrance 

 is guarded by rows of feather- like setae which extend from the 

 side across the opening. Over the bottom of the pit are several 

 parallel rows of delicate simple setae, the bases of which are in 

 close association with sense cells, and the nerve fibrils proceeding 

 from these cells form the auditory nerve, which goes to the brain. 

 These delicate setae it may easily be imagined will respond to 

 vibrations traversing the water : and the information thus conveyed 

 to the animal may warn it against the approach of its enemies, 

 though these vibrations would not be perceptible to our ears as 

 sound. But experiments have shown that as is the case with so many 

 ears, another and different function is carried out by the organ 

 to which the perception of vibrations may be quite subsidiary, viz. 

 the perception of the position of the animal with regard to the 

 vertical or the perception of balance. These experiments were 

 carried out on the sea crayfish Palinurus. The ear-pit contains 



