vi EYES OF ARTHROPODA 239 



is to ensure that the only rays that reach the individual retinula are 

 those whose direction is approximately that of its own axis. All other 

 rays, coming in at a considerable angle to this axis, are shunted off into 

 the pigment cells and there absorbed. 



It is clear that in this radiate type of eye we have to do with an 

 organ that carries out the function of vision in a manner totally different 

 from that of the more usual camera type of eye where an image is 

 formed, as in a photographic camera, by a lens and then cast upon a 

 sensitive screen the retina. The vision of the radiate eye is of what is 

 called the mosaic type, there being formed not a continuous optical 

 image but a collection of separate light impressions from the outer 

 world, each coming in along the axis of a vitrella. 



A definite mechanism exists for the weaving together of these separate 

 impressions, for the group of nerve fibres which pass brainwards from 

 each retinula separate from one another and then become collected into 

 new groups, the fibres composing which, although the same in number 

 as those of the original group, are derived not from a single retinula but 

 from a group of neighbouring retinulae. 



In spite of the existence of this mechanism for weaving together the 

 isolated impressions into a continuous whole it is quite incredible that 

 that impression can, as a representation of the outer world, be otherwise 

 than exceedingly crude compared with that given by the camera eye. 

 In all probability the special efficiency of the radiate eye lies not in 

 forming a picture but in the detection of objects close at hand which 

 are in motion relatively to the eye, either because the object is itself 

 moving or because the arthropod is moving with regard to it. The 

 same effect will be produced if the arthropod while remaining in one 

 spot rotates its head as a Dragon-fly may be seen to do when on the 

 outlook for mosquitos. 



ARTHROPODAN TYPES OF SPECIAL INTEREST 



The main subdivisions of the phylum Arthropoda have been indicated 

 on p. 209. While there is no necessity for the elementary student to go 

 into the details of the classification of the group it is of importance that 

 he should have a general idea of the features of special interest in 

 connexion with various types of arthropod. 



I. The group PROTARTHROPODA comprises the genus Peripatus to 

 the morphologist the most important of all arthropods, for it has lingered 

 on until the present day as the sole representative of those annelid-like 

 creatures that were gradually developing the characteristics of arthropods. 



