446 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



have several hundreds ; they are, as a rule, best de- 

 veloped in the region of the cerebral ganglia, and, in 

 some cases, even in these low forms, they are found 

 on the tentacles ; pigment cells are here also separate 

 from sensory cells, and the latter are continued into 

 nervous filaments, which pass to the optic nerve. 

 They are turned towards all directions, but exhibit an 

 advance in differentiation by lying below the epithe- 

 lium which invests the body. Pigment spots are not 

 confined to the adult forms, the larva of the liver- 

 fluke, for example, having on its back two curved 

 patches, the convex sides of which are opposed to and 

 placed close to one another. 



In higher groups, the number of eyes ordinarily be- 

 comes reduced, but even among the Polychsetous 

 Annelids we find a form (Polyophthalmus) in which 

 a pair of eyes is developed on every segment, in ad- 

 dition to those on the head. This fact, especially when 

 taken into consideration with the presence of eyes in 

 the last segment of the body in Fabricia and some 

 other worms, is very significant, as showing us that 

 sensory organs, which are essentially of epiblastic 

 origin, may be developed and retained on any part of 

 the body in which their presence is useful to their 

 possessor. 



When the eyes become reduced in number, there 

 may be several pairs in the more anterior region of 

 the body, as in the leech, which has ten pairs ; or they 

 may be found on the tentacles, as in Branchiomma, or 

 on the gills, as in Sabella. The next step in the re- 

 duction is seen in the scorpion and other Arthropods, 

 where there are a pair of " compound " and several 

 pairs of " simple " eyes ; and the final step is reached 

 in the higher members of all groups, where the eyes 

 are two in number only ; in various Entomostraca 

 (e.g. Leptodora) the two eyes become fused in the 

 adult. 



