THE EYES OF TRILOBITES 141 



pore. Holmgren says that in the Nauplius eye there are, in his opinion, 

 no less than five retinas. The five pits in Tretaspis, therefore, might be 

 interpreted as representing two pairs of ocelli, one pair in front of and the 

 other behind the central opening of the common sac. The central pit 



^.nM! 



Fig. 103. 

 Dorsal aspect of a Trilobite, Dalmanites socialis, an extinct arthropod, peculiar 

 to and characteristic of the palaeozoic rocks, the class being especially abundant 

 from the upper Cambrian to the Carboniferous. 



c. sh. : cephalic shield. m.c. : movable cheek. 



e. : situation of eye. p. : pygidium. 



: fixed cheek. pi. : pleura. 



; frontal suture. th. : thorax, 

 glabella. 



f.s. 

 gl. 



(After Gaerstaecker, redrawn from Textbook of Zoology, Parker and Haswell.) 



may, however, represent a pair of ocelli which have fused in the median 

 plane, as commonly occurs in the single median eyes of insects and some 

 crustaceans. In this connection it is interesting to recall the conditions 

 found in the freshwater mites (Hydrachnida) (see p. 129), more particularly 



