CHAPTER I I 



THE EYES OF ARTHROPODA 



This great class comprises the Crustacea, Onychophora (Peripatus), 

 the centipedes and millipedes, insects, and arachnids. Hitherto the eyes 

 with which we have been dealing have been chiefly of the simple type. 

 We have now to consider, in addition to the simple eyes of the ocellar 

 type, the laterally placed, paired, compound eyes. These attain a very 

 high degree of differentiation and specialization in certain insects and 

 Crustacea, and their structure has attracted the attention of some of the 

 most distinguished pioneers in the domain of microscopical zoology. 

 Owing to the vastness of the subject, the account which we are able to 

 give will be only in the way of a brief sketch ; this, however, we hope will 

 indicate the lines along which devolution and evolution of the simple 

 median type of eye and the highly complex lateral eyes of this class have 

 taken. Although many of these changes and developments, when 

 considered from the standpoint of the genealogical history of eyes in 

 general must be regarded as having occurred in types of animal which 

 have branched off very widely from the vertebrate stem, they nevertheless 

 help us to understand something of the history of the " pineal eye," and 

 incidentally many interesting points of general biological interest. 



Eyes of Crustaceans 



One of the most interesting Orders of the Class Crustacea is that which 

 includes the species Apus (Fig. 67) and Lepidurus (Fig. 68). In these 

 small fresh-water animals the back is covered by a shield which resembles 

 the dorsal shield or carapace of Limulus poly phemas, the king crab (Fig. 69). 

 In both, median paired eyes and lateral paired eyes are present. The 

 median paired eyes are of simple type ; the lateral eyes are compound. 



In Apus the median eyes consist of 3 or 4 groups of large, clear sensory 

 cells surrounding a mass of pigmented tissue (Figs. 247, 248, Chap. 24, 

 pp. 350, 360) ; together they appear under a low magnification as a single 

 black spot and hence are often described en bloc as " the median eye." 

 The compound lateral eyes which have been described by Bernard are 

 covered by a transparent cuticle forming the cornea, beneath which is a 

 narrow space, the water sac, which opens on the exterior by a pore. The 



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