NATURE OF PAIRED EYES. 349 



two lateral, closely aggregated together and each with a pigment 

 mass in relation with it. In the Corycaeidae large dorsal cuti- 

 cular lenses are developed * in relation with the paired lateral 

 elements of the eye ; but our knowledge of the morphology of 

 the median eye is very defective. 



The structure of the paired eyes has been already described 

 (p. 329). In many Branchiopoda they sink into the body and 

 are covered by a fold of the epidermis (Grobben). 



The nature of the paired eyes. The morphological nature of 

 the compound eyes of Crustacea has been much discussed, and 

 the evidence still appears to be conflicting. In the Decapoda 

 and other groups of higher Crustacea they are mounted on 

 jointed stalks and have therefore, to this extent, the character 

 of limbs. As has recently been clearly shown by Herbst,f 

 they may be replaced after injury by an antenna-like appen- 

 dage. On these grounds it has been urged that the crus- 

 tacean paired eye is, in fact, the modified appendage of the 

 first segment of the head. 



On the other hand the eyes are sessile in Trilobites, an 

 ancient and apparently primitive form of the crustacean stock. 

 The compound eyes of Insects have a similar structure to those 

 of the Crustacea and they are apparently derived from the 

 groups of simple ocelli such as we meet with in the Myriapoda, 

 and the Collembola. If these are to be regarded as limbs 

 they represent the arthropod appendage in an extreme modi- 

 fication indeed. Moreover the preantennary segment, of which 

 the protocerebrum is the neuromere, bears a vestigial appen- 

 dage in the embryo of Scolopendra (Fig. 229, 1). 



The argument for regarding the paired crustacean eye as a modified 

 limb, which appears at first sight to be most cogent, is that emphasized by 

 the experiments of Herbst. The phenomenon is apparently allied to 

 that of homoeosis.J In the same way, to take one of many instances, 

 the first antenna of an Asellus may, as an individual variation, assume 

 the character of a mandible. The force of this evidence in its bearing on 

 the appendicular nature of the crustacean eye depends on the extent to 

 which homoaosis is confined to truly homologous parts. The appar- 

 ently analogous case, quoted by Bateson (I.e. p. 148), in which the hind leg 



* Hartog shows that these are present also, though small, in relation 

 with the lateral elements of the eye of Cyclops. 



f Archiv. f. Entwickelungsmechanik d. Organismen, vols. 2, 9 and 13. 



j Bateson, Materials for the Study of Variation, p. 84. Homoeosis is 

 defined as " the assumption by one member of a meristic series of the form 

 or characters proper to other members of the series." 



