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THE EYES OF ARTHROPODS. 



In forms like Acilius, that give us a most detailed picture of ancestral con- 

 ditions, not a trace of the lateral eyes appears till late in the larval stages, when it is 

 impossible to certainly determine their relations to the cephalic lobes, or to other 

 segmental structures. They are first seen on the haemal side of the head of the 

 oldest larvae, median to the ocelli. The latter, during the metamorphosis, are 

 torn away from the ectoderm, apparently by the relative shortening of the optic 

 nerves, and are finally lodged on the surface of the optic ganglion, where they may 

 be seen in a degenerate condition, long after the lateral eyes have become func- 

 tional. In the early embryonic stages of insects that do not pass through a 

 metamorphosis, and in many Crustacea, the lateral eyes are seen on the posterior, 

 lateral margins of the cephalic lobes, just lateral to an infolding that gives rise to 

 the optic ganglion. Here also their relation to the metameres has not been 

 determined. 



Llmulus is the only form in which the larval ocelli, frontal ocelli (olfactory 

 organs), and the lateral eyes, are all present at the same time in an early embryonic 

 stage. Here it is clear that the lateral eyes arise from the cheliceral or first 

 thoracic segment. (Figs. 141 and 142.) 



I see no serious objections to regarding the lateral eyes of insects as also 

 belonging to the first appendage bearing segment, and if the "organ of Tomos- 

 vary," in the myriapods represents the rudiment of the lateral eyes, as I have 

 suggested, 1892, then that also would have a similar position, since it is situated 

 at the base of the antennae, and its nerve is attached to the ganglion of the larval 

 ocelli in the same way the compound eye-nerve is in Acilius. (See optic ganglion 

 in Acilius.) 



These facts indicate, therefore, that the lateral eyes of arthropods stand 

 serially behind both the primitive cephalic lobes and the larval ocelli, and belong 

 to the most anterior appendage-bearing segments of the primitive body or thorax. 

 I can find no evidence in the structure or development of the lateral eyes to indi- 

 cate that they are modified appendages. 



B. Development. Although the lateral eyes are often post-embryonic 

 structures, they may, in some forms, arise during the embryonic stages. 



In such cases, Vespa, Astacus, Limulus and others, the lateral eye placodes 

 lie on the external margin of a deep infolding which gives rise to the optic ganglia, 

 in the same manner that the infolding in Acilius gives rise to the ganglia of the 

 larval ocelli. (Fig. 14.) The lateral eyes, however, are never involved in 

 this infolding. It soon closes, and the placodes move away from" the margin of 

 the cephalic lobes onto the posterior haemal surface of the cephalothorax (Limulus, 

 many trilobites and merostomes), or in some cases, onto its anterior margin, 

 or they may remain in their original position on the neural surface, (Cladocera). 

 (Fig. 78.) The position of the lateral eyes in the adult, therefore, varies greatly, 

 and is either determined by the prevailing position of the animal in relation to 

 the source of light, or the location of the eyes determines the position of the 

 animal. 



