ACCOMMODATION IN INVERTEBRATES 



591 



A different type of muscular mechanism appears to occur in the Copepod, 

 Copilia (Fig. 139) ; the long slender muscle running along the side of the elongated 

 eye may not only move this organ in different directions but also act by altering 

 the distance between the lens and the receptor elements and thus provide an 

 accommodative adjustment. This is reminiscent of the way in which Cyclo- 

 stomes accommodate.^ 



A unique method appears to be present in the elaborate eyes of certain 

 Polychietes such as Alciopa (Fig. 112). It is said that stimulation of the secretory 

 cell increases the volume of the " distal vitreous " lying immediately behind 

 the lens, and it has been suggested that this pushes the lens forwards to accommo- 

 date the eye for near vision. In this eye there is in addition an accommodative 

 muscle similar to that in Cephalopods the contraction of which should also be 

 effective (Demoll, 1909 ; v. Hess, 1914). 



These active mechanisms, however, are exceptional. More usually, accom- 

 modation is achieved by the static device of the presence of two optical systems 

 in different parts of the eye. The simplest example of this is seen in the ocelli 

 insects. In the grasshopper, for example, there is a double curvature on 

 the proximal surface of the corneal lens which thus acts after the manner 

 of a bifocal spectacle lens and seems to be capable of producing two images at 

 different distances (TumjDel, 1914). 



By its nature the optical arrangements of the compound eye do not admit 

 accommodative adjustment, but this is rendered unimportant in the mosaic 

 type of vision. It would seem, however, that the different optical configurations 

 seen in different segments of certain compound eyes which are so arranged that 

 in one region there are short ommatidia and powerful lenses and in another 

 region long ommatidia and weak lenses, may provide alternative focusing 

 mechanisms. This is seen in its most dramatic degree in composite compound 

 eyes such as those of some Ephemeroptera and Diptera (Dietrich, 1919) and some 

 Hemiptera (Weber, 1934) (Fig. 140), and of certain pelagic Schizopods wherein 

 one part is adapted for near and the other for distant vision (Fig. 141) (Hesse, 

 1908). 



Finally, two separate eyes may exist, one optically adapted for distant 

 objects and the other for near. This is exemplified in the median and lateral 

 ocelli of spiders, 2 while the same expedient is also adopted in the dorsal and 

 ventral compound eyes of the whirligig beetle, the former being adapted for 

 aerial and the latter for aquatic vision (Fig. 231;. 



Cardium 



Copilia 



Grasshopper 



Wliirligig beetle 



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1 p. 644. 



- p. 580. 



