590 



THE EYE IN EVOLUTION 



the adjective, incidentally, applies not to the habits of the insect but to its 

 characteristic stance with its front legs raised as if in an attitude of prayer. The 

 female is a particularly anti-social creature who will eat anything in sight, 

 including her mate. Since she can only see moving objects, the male approaches 

 her with staccato movements, standing motionless whenever she looks in his 

 direction, exactly in the manner of the children's game. Grandmother's Footsteps. 

 Fortunately, the male has better vision than the female and usually manages 

 to approach her in this cautious manner until he can leap upon her ; but 

 the end is usvially the same because he is generally eaten either while mating 

 is in progress or after it is finished. 



ACCOMMODATION IN INVERTEBRATES 



Cephalopod, 

 Loligo 



The relative simplicity of the eyes of Invertebrates would not 

 lead us to expect elaborate accommodative facilities ; from the 

 functional point of view, of course, the degree of visual acuity of most 

 types would not merit a complicated mechanism of this nature. In 

 rare cases a muscular apparatus provides an active method of accom- 

 modation somewhat analogous to that characteristic of Vertebrates. 

 An exceptional device is a forward movement of the lens by increasing 

 the contents of the globe by secretory activity. More often, however, 

 any accommodation that is present is static in nature and depends 

 on the provision of different optical systems in the same eye or in 

 different eyes, one being adapted for distant vision and the other for 

 near. 



An active muscular apparatus to produce an accommodative change of 

 focus is seen in its most elaborate form inainly among Molluscs ; it acts pri- 

 marily by compressing the globe, that is, altering the position of the lens second- 

 arily, a method of accommodation, incidentally, adopted by snakes.^ Such an 

 accommodative mechanism is seen in its highest form in the eyes of Cephalopoda 

 (Figs. 113, 114). Beer (1897), Heine (1908) and Pflugk (1910) considered the eyes 

 of Cephalo23ods to be normally myopic (— 2 to — lOD), but v. Hess (1909) found 

 them to be emmetropic or slightly hypermetropic. This author concluded that 

 a considerable degree of amplitvide of accommodation is effected by the for- 

 ward displacement — not the deformation — of the lens, the mechanism being 

 the relatively simple one of compression of the globe by the contiaction of the 

 ciliary muscle, an action which raises the intra-ocular pressvire so that the 

 vitreous pushes the lens forwards passively, thus producing a positive accom- 

 modation of 10 to 14 dioptres (v. Hess, 1909; Alexandrowicz, 1927) ; this effect 

 can be abolished by atropine (v. Hess, 1909-12) and augmented by electrical 

 stimulation of the cerebral ganglion (Magnus, 1902). 



A somewhat similar method is seen in the Heteropod, Pterotrachea (v. Hess 

 and Gerwerzhagen, 1914). The accommodation of the pulmonate, Onchidium, 

 is closely allied : a muscular collar surrounds the distal part of the eye which, 

 on contraction, alters the shape of the globe in an analogous manner. In the 

 cockle, Cardium, the whole globe is invested with muscvilar fibres the contraction 

 rif which may serve as a similar and very primitive accommodative device. 



648. 



