VISUAL TISSUES 



2 4 I 



\ 



vta. c. 



The eyes of all higher creatures are organized to form a single image, 

 and it can be demonstrated that an image is formed by most of them. 

 An owl's or other bird's or mammal's eye, when the posterior side is 

 cleaned of fat, muscles, etc., and the corneal surface is pointed at a bright 

 window, shows a beautifully distinct picture of the window on its retina 

 from the rear. We also know that this image is transmitted to the 

 brain and consciousness in the case of our own eye. The question arises 

 as to how much of an image is formed by the compound eye. It has 

 been concluded that no recognizable image is produced in some cases; 

 while to see a male Papillio dart at a colored paper representation of 

 its mate, convinces one that this insect, at least, can see and recognize 

 form. With many other insects the eye probably does not record a 

 form or the brain 

 does not remember 

 it, but it is very quick vis. 

 to perceive any rela- 

 tive motion in the 

 objects around it. 



One more com- 

 pound eye with sepa- 

 rate lenses may be 

 studied in a serpulid 

 worm, Branchiomma. 

 Figure 214 represents 

 this eye a trifle dia- 

 grammatically. The 

 visual cells have de- 

 veloped out of every 

 third epithelial cell, 

 in section, and the 

 intervening cells are 

 pigment (iris) cells. 

 The visual cell of this eye is unusual in that it secretes a lens in its 

 distal cytoplasm as well as forming a nerve and visual cell-organ. 

 Besides doing all this it forms or takes part in forming the cuticle. These 

 various functions are the separate duties of differentiated cells in higher 

 forms, as the arthropods. 



The more complete yet simpler eyes, with single lens and a retina 

 which receives an image, are found among other worms and mollusks. 

 One has already been alluded to, but not described, in a medusa. 



No worm eye is to be found that is as low in its organization as that 

 of many mollusk forms such as Solen, etc. On the other hand, the 

 mollusks have the greatest variety, some of which are among the highest 



hyp. c. 



FIG. 214. Eye from the serpulid worm, Branchiomma. cu., 

 cuticle; hyp.c., hypodermal cells; sup.c., supporting cells 

 (slightly modified hypodermal cells) ; vis.c., visual cells (highly 

 modified hypodermal cells); vis.r., visual rods appearing as 

 curved hairs; /., lens formed by visual cells. (After HALLER.) 



