358 THE MAINTENANCE OF THE INDIVIDUAL 



Compound Eyes 



The intergradation from the type of photosensitive cells mentioned 

 above, to a primitive eye, or eye spot, is a gradual one. One of the 

 first steps in the production of a simple eye spot appears to be the 

 concentration at a given point of a number of light-sensitive cells con- 

 nected with nerves. From such simple beginnings two types of eyes 

 have been evolved in the animal kingdom, the compound eye of the 

 insects and Crustacea, and the camera eye of certain molluscs and the 

 vertebrates. The compound eye is composed of a varying number of 

 complete individual eyes called ommatidia. Each ommatidium is di- 

 rectly connected with the brain and produces a separate image that, 

 joined to others, gives a unified picture. It has been ascertained by 

 counting the exposed surfaces, or facets, of the ommatidia that there 

 may be present any number from a few dozen up to several thousand. 

 Some ants have about fifty, while the swallowtail butterfly has seven- 

 teen thousand, and dragonflies still more in each eye. The walls of 

 each ommatidium are surrounded with pigment cells that absorb all 

 tangential rays, consequently only those rays which penetrate straight 

 in through the facet reach the sensory areas located in the retinular, 

 or photoreceptive, cells. On account of this restricted intake, each 

 ommatidium receives for interpretation only a small portion of the 

 rays entering through the cornea. It is believed that there is no 

 marked overlapping of images since each image is recorded in a differ- 

 ent spot, the end result being a series of small images one next the 

 other, which act to produce the completed picture, called an erect 

 mosaic (see figure, page 206). 



Camera Eyes 



The camera type of eye in invertebrates reaches its peak in the 

 molluscan squid and, among the vertebrates, in the human eye. 

 These two types offer a good illustration of analogous structures. A 

 study of the development of these two types of eyes shows that the 

 position of certain elongated cells of the retina, called the rods and 

 cones, are reversed in the two forms, and consequently while their 

 function is in general the same, or analogous, their type of structure, 

 or homology, is different. The vertebrate eye is almost spherical, 

 and fits into a funnel-shaped socket of bone, called the orbit, while 

 the stalklike, optic nerve connects the eye directly with the brain. 

 Free movement is made possible by means of six small muscles which 



