PHYLUM CHORDATA 



413 



recording images of distant and near objects by changes in the 

 convexity of the lens caused by its own elasticity, and the pull 

 exerted upon it by the elastic choroid coat and the ciliary 

 muscles (C.P.). In viewing near objects the ciliary muscle 

 counteracts the pull of the choroid coat and allows the lens to 

 assume a more convex shape, whereas distant objects are made 

 distinct by the flattening of the Jens. 



The eye is moved by six muscles; four stra'ight (reclus) and 

 two oblique. Folds of skin, the eyelids, protect the eye in higher 

 vertebrates. There may be three eyelids : an upper and a lower 

 lid which act vertically, and a lateral lid (nictitating membrane) 

 which moves outward from the inner angle of the eye. In some 

 reptiles the eyelids are transparent and fused over the eye. 

 Terrestrial vertebrates have lacrymal glands in connection with 

 the eye, the secretion from which keeps the surface of the eye- 

 ball moist and washes away foreign particles. 



Reproductive System. The sexes of vertebrates, with few 

 exceptions, are separate. The reproductive organs arise in 

 close connection with the excretory organs, and the excretory 

 ducts may serve to carry germ-cells to the exterior. Fertiliza- 

 tion takes place in some AMPHIBIA and most fishes after the 

 eggs are extruded. In other vertebrates fertilization is internal. 

 Most vertebrates lay eggs, i.e. are oviparous, but many of them, 

 especially mammals, bring forth their young alive, i.e. are 

 viviparous. 



