THE ORGANS OF SENSATION 175 



cornea, or with a cornea formed by the integument over a 

 simple, or compound eye, developing a transparency, and fre- 

 quently a thickening, of its convexity which transforms it into 

 a lens : these are invariably connected with the nerve-centres. 



All free molluscs (except those that live in burrows) have 

 visual organs ; even the fixed molluscs have in some instances 

 rudimentary eyes during the larval stage, although these may 

 disappear later. 



Well-developed eyes are met with on the sheath margin of 

 many of the Lamellibranchiata ; the eyes are placed in pairs in 

 the gastropoda, the most typical organs being met with in 

 the Cephalopoda. The eye of the Gastropoda consists of a 

 globe with retina, pigment layer and cornea, while the Cephalo- 

 poda possess a globe with cornea, lens and optic nerve ganglia, 

 lying in an orbit-like space ; in many ways the eye resembles a 

 vertebrate's eye. 



The eye of all Vertebrates has essentially an identical ar- 

 rangement of integument and nerve elements ; first, the primitive 

 optic vesicle appears as a process from the fore brain, and grow- 

 ing outwards against the integument develops an invagination of 

 its anterior wall to form the posterior optic cup (secondary optic 

 vesicle). Later on, from the horny layer of the skin, there is 

 formed a lens which separates from the epidermis, while the 

 proliferating tissues behind the lens have developed into the 

 vitreous ; the inner layer of the secondary optic vesicle becomes 

 the choroid, and the outer the sclerotic. The transparent 

 cornea unites with the conjunctiva, and inside the secondary 

 optic vesicle are developed the retina and tapetum. This latter 

 occurs only in fishes, in ostriches and certain mammals, not 

 being found in man. 



The cornea of fishes and many amphibia is flat, while in snakes, 

 crocodiles and the majority of birds it is curved. The relation 

 of the antero-posterior diameter to the transverse varies accord- 

 ing to the different species of Vertebrates ; in birds, as a rule, 

 an anterior and posterior segment of the eye can be recognised, 

 and in birds of prey the antero-posterior diameter is much 

 greater than the transverse. The long axis is the shorter, 

 however, in aquatic and long-legged birds, and among mammals 

 in the Cetacea, ruminants and Equidae (Ungulates). In most 



