238 THE PINEAL ORGAN 



Before passing on to the description of the pineal system of Reptilia 

 it will be worth while to recall some of the effects which ensue from the 

 disuse of the lateral eyes in animals which have for many generations lived 

 in situations where they are completely cut off from light and are unable 

 to use the eyes as a guide to locomotion or the capture of food. The 

 structural degeneration of the lateral eyes under these circumstances is 

 very similar to that which has occurred in the pineal sense organs. In 

 both cases there is a withdrawal of the unused sense organ beneath the 

 superficial structures, a disappearance of the refractile mechanism and of 

 the sensory or optic nerve-fibres which connect the eye with the central 

 nervous system. Examples of these degenerative changes are found in 

 widely different classes of animals and in both vertebrates and inverte- 

 brates. They are well known in such cave-dwelling animals as Proteus 

 anguineus ; burrowing animals such as the mole or common earth worm, 

 or as we have previously mentioned Balanoglossus and Amphioxus which 

 bury themselves in the sand. Other instances occur in the deep-sea 

 fishes which live in total darkness, 2,000 to 3,000 fathoms beneath the 

 surface of the ocean, and there are also the parasitic worms and other 

 animals which live imbedded in the flesh, tissues or organs of their hosts, 

 and those which in their adult state become fixed like the barnacles to rocks 

 or other objects, and which though possessing motile processes or cilia are 

 incapable of locomotion and become blind. 



One of the most interesting examples of such structural degeneration 

 is the olm, or Proteus anguineus (Fig. 159, p. 225). It lives in the sub- 

 terranean streams and pools of Carniolia, Carinthia, and Dalmatia, and 

 resembles a lizard, but has external gills uncovered by an operculum ; 

 these may be removed without causing the death of the animal, which 

 has lungs like those of other amphibia. The colour of the animal has 

 been described as being of a " fleshy whiteness and transparency," and 

 in this respect as well as in the retention of its external gills it resembles 

 the Mexican axolotls (Amblystoma tigrinum) which also retain the larval 

 condition of their gills. 



The general appearance of Proteus anguineus and the condition of the 

 eyes have been well described by Karl Semper (1883). The eye is com- 

 pletely buried beneath the superficial structures, being entirely covered 

 by the skin. It has with the exception of the lens and the adjoining ciliary 

 apparatus and iris all the characteristic parts of the eye. The development 

 of these parts has, however, been arrested at an early embryonic stage. 

 Inversion of the retina has taken place, and there is a deposit of pigment in 

 its outer layer. Notwithstanding the imperfect development of the eyes, 

 Semper was able to prove from observations he made on a family of 

 Proteus that he kept for four years that the creatures were " highly sensitive 



