THE WONDER OF LIFE 525 



by the influence of gravity. The lantern is then retracted 

 and the teeth swing forward into position for initiating a 

 new lurch. 



Eyes that shine at Night. — Every one knows the 

 gleam of a cat's eyes when a light catches them in the 

 darkness. This appears to be due to reflection from a 

 layer behind the retina called the choroid tapetum. This 

 layer includes numerous flat cells packed with crystalloid 

 bodies which act Uke a mirror. In some beetles and moths 

 the eyes shine hke rubies when they are obhquely illxmiined 

 at night. Prof. Bugnion has recently studied the eyes of 

 one of the hawk-moths, Sphinx euphorbise, and finds 

 that the retina is very thick and infiltrated with a rose- 

 coloured pigment, ' erythropsin.' Part of the retina 

 forms a tapetum, and the reflection is due to a network of 

 silvery air-tubes or tracheae, helped to some extent by 

 movement of the retinal pigment. It is probable that the 

 reflection of the light rays from the tapetum is advan- 

 tageous, since the visual cells are thus affected twice 

 instead of once. 



The Chick's Egg-Tooth. — An adaptation that gives 

 one pause is the ' egg-tooth ' found at the tip of the bill in 

 many young birds, and used by them to break a way through 

 the imprisoning egg-shell. It is a hard thickening of horn 

 and lime at the tip of the bill, and since it develops before 

 the horny ensheathment of the beak it may be a residue 

 of a very ancient scaly armature in Reptihan ancestors of 

 birds. Be this as it may, the instrument is an effective 

 one, and it is used only once ! What happens is this : 

 the young bird ready to be hatched thrusts its beak into 

 the air-chamber that forms at the broad end of the egg ; 

 air rushes down the nostrils and fills the lungs for the first 



