THE PRAWNS. 165 



equal in size, and forming an angular band of spots. 

 The distinctions drawn from tlie form and dentition 

 of the rostrum, and the gibbous carapace of P. sqiciUa, 

 I need not speak of particularly, as these are suffi- 

 ciently appreciable in cabinet specimens. 



When viewed with a candle, the eyes of the PraAvn 

 reflect the light with a glare exactly like that seen in 

 a cat's eyes under similar circumstances. The light, 

 in the case of the Prawn, is seen only when the candle 

 is held between the beholder and the insect, and 

 becomes brighter and larger the more nearly the flame 

 of the candle is brought to the line which unites the 

 observer's eyes and the object. It might seem as if 

 nothing could possibly be discerned when the flame 

 is absolutely in this line, but it is not so ; both eyes 

 being open, the line of vision of each eye passes on 

 one side of the candle, and we discern the two eyes of 

 the Prawn, like t^vo little globes of fire. 



In the case of the cat the phenomenon is said to be 

 produced by the choroid coat at the bottom of the 

 eye {tapetum lucidum), reflecting from its polished 

 metal-like sm'face the entering rays, and converging 

 them by its concavity, as if from a concave mirror. 

 This is simple and perfectly intelligible ; but I do not 

 see how a similar effect is produced in the compound 

 eyes of the Crustacea, each of which is composed of 

 a great number of conical lenses with apices inward. 

 What is there in this structure that can represent 

 a concave mirror ? Yet no one can look at the sight 

 I am speaking of, without feeling certain that the 

 optical process is one of concave reflection and con- 

 vergence of the rays ; and that the reflecting surface 



