PEOPLING OF LAND AND SEA 151 



flexing movements thereof. This last method of locomotion 

 is not unknown among the crawling Decapods, but they employ 

 it less often. Their abdominal appendages are not utilized in 

 swimming, and the creature progresses by the aid of ten 

 strong legs borne on its thorax. The swimming Decapods keep 

 below the surface most of the time, and move with great agility ; 

 the crawlers, on the contrary, hardly leave the bottom, with 

 which their feet are most generally in contact, and which they 

 are perpetually feeling, so to speak. The deep-sea adaptations 

 of these two types of Crustaceans takes place, consequently, 

 along two opposite lines. The antennae of the swimming 

 Crustacean become fine and exceedingly elongated, so as to 

 serve as tactile organs x to warn the creature of the least 

 obstacle, and their eyes become greatly enlarged. The 

 antennae of the walking Crustacean, on the contrary, remain 

 relatively short, and their powerful thoracic appendages dis- 

 appear, as also their eyes, now unnecessary owing to the 

 creature's extreme caution in moving. 2 The increase in size of the 

 eyes of the swimming Crustaceans appears at first somewhat 

 paradoxical. But the darkness of the great depths is not absolute. 

 Many organisms become light-producing. Among these are the 

 Gorgonid Polyps, Crustaceans like Gnathophausia and Euphausia, 

 which bear luminous organs on their appendages, and numerous 

 swimming Decapods ; certain Squids are provided with 

 veritable light-projectors ; and many Fishes 3 have luminous 

 organs situated either on the head or in series on the sides of 

 the body, like the organs of the lateral line. We are unable to 

 say whether it was the darkness that caused the elongation of 

 the appendages and stimulated the development of the luminous 

 organs. Vire, however, has shown that the appendages of 

 certain Crustaceans kept in obscurity become very much 

 elongated ; and this happens likewise in the case of certain 

 Insects inhabiting dark caves. It is probable that the constant 

 use these animals make of their appendages for palpating 

 their surroundings has contributed to this elongation ; and it 

 is quite normal that where no stimulus occurs the eyes should 

 disappear. As to the frequency of luminous organs among 

 deep-sea organisms, one might say with the defenders of the 



1 Nemalocarcinus gracilipes, Pandalus, Benthesicymns, etc. 



2 Pentacheles, Nephropsis, Galathrodes, etc. 



3 Chauliodus, Stomias, Malacosteus, etc. 



