SAND-HOPPERS 



153 



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fresh-water creature common in weedy streams, which 

 has no English name (except the general one of " fresh- 

 water shrimp "), and is called by naturalists Gammarus. 



In the open sea there are many hundreds of kinds 

 of small crustaceans resembling the sand-hoppers in 

 their compressed (not flattened) shape of body and in 

 the details of their legs and the grouping of the joints 

 of the body. 

 Many of the 

 smallest crusta- 

 ceans which 

 swarm in the 

 surface waters of 

 the sea and form 

 part of that float- 

 ing population, 

 mostly of small 

 transparent or 

 iridescent and 

 blue creatures, 

 which we call the 

 " plankton," or 

 " surface - float- 

 ing " population, 



and may be gathered by towing a very fine net behind a 

 boat on a quiet day, can produce flashes of light which are 

 vivid enough when seen at night. They contribute, to- 

 gether with jelly-fish and the teeming millions of minute 

 bladder-like Noctiluca, and other unicellular animalcules, 

 to produce that wonderful display seen from time to time 

 on our coasts, and called " the phosphorescence of the 

 sea." These minute crustaceans produce flashes of light 

 by suddenly squeezing from pits or glands in the skin 

 a secretion which is chemically acted on (probably 



a,. 



Fig. 20. — a, Talitrus locusta, b, Orchestia littorea, 

 the two common kinds of " sand-hopper. " Of the 

 natural size, c, A kind of small lobster which 

 burrows in the sand, Callianassa subterranea. 

 About two-thirds the natural size, linear. 



