THE THREE-SPIN 7 ED STICKLEBACK 245 



plates are entirely wanting and the snout is longer 

 than in our Stickleback, whilst the Algerian Stickle- 

 back (G. algeriensis) may be shortly characterized 

 as a gymnurus form with not more than four small 

 plates on each side, the spines very small, the pelvis 

 very short and broad, and the vertebrae reduced in 

 number, only twenty-nine instead of thirty-one to 

 thirty-three. 



Four-spined examples are not very uncommon, 

 and have been wrongly described as distinct species ; 

 a series from Newark sent to me by Mr. Littler 

 includes not only a four-spined specimen but also a 

 two-spined one, the latter being a great rarity. 



As the Three-spined Stickleback is found both 

 in fresh and salt water, some curious experiments 

 have been made to ascertain the result of sudden 

 transference from one medium to the other. These 

 seem to show that, as might be expected, when 

 Sticklebacks accustomed to live in fresh water are 

 suddenly placed in salt water they do not long 

 survive the change ; on the other hand, estuarine 

 examples of any of the varieties can be readily 

 transferred suddenly from salt to fresh water and 

 vice versa, without seeming to notice the difference. 



The Three-spined Stickleback lives in shoals and 

 is especially abundant in small streams, ditches, 

 and ponds. The shoals are sometimes very large, 

 and as an example of the extraordinary numbers 

 attained we may quote Pennant, who in 1776 wrote : 

 " Once in seven or eight years amazing shoals 

 appear in the Welland and come up the river in the 

 form of a vast column. The quantity is so great 

 that they are used to manure the land, and trials 

 have been made to get oil from them. A notion 



