SOME OTHER CRUSTACEA. 213 



Turn now to the telson, or last segment of the body, and 

 you will find that this is deeply cleft at its tip (t in Fig. 61), 

 and bears twenty-one to twenty-seven spines on either side. 

 Note at the same time the curious ear (o) in the swimmeret. 

 Minute points of no importance you will probably think 

 these, but your respect for them will probably increase 

 when you examine specimen after specimen and find them 

 constant, true indices of those subtle undefinable characters 

 which make up the species M. flexuosa. There are few 

 more striking illustrations of what is meant by the con- 

 stancy of nature, than the characters of nearly related 

 species like those of the genus Mysis. In many cases the 

 species is defined by the relative size of two structures, or 

 by the number of spines borne by an organ. What invisible 

 force is it that limits the growth of the antennal scale in 

 M. flexuosa when it is twice as long as the stalk of the 

 antennule, and allows that of M. vulgaris to grow till it is 

 four times as long? Why should the latter never have 

 more than twenty-five spines on its telson when the former 

 may have twenty-seven ? When these and similar questions 

 crowd upon you, then the fascinations of species work will 

 become clear. One would not of course deny the existence 

 of variability here, as elsewhere, but very little species work 

 will serve to convince you of the essential constancy upon 

 which the variability is superimposed. 



The characters given above will be found sufficient to 

 identify M. flexuosa. Another species, smaller in size and 

 much less common, may be sometimes found with it. This 

 is M. vulgaris, which, though occasionally found in rock 

 pools, is typically an inhabitant of tidal rivers and estuaries. 

 It is most likely to be found in the pools left by the ebbing 

 tide on those mud flats which in Northumberland are called 

 "slakes"; or sometimes occurs in myriads at the edges of 

 tidal rivers. This species may be recognised by the fact 

 that the antennal scale has no spine, is furnished with setse 

 all round, and is four times as long as the peduncle of the 

 antennules. The telson is not cleft, and ends in four spines. 



There are a great many other species of Mysis found 

 more or less commonly on our shores, but for these reference 

 must be made to Canon Norman's papers. I shall mention 

 one more only, which I have found to be not infrequent on 



