OF SEA- ANEMONES. 77 



are frequently so numerous and so different in 

 appearance that it becomes necessary to view them 

 collectively and together with individuals of other 

 species, in order to define their proper position and 

 determine their relationship one to the other. Thus, 

 too, a long series of observations on their habits, 

 colouring, size and the like becomes equally neces- 

 sary to ensure a correct judgment on the subject. 

 For instance, some so-called species of Actinia have 

 no other appreciable difference from other species 

 than that of colour. But if it can be shown that in 

 the same individual the colour varies at different 

 times so greatly as to bring it within the limits of 

 either division, or that specimens occur which com- 

 bine the distinctive colouring of each assumed class, 

 in such a case the artificial arrangement would fall 

 to the ground. 



The necessity of continual observation is strikingly 

 shown in the case of Anthea cereiis. We have 

 stated that it is generically separated from the 

 Actinia, because it does not draw back its tentacles 

 within its body, after the fashion of the latter class. 

 Yet I have observed a colony of these animals in a 

 shallow pool, under the influence of a powerful sum- 

 mer sun, who, with few exceptions, had retracted 

 their tentacles within the margin of their bodies till 

 nothing but the purple tips were left exposed. Still, 

 prolonged observation confirms the fact that it is not 

 their habit to draw in their tentacles under the same 



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