70 N. AnnANDALE : The Fauna of Brackish Ponds. [YOL. I, 



degeneracy of the basal disk in the pond race. I do not know of 

 any other form of the genus in which these characters are so 

 strongly marked; but many instances among the Actiniaria could 

 be adduced in which there is a considerable tendency to variation 

 as regards them. Anyone who has observed living examples of 

 the common British Sagartia troglo4ytes from different parts of 

 the country, or even from different situations in the same loca- 

 lity, must have been struck by the differences they exhibit as 

 regards the form of the column and the relative proportions of 

 its base. Those individuals which have been extracted from small 

 crevices in rocks have a long, thin column and a base with a 

 small transverse diameter, while those from pools with smooth 

 bottoms are short and squat. In Gosse's History of the British 

 Sea- Anemones (i) figures are given of the species in the latter 

 condition. As regards outline at any rate, these figures are ac- 

 curate ; but they are as unlike as they could well be to some in- 

 dividuals I have seen. Moreover, I have noticed that in such cases 

 the column cannot adapt itself, except to a limited extent, to new 

 conditions, even although the individuals may be kept alive for 

 many years in captivity. Those individuals which have been 

 living in small round holes such as are a favourite station for the 

 species, cannot assume the depressed conical form that character- 

 izes those which have been fixed to a smooth surface ; but 

 those which have been taken from the latter situation are able to 

 elongate their columns considerably^ and to draw in the project- 

 ing margin of their bases. In other British species differences, 

 which may be local, have been recorded, e.g., Dixon (5) states that 

 specimens of 5. nivca from the east coast of Ireland are much 

 longer and more attenuated than those described from Torquay, 

 on the south coast of England, by Gosse. From Indian seas 

 Alcock (7) has described a variety of Sphenopus arenaceus in which 

 the base of the column is drawn out into a relatively long and 

 narrow peduncle. 



In none of these cases has the basal disk become degenerate to 

 the same extent as it has done in the tank form of M. schillerianum , 

 for there is no basal disk in the genus Sphenopus ; but in other 

 respects the variation seems to be of a similar nature. It must 

 be remembered, moreover, that there is a great difference, in res- 

 pect to the condition of the base, between the young and the adult 

 of M. schillerianum var. exul, as well as in respect to the proportions 

 of the column. It must further be borne in mind that this Actinian 

 lives in a medium the chemical constitution of which is different 

 from that of the medium proper to its class, and there is ver}^ 

 good reason to believe that a chemical stimulus may be a powerful 

 one in matters of variation. The particular direction which evolu- 

 tion has taken in respect to this isolated race, moreover, is one 

 which has not been uncommon in the history of the sub-class to 

 which M. schillerianum belongs, for we get forms as distinct from 

 one another morphologically as Edwardsia, Cerianthus and Peachia 

 all adapted in a similar manner to become burrowing animals, and 



