60 JOURNAL OF MARINE ZOOLOGY AND MICROSCOPY. 



crustaceans profit largely under natural conditions from the flotation 

 support afforded by the surface film. Nebalia especially utilizes it, 

 and indeed seems to require to exert a special effort in order to 

 break through the film when wishful to descend. The Amphipod 

 Mmra (various species) is another similar instance out of several 

 other genera of Amphipods. Indeed there are quite a number of 

 small crustaceans which may be occasionally seen floating on the 

 surface by this means. Conspicuous among these others are the 

 Cypris or pupal larvae of the Barnacle (Balanus) and the fine Ostracod, 

 Asterope marios. To the former this habit is a very valuable one, as 

 it enables them to be lodged by the receding tide on the higher rocks 

 of the shore, just the situation suitable to their requirements ; the 

 presence of numerous oil-globules in these larvae, is of much interest 

 in this connection, and must be of the utmost use to the animals in 

 bringing and keeping them within the influence of surface tension. 

 The nauplii of this creature, though much smaller, are scarcely ever 

 seen on the surface film, as they keep ata lower level ; it is obvious that 

 were they to rise into the surface film they would experience great 

 danger of being cast ashore, a fatal accident for them but just the 

 contingency required by the Cypris-larvae. 



While in Nebalia, Mmra, the Cypris-larvae of Balanus and 

 some others, the habit is adopted, I believe, with a definite 

 object, there are many instances where it is practically certain that 

 the flotation is not voluntary and is accidental, the animal having 

 either jumped or been splashed on to the surface, where it is retained 

 by tension of the water-film, till, by a vigorous effort it breaks through 

 and is able to descend beneath the surface. 



In confinement, I have several times noticed anemones, chiefly 

 Actinoloba dianthus, floating on the surface, foot upwards, suspended 

 by the action of surface tension: This obviously is a ready mode of 

 transport but how far it is accidental and induced by the unusual 

 nature of its environment is difficult to determine, though as 

 Actinoloba often frequents quiet land-locked localities, e.g. South- 

 ampton Water, where perfectly smooth water is of common occurrence, 

 it may be that it is a natural habit. Actual observation in such 

 localities is therefore required to determine the question. 



As to the Molluscs, the smaller ones, such as Elysia, which 

 commonly frequent tidal pools where the water is normally quiet, 

 certainly do make use of this mode of progression, but it is more 

 than doubtful if the large Doris do under purely natural conditions. 



It is, however, among the Crustaceans that real service is 

 obtained by animals from the phenomenon of surface tension. To 

 the other groups it is at most but of very occasional use ; to Nebalia, 

 Cirripede pupal larvae, &c, it is of constant value, and of the highest 

 importance in the routine of their life history, indeed as regards the 

 Cypris larvae, very much emphasis must be laid upon the correlation 

 of this peculiar flotation with the special habitat of the adult animals. 



