136 SOME ASCIDIANS FROM THE ISLE OF WIGHT. 



Now no one can liave mucli attended to tlie conditions of existence 

 in tlie littoral zone without having been impressed by the extent of 

 the disturbing forces which littoral animals have to resist, if they 

 are to survive in that locality. They are battered by the waves 

 almost incessantly, and cannot exist without special means of defence. 

 This defence in many groups is ensured by the development of 

 strong adhesive or clinging organs, the prevalence of which among 

 littoral animals shows, by a reversal of the argument, the extent of 

 the disturbing forces that play around them. 



Tunicates are essentially plastic creatures, for the structure and 

 mode of development of their tests renders their external form 

 easily modifiable. It would, therefore, be extremely improbable to 

 find that the larvae of Ascidia mentula, when carried by in-flowing 

 tidal currents from deeper water into the littoral zone, would grow 

 in quite the same way in one place as in the other. The incessant 

 motion of the water would necessitate, and indeed frequently bring 

 about, as growth proceeded, a larger area of attachment than would 

 suffice to resist the comparatively feeble currents of deeper water. 



The results of such a process would be (1) Hancock's Ascidia 

 ruhiciuida, which is merely the red-coloured variety of A. mentula 

 adapted to a littoral existence j (2) my specimens from the Isle of 

 Wight, which are merely the pale variety of A. mentula adapted to 

 a littoral existence upon a comparatively smooth surface of rock ; 

 (3) Hancock's A. rohusta, which is a pale reddish variety modified 

 in its mode of attachment by tidal influences, and in its general 

 shape by the irregularity of the surrounding objects {' roots ' of 

 Laminaria digitata) . 



Even Miiller a hundred years ago recognised the plasticity of 

 form in his species, for, referring to the oral and cloacal apertures, 

 he says : — >" Pro figura massae, quae ab adjacentibus corporibus 

 determinatur, vel utraque lateralis, vel altera plerumque terminalis.'^ 



If it should be objected that the Mediterranean zoologists can 

 supply little or no evidence of variability in the extent and mode of 

 attachment in their specimens, the fact is rather in favour of my 

 contention than against it ; for the causes to which the variation 

 has been here atti'ibuted are absent in the Mediterranean, 

 where the tidal oscillation, with its accompanying disturbance of the 

 sea-bottom, is so small that it may practically be neglected. 



With regard to internal structure, the differences between the 

 Isle of Wight specimens and those described by the Mediterranean 

 zoologists are very slight and unimportant. 



For a comparison of the descriptions of Mediterranean forms 

 shows that variability is not confined to points of colour and 

 external form. Traustedt gives the number of tentacles as from 



