SOME ASCIDIANS PROM THE ISLE OP WIGHT. 149 



adapted to a littoral existence ; (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. robusta, 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 attributed are absent in the Mediterranean, where the tidal 

 oscillation, Avith 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 78 to 85 in Neapolitan 

 specimens, while Heller, who also examined the species in great detail, 

 ascribes from 30 to 35 to Adriatic examples. There are 30 in one of 

 mine, 40 in the other. Herdman's Ascidia lata (3£ inches long ; one 

 specimen) possessed from 16 to 20, and the species was defined upon 

 the ground of this difference* and of a peculiarity in the aperture of 

 the dorsal tubercle. 



Take again the dorsal lamina. Heller unfortunately gives no 

 details upon this point, but Traustedt and Roule agree that the 

 lamina is strongly pectinated. In Roule's specimens the right face 

 of the lamina is also provided with a few smaller " languettes." On 



* Since the above was put in type, I have been enabled to examine some 

 specimens of A. mentula, which were dredged in Loch Long and are now 

 under Mr. Hoyle's charge in the Manchester Museum. The number of 

 tentacles is so variable as to be only 18 in an individual 4| inches long, while 

 it is nearly 40 in an individual 3 inches long. 



