148 WALTER GARSTANG, M.A. 



The Ascidia mentula of authors is an inhabitant of the deeper waters, 

 and is found attached usually to stones and shells by its base and a 

 very little of the left side. Adhering in this way, it is obvious that 

 it has an erect position upon the sea-bottom. Now the three species 

 named above were distinguished by Hancock from Ascidia mentxda 

 partly on account of the mode of their attachment ; A. rubrotincta 

 adhered " by the middle portion of the side," A. rubicunda " by the 

 whole side with imperfect marginal expansions," A. robusta " by the 

 whole side, but [was] sometimes much distorted, and with adherent 

 root-like prolongations." 



These three " species " were all taken from between tide-marks, the 

 first at Guernsey, the second at Tobermory (Isle of Mull), Portaferry 

 (Strangford Lough), and Bertraghbuy Bay (Connemara), the third at 

 Herm. 



The Isle of Wight specimens also were attached by the whole or 

 the greater part of the left side, and they also were taken from a rock 

 at low water. 



Now no one can have much attended to the conditions of existence 

 in the 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 pro- 

 ceeded, 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 

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



