CRENELLA. 201 



foot is white, very narrow, and finger-shaped, moderately 

 pointed; but when fully extended it takes the form of 

 a very narrow flat tape, with a slight brown line running 

 longitudinally from its base to the point : it is protruded by 

 the animal close by the anterior side of the byssus. As an 

 organ of locomotion it can be of little use until the animal 

 liberates itself, which it has, we believe, the power of doing 

 to change place : it may be of use as an organ of tact. 

 Change of place can only be effected when it is fixed to 

 stones, and not when buried in the coriaceous coat of the 

 Ascidia. Its mode of operation to detach itself from 

 stones, &c., is to discard the old byssus and spin another 

 elsewhere, as we have seen. There are on each side of the 

 body two pale orange-coloured branchiae, smooth on all 

 surfaces ; also two subtriangular, long, pointed, pale brown 

 pectinated, pendulous palps." 



0. marmorata is common wherever Ascidia are found, 

 all round our coasts, and in many places is very plentiful, 

 enjoying a range in depth of from low-water mark to forty 

 fathoms. It burrows in the tests of tunicated mollusks, 

 especially Ascidia mentula. " In the skin of Cynthia 

 tuherosa and Ascidia sordida, especially the latter " 

 (Alder). " In the coats of tunicated mollusks generally, 

 both simple and compound " (W. Thompson). It ranges 

 throughout the European seas, and is an ancient species, 

 dating its appearance within our own area from the epoch 

 of the coralline eras'. 



VOL. II. D D 



