ASCIDIA. 149 



tions how scanty is our knowledge of the objects of Creation, how fruit- 

 less our conjectures, how fallacious all our reasoning from analogy. 



But this, at the same time, offers the strongest inducement to search 

 for the truth, by steady perseverance in observation and experiment. 



The Ascidia papilla is the most common of any of the tribes inhabit- 

 ing our Scotish seas. Almost every substance is readily, and many of 

 them most profusely, invested by it. Vegetables, zoophytes, wood, shells, 

 rocks, and stones, serve alike for its abode : Nor does any of these seem 

 to be chosen in preference. Here it is affixed, solitary, associated in 

 groups, or conglomerated in masses. 



The single animal is of an irregular hemispherical form, somewhat 

 elongated upwards, with two tubular projections and orifices above, like 

 the rest. Under this aspect, it is of all different dimensions, within nine 

 lines in diameter and nine in height, — though seldom reaching that size — 

 and it is of all different shades, from dingy white and carmine to tile-red, 

 or somewhat brighter. However, it is commonly seen as a low spherical 

 segment, with two nipples, and adhering by the base. Larger specimens 

 are usually inclined, not upright ; and in these, both orifices are found to 

 be quadrangular. Infinite discrepancies occur, both in size and colour. — 

 Plate XXXVII. figs. 1, 2. 



Where dwelling in society, this Ascidia is dispersed over the surface 

 of shells : if in groups, five or six are incorporated together as a common 

 mass, or there may be a congeries of thirty, intimately united. 



One particular group consisted of at least fifty Ascidia?, adhering to 

 each other, and covering an area equal to fifteen lines square. This group 

 was three lines in thickness, in some parts comprehending two Ascidia', 

 that is, some individuals over some others. The congeries was profusely 

 overspread by the Pedicellina, and the whole surface darkened by muddy 

 matter. The vicinity of mud seems congenial to all the species. 



On vitiation of the water, the Ascidia merely swells and stretches 

 upward, as if to escape the deleterious influence ; thus evincing its in- 

 ability to detach itself — though some animals contract closely, as the 

 Actinia, Solen, and others, while enjoying the faculty of quitting their 

 place : Thereby manifesting a very low degree of instinct. 



