ASCIDIA. 39 
and often most gorgeous colouring. One of the commonest 
species of our seas, Ascidia virginea, O. F. Miller, has the 
form of a short and somewhat irregular cylinder, with 
rounded ends, one of which is attached to a stone or dead 
shell, or some other object at the bottom, while the other 
end is directed upwards into the water, and bears two 
short projections, each terminated by an opening (see 
fig. 1, Pl. II.). This species is of a dull grey colour; it is 
usually found in from five to twenty fathoms of water, and 
is in some places so abundant that the naturalist’s dredge 
may come up absolutely filled with it. An average size is 
an inch and a half in length, an inch in breadth, and half 
an inch in thickness; but in a dredgeful one usually finds 
all sizes, from a quarter of an inch to two inches in length. 
Our largest British species, Ascidia mentula, O. EF. Muller 
(see Pl. I.), measures from three to six inches in length, 
and is usually found on a muddy bottom in from ten to 
fifty fathoms. Hither of these two species of Ascidia will 
serve very well as the type of a simple ascidian, and the 
following description in nearly all details applies to both. 
EXTERNAL APPEARANCE. 
The two openings at the upper end, although they 
appear very similar at first sight, are really different, and 
can readily be distinguished with a little practice. One of 
them (Br. in fig. 1, Pl. I1.), is higher, or more nearly 
terminal in position than the other (At.), which may be 
placed some way down one edge; the former is the 
Branchial, and the latter the Atrial aperture. A close 
examination will show that the margin of the branchial 
aperture is cleft into eight small projections, or lobes; 
while the atrial aperture is bounded by six lobes only (see 
also Pl. I.—as an individual peculiarity this specimen 
has a seventh small lobe at its atrial aperture). Conse- 
