and often most gorgeous colouring. One of tlie commonest 

 species of our seas, Ascidia vlrglnca, 0. F. Miiller, 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, PI. 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 dredge ful one usually finds 

 all sizes, from a quarter of an inch to two inches in length. 

 Our largest British species, Ascidia mentula, 0. F. Muller 

 (see PI. I.), measures from three to six inches in length, 

 and is usually found on a muddy bottom in from ten to 

 fifty fathoms. Either 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. 



EXTEENAL ApPEAEANCE. 



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, PI. II.), 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 PI. I.- — -as an mdividual peculiarity this specimen 

 has a seventh small lobe at its atrial aperture). Conse- 



