40 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
quently, we can distinguish the apertures both by their 
position and by the number of their lobes; and this is a 
very important matter, since it enables us to recognise 
-the different regions in the body of the Ascidian; and also 
‘because the exact number of lobes is a characteristic of 
certain families and genera of Ascidians, e.g., Molgula 
has six branchial and four atrial lobes, while Ascidia 
has eight branchial and six atrial. Plate I. shows the 
exact shape of the lobes and the apertures in Ascidia 
mentula. | 
The branchial aperture (Br.) indicates the anterior end 
of the body, the region which corresponds to the head end 
of a man, a dog, or a fish; and, consequently, the opposite 
part, which is attached to the stone, is the posterior end. 
Then, the atrial aperture (At.) 1s invariably placed to the 
dorsal side of the branchial; hence, that side of the body 
near which the atrial aperture is situated corresponds to 
the back of the man, dog, or fish; while the opposite side — 
is, of course, ventral. 
- Now that it is known which is the anterior end, and 
which is the dorsal surface, or back, it is easy, by placing 
the Ascidian alongside oneself, and comparing the parts, 
to determine which is the right-hand side and which is the 
left. In Pl. I. and in Pl. IL., figs. 1 and 4, it is the mght 
side of the specimens which is shown. The area of attach- 
ment at the posterior end frequently involves a certain 
amount of the left side. 
THE TEST. 
The outside of the Ascidian—all that is visible, unless 
one looks into the expanded apertures—is formed by a 
stout, gristly, translucent layer called the Test or Tunic 
(hence ‘‘Tunicata,” the name of the group), which 
is notable amongst animal structures for containing 
