ON THE ANATOMY AND DEVELOPMENT OF PYROSOMA 327 
adocth of an inch. This results from the circumstance that the test 
is thickened at the margins of the mouth, so as to diminish its 
aperture to this extent; and it is at the same time puckered, so that 
when viewed from without,a number of fine grooves appear to radiate 
from the lips of the aperture. These must not be confounded with 
certain fine fibres which radiate from the outer margin of the 
sphincter into the test, and are perhaps muscular (PI. XXX. 
[Plate 29] fig. 6). 
The test ceases to be traceable upon the walls of the oral cavity a 
little within the sphincter ; and where it ends, the inner tunic is pro- 
duced inwards into a broad fold with lobed edges, which takes the 
place of that circlet of tentacles which is found in this position in 
most other Ascidians. I shall therefore term this the éextacular 
fringe. It is divided altogether into thirteen lobes, of which twelve, 
though irregular, are tolerably similar and roughly symmetrical, while 
the thirteenth is situated in the middle of the hemal half of the 
ccirclet, and is very different in form and size from the rest. It is, in 
fact, three or four times as long as they are, and is divisible into a 
broad trilobed base, shaped somewhat like an ace of clubs, and a 
narrow fringe-like terminal portion. This may be distinguished by 
the title of the Aemal tentacle (P|. XXX. [Plate 29] figs. 6, 6a). 
The form, size, and relative position of the oral aperture remain 
the same in ascidiozooids which have the oral aperture mounted upon 
a very short cone; but as the cone enlarges, its hamal grows faster 
than its neural side, and finally becomes prolonged into the labial 
process, which bends over at a right angle to the direction of the 
axis of the zooid Concomitantly with, and apparently as a result 
of, this development of the labial process, the plane of the oral 
aperture gradually shifts, until, in the first place, it lies parallel with 
the axis of the zooid, and then continuing to turn, as it were, on its 
hemal margin, it eventually takes up a position perpendicular to the 
axis of the ascidiozooid again, but exactly the reverse of that which 
it had at first. The labial process so completely overhangs the oral 
aperture when this stage is attained, that the free access of the water 
to the interior of the zooid must, one would think, be somewhat 
impeded. 
Two very delicate muscular bands, attached to the inner tunic, 
succeed one another at short intervals behind the aperture of the 
mouth, within which the buccal cavity rapidly widens, until it attains 
1 Tn the figures given by Lesueur and by Savigny, the axis of the labial process is parallel 
with that of the body. In most of the ascidiozooids of my specimen, the end of the process is 
turned tuwards the hzemal side, but in some it is bent the other way. 
