AND DEVELOPMENT OF PYROSOMA. 
203 
and the whole ascidiariuni may be regarded as a succession of tiers of ascidiozooids 
♦enveloped in a common test. 
The extreme apex of the cone (PL XXX. fig. 5) is formed by only four ascidiozooids 
ranged round a common point. In the next tier there are at least twelve, and the number 
increases until, in the widest part of the ascidiarium, there are between thirty and forty 
in a tier. It should be understood, however, that there is nothing very regular in tin- 
arrangement of these tiers, and that the zooids in any given tier are of very various 
sizes and degrees of development. 
The Ascidiarium presents for study (1) the ascidiozooids, and (2) the common test which 
envelopes them*. 
The Ascidiozooids. — In investigating the structure of the ascidiozooids, an example 
from the middle region of the ascidiarium may most conveniently be Belected for study. 
Such an ascidiozooid is represented in longitudinal section in PI. XXX. Jig, 1, in trans- 
verse section in fig. 2, and from above and partly in section in tig. 3. 
It is somewhat irregularly fusiform, a good deal longer than deep, and deeper than 
broad. Its outer extremity exhibits the oral aperture, win eh lies upon the haemal ride of 
one of the above-mentioned conical protuberances, and is overhung by a tongue-like 
process of the test— the labial process, by whose outgrowth, indeed, its relations and 
appearance have become so completely altered, that it will be better to become acquainted 
with the character of the oral aperture in a less modified specimen. On examining one 
of those oral apertures, in fact, which are hardly, or not at all, raised above the general 
level of the outer face of the ascidiarium, the plane of the oral aperture is seen to be 
perpendicular to the axis of the body (taking a line drawn from the oral to the cloaeal 
aperture as that axis). A circular sphincter, composed of a band of unstriped muscular 
fibres, surrounds the oral entrance, being attached where the lining membrane of the ali- 
mentary tract (inner tunic : see note, p. 202) and the integument (outer tunic) pass into 
one another. The inner diameter of the circular sphincter is jfeth of an inch; but the 
diameter of the oral passage itself is far less, amounting to not more than ToW* 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 t he same time 
puckered, so that when viewed from without, a number of fine grooves apj, ir 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 (PL XXX. 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 produced inwards into a broad fold with 
lobed edges, which takes the place of that circlet of tentacles which is found in this posi- 
tion iu most other Ascidians. I shall therefore term this the tentacular fnnge It is 
divided altogether into thirteen lobes, of which twelve, though irregukr, are tolerably 
similar and roughly symmetrical, while the thirteenth is situated in the middle of the 
haemal half of the circlet, and is very different in form and size from the rest. It is, m 
* la the present memoir I propose to confine myself as nearly as may be practicable to anatomical and embryolo- 
gical details, reserving the many interesting histological peculiarities of Pyrotoma for a future occasion^ 
Z E Ik 
