326 ON THE ANATOMY AND DEVELOPMENT OF PYROSOMA 
From what has been said, it follows that each fully-formed 
ascidiozooid must be equal in length to the thickness of that part of 
the wall of the ascidiarium in which it occurs; and the whole 
ascidiarium may be regarded as a succession of tiers of ascidiozooids 
enveloped in a common test. 
The extreme apex of the cone (Pl. XXX. [Plate 29] 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 the 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 Ascidiozootds.—In investigating the structure of the ascidio- 
zooids, an example from the middle region of the ascidiarium may 
most conveniently be selected for study. Such an ascidiozooid is 
represented in longitudinal section in Pl. XXX. [Plate 29] fig. 1, 
in transverse section in fig. 2, and from above and partly in section in 
fig. 3. 
It is somewhat irregularly fusiform, a good deal longer than deep, 
and deeper than broad. Its outer extremity exhibits the oral 
aperture, which lies upon the hemal side of one of the above- 
mentioned conical protuberances, and is overhung by a tongue-like 
process of the test—the /adial 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 cloacal 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 alimentary tract (inner tunic: see note, p. 325) and 
the integument (outer tunic) pass into one another. The inner 
diameter of the circular sphincter is z}gth of an inch ; but the diameter 
of the oral passage itself is far less, amounting to not more than 
1 In the present memoir I propose to confine myself as nearly as may be practicable to 
anatomical and embryological details, reserving the many interesting histological peculiarities 
of Pyroscma for a future occasion. 
