202 PROFESSOR HUXLEY ON THE ANATOMY 
aperture, the oral opening of an ascidiozooid ; and there are other, similar, apertures 
dispersed hetween the eminences. In the specimen under description, the ascidiozooids 
are almost colourless, or* have at most a very pale brownish hue ; but how much of this 
colourlessness may he due to the action of the spirit, I do not know. 
Such is the general appearance of the ascidiarium. To examine its internal structure, 
it is expedient to make sections with a razor in various directions. Although not abso- 
lutely necessary, I found it extremely advantageous to treat these sections with glycerine, 
or with a mixture of gum and glycerine — a process which not only has the advantage of 
rendering the tissues extremely transparent, but of preserving the preparations for a 
very long time unchanged*. It might have been reasonably expected that the tissues 
would undergo serious distortion in such a medium, but this is not the case ; on the 
contrary, the most delicate structures, such, for instance, as the cilia upon the branchial 
sac, are most exquisitely exhibited in glycerine preparations. As I have said above, I 
have often had occasion to remark the perfection with which the tissues of the Ascidians 
generally are preserved by strong spirit, and the subsequent addition of glycerine seems 
only to increase the transparency of such preserved specimens, without otherwise altering 
them . 
When a segment is cut out of the ascidiarium of Tyrosoma and examined from the 
inner or cloacal side, the surface presented to the eye is seen to be* tolerably smooth, or at 
most minutely mammillated, and to present numerous small apertures, each of which 
corresponds with, and is opposite to, one of the apertures upon the outer surface : while 
the latter, in fact, is the oral, the former is the atrial t orifice of one of the ascidiozooids. 
In a thin vertical and radial section (PI. XXX. figs. 1 & 4), the orifices are seen to be 
connected together by a comparatively wide, somewhat oval cavity, composed of the 
branchial chamber and the atrium of the ascidiozooid, which are separated from one 
another only by the perforated branchial sac, stretched like a bag-net from one wall of 
the cavity to the other. It would be a difficult operation to perform, but a fine hair might 
be passed in at the oral and out at the atrial aperture, through one of the meshes of the 
branchial sac, without injuring any organ. 
Prom 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 ; 
« 
Some which have now heen more than a year in my possession exhibit no alteration 
t M. Milne-Edwards, in his " Observations sur les Ascidies Compose'es," 1839, describes the cavity which surrounds 
the branchial sac, and into which the branchial currents flow, as the ' chambre thoracique ; ' that part of it which 
receives the faeces and generative elements he terms the ' cloaca,' while he retains the name of ' anus ' for the external 
aperture of this cloaca. From experience of the inconvenience of this phraseology, I was led some years ago ( re- 
searches into the Structure of the Ascidians," Reports of the British Association, 1852) to propose the term atrium 
to indicate the • thoracic chamber,' and to reserve the term cloaca for the chamber common to several or many 
ascidiozooids, as in Botryllus, &c. The aperture of the atrium may be termed the atrial aperture. The membrane 
which lines it, and which was in part distinguished by Milne-Edwards in the memoir cited, is the atrial tunic- 
The cellulose integument of an Ascidian is for me the test. The body-wall which underlies and gives origin to this 
test, I term the external tunic. The proper wall of the alimentary canal (with Milne-Edwards, I regard the branchial 
sac as a dilated pharynx) is the internal tunic of the body. For the meaning of any other terms not explained in tne 
text, I must refer to my " Memoir on Salpa and Pyrosotna" already cited. 
