ON THE ANATOMY AND DEVELOPMENT OF PYROSOMA = 325 
various directions. Although not absolutely 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 Pyrosoma 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? orifice of 
one of the ascidiozooids. In a thin vertical and radial section (PI. 
XXX. [Plate 29] 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 meh 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. 
1 Some which have now been more than a year in my possession exhibit no alteration. 
2 M. Milne-Edwards, in his ‘‘ Observations sur les Ascidies Composées,” 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 (‘Researches into the Structure of the Ascidians,” Reports of the British Association,. 
1852), to propose the term a/rzzm 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 maybe termed the atrial aperture. The membrane which lines it, 
and which was in part distinguished by Milne-Edwards in the memoir cited, is the a¢riat 
tunic. The cellulose integument of an Ascidian is for me the /es¢. The body-wall which 
underlies and gives origin to this test, I term the externa! tunic. The proper wall of the 
alimentary canal (with Milne-Edwards, I regard the branchial sac as a dilated pharynx) is 
the zvdernal tunic of the body. For the meaning of any other terms not explained in the 
text, I must refer to my ‘‘ Memoir on Sa/¢a and Pyrosoma” already cited. 
