202 PROFESSOR HUXLEY ON THE ANATOMY 



aperture, tlie oral opening of an ascidiozooid ; and there are other, similar, apertures 

 dispersed between 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 be 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 Aseidians 

 generally are preserved by strong spirit, and the siibseqixent addition of glycerine seems 

 only to increase the transparency of such preserved specimens, Avithout otherwise altering 

 them. 



When a segment is cut out of the ascidiarium of Fyrosoma 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 waU 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. 



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 waU 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 Composees," 1833, 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 Aseidians," 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 the 

 text, I must refer to my "Memoir on Salpa and Pijrosoma" already cited. 



