3604 ON THE ANATOMY AND DEVELOPMENT OF PYROSOMA 
end of this diameter and, consequently, on that face of the ascidio- 
zooid which is adjacent to the ovisac and cyathozooid. 
At the commencement of the series of changes here indicated the 
ascidiozooids are, individually, much smaller than the combined cya- 
thozooid and ovisac ; but as development advances, the latter diminish 
while the former increase ; and as, by the increase of size of the 
ascidiozooids, the interval between them becomes both relatively and 
absolutely less, they, at last, completely hide the combined cyatho- 
zooid and ovisac from view, so that it is not always an easy matter to 
find the latter (Plate XXXI. [Plate 30] fig. 15). The test increases, 
concomitantly with the ascidiozooids, enveloping them and filling up 
their intervals so as, finally, to form a spheroidal investment for the 
entire tetrazooidal foetus (figs. 14 & 15). 
During the whole of these changes and until the feetus attains a 
diameter of th of an inch, it remains within the mid-atrium of the 
parent, which, at last, it completely fills. With Savigny, I am unable 
to understand how it escapes, unless indeed it becomes freed by the 
destruction of its parent. For it seems quite impossible that the 
foetus should find a way open to it by any conceivable amount of 
dilatation of the atrial aperture. Nor does one ever find a fully 
formed ascidiozooid without a foetus in its mid-atrium. And if, at 
the same time, it is recollected that only one ovum ever comes to 
maturity in an ascidiozooid, so that when the fcetus has arrived at its 
full development the parent’s “occupation is gone,” it seems less 
improbable that the destruction of the latter should be involved in 
the maturity: of its offspring. 
Such is a general description of the changes in the size, form, and 
position of the chief constituents of the fcetus, in virtue of which 
it assumes its final characters. It now becomes necessary to trace 
the internal modifications which each of these constituents undergoes. 
1. The Cyathosooid—tIn my brief preliminary sketch of the deve- 
lopment of Pyrosoma (‘Annals of Natural History’ for January, 
1860), I have termed this part the “rudimentary cloaca ;” but it 
would have been a more accurate account of the matter, if I had 
called it the ‘mould’ or ‘ forerunner’ of the cloaca. Rudiment of the 
cloaca, in the strict sense of the words, it is not ; for, as we shall see, 
the atrial apertures of the ascidiozooids never really open into it. 
When the cyathozooid is first distinguishable as a separate seg- 
ment and traces of structure are discernible in it (Plate XXXI. 
[Plate 30] fig. 11), it presents, when viewed from above, near that 
edge which is most distant from the first isthmus, a rounded depres- 
sion, Viewed sideways, the blastoderm appears to be divided into 
