AND DEVELOPMENT OF PYROSOMA. 
233 
dicular lip 4 Joth of an inch deep. The aperture leads into a wide cavity about as deep 
as the lip (y), into which the prolonged tongue of the test projects. The canal which 
traverses the centre of this tongue, and which consequently must place the cavity of the 
cyathozooid in communication with the exterior, appears very distinct. The appendix (h) 
has the form of a curved tube, with its concavity turned towards the cavity of the cyatho- 
zooid. Its anterior end is slightly enlarged, while its posterior end, also a little dilated, 
is seated upon a slight prominence : both ends seem to be open. 
On one side of this appendix, a canal (0), xyoth of an inch long by 9+oth wide, passes 
obliquely towards the cavity of the cyathozooid and apparently opens into it. Posteriorly 
it is continued, at an obtuse angle, into a similar tube having about the same length, and 
eventually passing into the first isthmus, now -fi^th °f an mc ^ long. It will be observed 
that, notwithstanding the advanced condition of the ascidiozooids in this foetus, their 
upper extremities do not rise so high as the level of the middle of the ellipsoid formed 
by the cyathozooid and ovisac. The point at which their atrial apertures will eventually 
be formed, consequently, can hardly be so high as the lower end of that ellipsoid. 
As has been already hinted, with the advance of the foetuses in size all their relations 
become changed. The ascidiozooids, instead of presenting a fraction of the length of the 
combined ellipsoidal cyathozooid and ovisac, and occupying only a small portion of the mass 
of the foetal spheroid, gradually become fully thrice as high as the ellipsoid in question, and 
form by far the greater proportion of the mass of the spheroid (fig. 15). The ovisac and 
cyathozooid, again, diminish, not only relatively but absolutely (fig. 16), inasmuch as their 
largest diameter does not eventually amount to more than ^jth or -g^jth of an inch, while 
the lip and the internal cavity of the cyathozooid become less distinct structures than 
before . 
But the most curious change is that which has taken place in the test in the vicinity of 
the cyathozooid. It has, as it were, separated itself from the latter, following the asci- 
diozooids as their vertical diameter increases, whereby the central tongue of the test is 
pulled completely out of the mouth of the cyathozooid, as one might pull a finger out of a 
glove (compare figs. 18 and 19, Plate XXXI.) . As a consequence of this operation a 
cavity, which gradually increases in dimensions, is developed between the outer surface 
of the cyathozooid and the inner wall of the test ; and as the atrial ends of the ascidiozooids 
ascend in consequence of the growth of the latter, they open into this cavity, which thus 
manifests itself as the cloaca (fig. 19) . The tongue-like prolongation of the test becoming 
pulled out and flattened as the cloaca widens, ultimately ceases to project into the cavity 
of the latter, and becomes converted into the lip of its aperture. In fig. 19 it still 
protrudes for some distance into the cloacal cavity. 
2. The Ascidiozooids. — From their small size, flattened form and general opacity, it is 
by no means so easy to trace satisfactorily the successive changes by which the other seg- 
ments of the blastoderm are converted into perfect ascidiozooids, as it is to follow out the 
development of the buds. Nevertheless, knowing the latter process, it is not difficult to 
interpret the appearances presented by the segments of the blastoderm, in the course of 
their development. 
When the blastoderm first becomes marked out into those segments which eventually 
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