ON THE ANATOMY AND DEVELOPMENT OF PYROSOMA = 3605 
two lamella, the separation between which is most marked imme- 
diately under the depression. In a line between the depression and 
the first isthmus a clear streak is visible, the first rudiment of what I 
shall term the appendix of the cyathozooid. As the development of 
the foetus progresses, the interspace between the two layers of the 
blastoderm enlarges and the depression becomes an opening, into 
which, however, the thick test is continued, projecting like a conical 
tongue into the interspace or cavity just mentioned, in such a manner 
as to leave but a narrow median passage, by which I conceive that a 
free communication between the cavity of the cyathozooid and the 
exterior must be effected (figs. 17 & 18). At the same time, the 
aperture is gradually shifted from the margin to the centre of the 
cyathozooid, so that, eventually, its middle corresponds to one pole 
of the foetus (fig. 14), and gives the latter the appearance of a cup, or 
ofan egg with its top cut off. Contemporaneously with these changes 
that streak which I have mentioned takes shape as a singular ap- 
pendage situated between the two layers into which the outer 
wall of the cyathozooid is differentiated, and a communication, 
which, I believe, existed from the first between the cyathozooid 
and the first ascidiozooid by means of the first isthmus, becomes 
patent and obvious. But a description of the structure of a more 
advanced cyathozooid will best render these changes intelligible. 
Fig. 14 represents a fcetus 75th of an inch in diameter. The 
cyathozooid and ovisac, taken together, have the form of an ellipsoid, 
truncated at that end which presents the aperture of the cyathozooid, 
and rounded at the other. The circular aperture of the cyathozooid 
(B) is z}sth of an inch across, and is bounded bya constricted perpen- 
dicular lip j3;th 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 cyatho- 
zooid in communication with the exterior, appears very distinct. The 
appendix (8) has the form of a curved tube, with its concavity turned 
towards the cavity of the cyathozooid. 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 (8), 735th of an inch long by 
qigth 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 73th of an inch long. 
It will be observed that, notwithstanding the advanced condition of 
