ON THE ANATOMY AND DEVELOPMENT OF PYROSOMA 369 
fashioned ; and the elaoblast is large, conspicuous, and composed 
of a reticulated tissue. 
In a foetus J,;st of an inch long, with ascidiozooids A,th of an 
inch long by ~;th of an inch from their oral to their cloacal extre- 
mities and nearly hemispherical in form (the flat side of the hemi- 
sphere being applied to the combined ovisac and cyathozooid), the 
isthmuses measure z}5th of an inch in Jength; and it is obvious that, 
while their central canals connect together the branchial sacs, the 
interspaces between their double walls place the sinuses of the ascidio- 
zooids in communication. There are ten branchial stigmata, of 
which the first and last are very small ; and six or seven longitudinal 
branchial bars have made their appearance. The mid-atrium is dis- 
tinctly developed below and behind the gastro-cesophageal part of 
the alimentary canal. The place of the atrial aperture is indicated 
by the union of the atrial and outer tunics, in a round spot at the 
posterior part of the mid-atrium. In fact, the whole zooid is nearly 
in the same condition as the bud represented in Pl. XXX. [Plate 29] 
fig. 22. The renal (?) organ has made its appearance as a patch of 
opaque yellowish cells. 
Ascidiozooids ,:nd of an inch long, by -};th from their oral to 
their cloacal apertures, which form part of a foetus ;.th of an inch 
long, and surround the lower half of the combined cyathozooid and 
ovisac, have ten or eleven stigmata and seven longitudinal branchial 
bars. The hemal tentacle is well developed, the rudiment of the 
ciliated sac is discernible, and between the aperture of the central 
canal of the isthmus and the cesophagus are two rudimentary lan- 
guets. The diapharyngeal band is very slender. The eleeoblast 
has a length of about ~;th of an inch. The rudiment of the atrial 
aperture (round, and about ;4,th of an inch in diameter) lies alto- 
gether below the level of the equator of the combined cyathozooid 
and ovisac. There is an indistinct appearance as of a small cavity 
between it and the latter organs. The posterior end of the endostyle 
appears quite distinctly to be continued back into the central canal 
of the isthmus. The rudiment of the heart is obvious, in close con- 
nexion with, and apparently developed from the wall of the branchial. 
sac ; and there are two slight papillary elevations in the place whence 
the stolons will be given off. 
In a fetus of about the same diameter as the preceding but whose 
ascidiozooids have a vertical diameter of #yrd of an inch, while the 
combined cyathozooid and ovisac are ~jth of an inch long, the 
neural boundaries of the ascidiozooids project a little way beyond 
the open end of the cyathozooid. The upper edges of their atrial 
VOL. II BB 
