AND DEVELOPMENT OF PYROSOMA. 
235 
number to nine or ten on each side and the middle ones occupy the whole depth of the 
branchial sac ; but there are, as yet, no longitudinal branchial bars. The mode of origin 
of the stigmata appears to be just the same as in the buds. 
The nervous ganglion makes its appearance as a thick mass of indifferent tissue between 
the isthmus and the oral aperture; and the contour of the gastro-intestinal part of the 
alimentary canal is discernible on that face of the embryo which is nearest the ovisac. 
The isthmuses have lengthened to 3-5 o^h of an inch. 
A foetus -oVfch of an inch lorn* whose ascidiozooids had a length of A-rd of an inch, 
presented the clear profile view of one of the latter, which is represented in PI. XXXI. 
fig. 13a. The central tube or canal of the first isthmus is obvious, and it opens freely 
into the branchial sac of the ascidiozooid between the ganglion and the oral aperture. The 
central canal in question is bounded by the inner tunic of the ascidiozooid, between which 
and the outer tunic is an interval which is connected, on the one hand, with the sinuses of 
the ascidiozooid, and on the other, with the space between the two walls of the cyathozooid. 
The ganglion is very distinct and occupies nearly the whole interval between the oral 
aperture and the isthmus. 
The diapharyngeal band, already visible in earlier stages, is continued from above the 
posterior half of the ganglion to the roof of the branchial cavity ; but its proportions are 
more slender, as it measures xh>th of an mch lon o» by -fgVoth of an inch thick. The oral 
aperture is not open, but its lobed tentacular fringe may be observed, the haemal tentacle 
being even now distinguished from the rest of the fringe by its length and form. 
Nine or ten branchial stigmata are discernible ; but there are, as yet, no longitudinal 
branchial bars. The intestine is completely fashioned ; and the ehcoblast is large, con- 
spicuous, and composed of a reticulated tissue. 
In a foetus -fat of an inch long, with ascidiozooids ^th of an inch long by ^th of an 
inch from their oral to their cloacal extremities and nearly hemispherical in form (the 
flat side of the hemisphere being applied to the combined ovisac and cyathozooid), the 
isthmuses measure <rbth of an inch in length ; 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 ascidiozooids in comnmnication. There are ten branchial stig- 
mata, of which the first and last are very small ; and six or seven longitudinal branchial 
bars have made their appearance. The mid-atrium is distinctly developed below and 
behind the gastroesophageal 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 con- 
dition as the bud represented in PI. XXX. fig. 22. The renal (?) organ has made its 
appearance as a patch of opake yellowish cells. 
Ascidiozooids - 4 \nd of an inch long, by -^th from their oral to their cloacal apertures, 
which form part of a foetus Ath of an inch long, and surround the lower half ot the 
combined cyathozooid and ovisac, have ten or eleven stigmata and seven longitudinal 
branchial bars. The hamial 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 oeso- 
phagus are two rudimentary languets. The diapharyngeal band is very slender. I he 
2 1 2i 
