368 ON THE ANATOMY AND DEVELOPMENT OF PYROSOMA 
developed more largely laterally than in the middle line, so that 
it appears, at first, as if it were composed of two distinct portions ; 
but its two moieties are really continuous with one another on the 
hzemal side of the alimentary tract. The position of the future oral 
aperture is just indicated in the middle of the exposed surface of 
the ascidiozooids in this stage; but I could not ascertain anything 
definite as to the condition of the intestine. Indeed, from the 
flattened form of the embryonic ascidiozooids and their close appo- 
sition to the ovisac, it is exceedingly difficult to decipher all the 
details of their internal structure. 
Ascidiozooids of j,th of an inch in length exhibit a well-defined, 
though not open oral aperture, s3,;th of an inch in diameter. The 
branchial stigmata have increased in 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 dis- 
cernible on that face of the embryo which is nearest the ovisac. The 
isthmuses have lengthened to ;1,th of an inch. 
A foetus },th of an inch long, whose ascidiozooids had a length 
of @srd of an inch, presented the clear profile view of one of the latter, 
which is represented in Pl. XXXI. [Plate 30] 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 con- 
tinued from above the posterior half of the ganglion to the roof 
of the branchial cavity ; but its proportions are more slender, as 
it measures ,1,th of an inch long, by ygygth of aninch thick. The oral 
aperture is not open, but its lobed tentacular fringe may be observed, 
the hemal 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 
