AND DEVELOPMENT OF PYROSOMA. 217 
Of the outer tunic all that requires to be said is, that it becomes relatively thinner as 
development goes on. In buds which are situated within a certain distance of the open 
end of the ascidiarium, and which have attained a length of p^-tli of an inch (fig. 21), the 
outer tunic of the neural wall of the atrium is raised into a slight rounded projection (r *), 
and in older buds (fig. 25) this gradually elongates, and extending towards the open end 
of the ascidiarium, and finally into the lip of the cloacal aperture, becomes converted into 
one of the stolons of the test. 
The atrial muscular bands are visible in buds not more than ^.th of an inch in length 
(fig. 23) ; the pharyngeal muscular bands, only in more advanced zooids. 
The tentacular fringe appears first as an inward thickening of the parietes of the 
mouth. The haemal tentacle is markedly the longer, even in such buds as that represented 
in fig. 24. The ganglion is discernible in buds / th of an inch long (fig. 20) as an 
opake oval mass situated between the peduncle and the oral end of the bud, and very 
much larger in proportion to the rest of the organism than afterwards. The ciliated sac 
appears as a short caecal diverticulum of the pharyngeal cavity, connected with the anterior 
and haemal side of the ganglion. 
A most curious structure is visible in buds y^-th of an inch long, and remains obvious 
until they have attained a length of -^th of an inch or thereabouts. For want of a better 
name, I will term this the ' diapharyngeal band.' In the section, fig 21, its upper part is 
visible, passing obliquely downwards and backwards from between the two middle bands 
of the endostyle; while, in figs. 22 & 23, its lower extremity is seen to end in the pha- 
ynx, immediately over the posterior moiety of the ganglion. The diapharyngeal band is 
hollow, and effects a communication between the haemal and neural sinuses; and if, as is 
possible, the heart of the bud has at this period but little functional activity, the ex- 
istence of this direct channel may facilitate the circulation of the blood. However this 
may be, this structure becomes longer and thinner as the development of the bud advances ; 
i 
and all that remains of it, in buds ^th of an inch long (fig. 24), is a small tubercle which 
lies over the posterior part of the ganglion. Eventually even this disappears. 
I have already spoken of the origin of the branchial stigmata. Fig. 21 represents an 
accidental, but very fortunate, longitudinal section of a bud -^-th of an inch long; the 
razor having passed rather to the right of the middle line above, rather to the left below. 
As it is seen from the right side, the inner surface of the left wall of the branehial sae is 
exposed to view. Of the eleven stigmata, those in the middle are the longest and most 
oval, those at the two ends of the series shortest and most rounded. They look clear in 
the centre, but on careful examination they are seen to be closed, the sheet of indifferent 
tissue which forms the innermost wall of the pharynx being continued over them. I am 
strongly inclined to think that it is this sheet of indifferent tissue which gives rise to the 
longitudinal branchial bars, for in more advanced buds (fig. 22), in which the median 
stigmata have undergone much elongation, the same layer is continued over their haemal 
and neural ends, while it has disappeared in the interval, except along three longitudinal 
lines, where it evidently forms the foundatiom of as many longitudinal branchial bars. 
In the more advanced stages, new stigmata are added to the anterior and posterior ends 
VOL. XXIII. 
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