ON THE ANATOMY AND DEVELOPMENT OF PYROSOM.\ 329 
fold, it appears (especially in the fresh state) like a strong hollow rod 
mounted upon a thin ridge-like plate. 
Transverse sections, however, demonstrate the true nature of this 
structure with perfect clearness (Pl. XXX. [Plate 29] fig. 8). The 
bottom of the diverticulum is seen to be occupied by two stout cords, 
formed of elongated epithelium-cells set perpendicularly to the axis 
of the cord. These cords are separated from one another by a slight 
interval. Externally and below they are in contact with two lateral 
cords of similar cells. Anteriorly the lateral pass into the middle 
cords, while the latter project beyond the anterior boundary of the 
groove-like entrance into the cavity of the endostyle (and, con- 
sequently, of the anterior ends of the lips or epipharyngeal folds 
which bound it) and, coated by a process of the inner tunic, constitute 
the free, rounded, anterior termination of the endostyle. 
Posteriorly, the same confluence of the median and lateral cords 
takes place; but here the endostyle extends much further beyond the 
limit of the groove and its bounding folds, and constitutes a free, 
hollow cylindroid or conical process, which, as we shall see, plays a 
very important part in the process of gemmation, where I shall have 
occasion to speak of it as the exdostylic cone. 
The kypopharyngeal band is not, as in many Ascidians, separated 
for the greater part of its length from the neural wall of the ascidio- 
zooid. On the contrary, in consequence of the position of the 
cesophageal aperture close to the neural wall of the branchial cavity, 
and the non-extension of the atrium forwards in the middle line, the 
hypopharyngeal band is represented only by the inner tunic of this 
neural wall, which lies parallel with the outer tunic, and is separated 
from it only by the neural sinus, which usually contains a great aggrega- 
tion of blood-corpuscles. These corpuscles are commonly aggregated 
more densely in the posterior two-thirds of the hypopharyngeal sinus, 
and not unfrequently are divided, more or less completely, into two 
lateral portions by a median clear space. When this state of things 
exists, the hypopharyngeal sinus, under a low power, presents exactly 
that appearance which is figured by Savigny as a siphon-like tube. 
The inner tunic of the hypopharyngeal band is produced in the 
middle line into eight slender conical processes—the /anguets, which 
are situated at tolerably equal distances from one another. Thus both 
the neural and the hemal walls of the pharynx are separated from 
the outer tunic in the middle line by nothing but the corresponding 
sinuses ; and the same holds good of the lateral wall in the region of 
the peripharyngeal ridge. But at any point behind this, either a vertical 
and transverse section, or a view from above, shows that the inner 
