AND DEVELOPMENT OF PYROSOMA. 205 



the neural side, the two peripharyngeal ridges pass on to the elevation of the inner tunic 

 in which the ciliated sac opens, and unite upon its posterior half, widening as they do so, 

 whence their junction forms a triangular area with its apex directed backwards. 



The very singular structure which I formerly termed the endostyle, and which I was at 

 one time inclined to regard as a kind of internal shell, is, in reality, a longitudinal fold or 

 diverticulum of the middle of the lisemal wall of the pharynx, which projects as a vertical 

 ridge into the hsemal sinus, but remains in free communication with the pharynx by a 

 cleft upon its neural side. In consequence of the thickness and opacity of the epithelium 

 which lines the fundus of this 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 (PI. XXX. 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, consequently, 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 enclostylic cone. 



The liypoflmryncjeal hand is not, as in many Ascidians, separated for the greater part 

 of its length from the neural wall of the ascidiozooid. On the contrary, in consequence 

 of the position of the oesophageal aperture close to the neural wall of the branchial 

 cavity, and the non-extension of the atrium forwards in the middle line, the hypo- 

 pharyngeal 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 aggregation of blood-corpuscles. These corpuscles are com- 

 monly aggregated more densely in the posterior two-tliirds of the hypopharyngeal sinus, 

 and not unfrequently are divided, more or less completely, into two lateral portions l^y 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 languets, which are situated at tolerably equal distances from 

 one another. Thus both the neural and the haemal 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 tunic (or pharyngeal wall) is separated from the outer tunic by a 

 more or less wide space, enclosed within a membrane which is totally distinct from l3oth 



