36 THE VOYAGE OF H.M.S. CHALLENGER. 



Ill Pyrosoma the sac is formed by a series of transverse interstigmatic vessels, and of 

 internal longitudinal bars intersecting at right angles. 



In Doliolum the stigmata may either be confined to the posterior region of the sac, as 

 in Doliolum miilleri, or may extend forward, as in Doliolum denticulatum. 



In Salpa the branchial sac is greatly modified and reduced to a branchial " band," 

 which represents the dorsal lamina. The systems of vessels uniting this dorsal band to 

 the endostyle are entirely absent, so that each half of the sac is converted into a single 

 large stigma. 



In the Appendiculariidaa there are only two stigmata, one on each side of the 

 endostyle, near the posterior end. They lead by atrial canals, which represent the 

 right and left sides of the peribranchial cavity, to the exterior. The endoderm cells lining 

 the branchial sac are ciliated, and larger cilia are present along tracts corresponding to the 

 peripharyngeal bands and the dorsal lamina, and around the edges of the two stigmata. 



The Peripharyngeal Bands (Pericoronal Ridges, Ciliated Arcs). 



The position of the peripharyngeal bands at the anterior end of the branchial sac has 

 already been pointed out. They lie at the base of the branchial siphon, and the anterior 

 band, or, as Julin calls it, the internal lip of the groove, forms a complete ring, bounding 

 the prasbranchial zone posteriorly. The posterior band or external lip, on the contrary, 

 is complete only at the sides. At the ventral end its right and left halves turn pos- 

 teriorly, and become directly continuous with the marginal ridges of the endostyle ; 

 while dorsally they are continued into the anterior end of the dorsal lamina. 



The peripharyngeal bands are formed of ridges of connective tissue continuous with 

 that of the mantle, and covered on their free surfaces with epithelium, which changes 

 gradually from low columnar in the groove to cubical and then squamous on the outer 

 slopes, where the ridges become continuous with the prasbranchial zone on the one 

 hand, and with the inner surface of the branchial sac, on the other. The cells covering 

 the floor of the groove, and those along the most prominent parts of the ridges, bear 

 very short cilia, while the anterior and posterior slopes of the ridges and the sides of 

 the groove are not ciliated. 



Tlie Endostyle (Hypobranchial Groove). 



This organ forms the ventral edge of the branchial sac, and extends from the 

 peripharyngeal bands anteriorly to the posterior end of the sac. It is a groove bounded 

 by parallel lips, often of considerable height, and jirojecting into the cavity of the 

 branchial sac (fig. 8). The groove is lined by a modification of the epithelium of the 

 interior of the branchial sac, and is in some parts cubical, and in others columnar and 

 ciliated. 



