XIII 



PHYLUM CHORDATA 



17 



- -^S 



stigmata (Fig. 718, digin.) arranged in transverse rows. Through 

 those the cavity of the pharynx coniniunicates with the atrial or 

 peribranchial cavity,^ which completely surrounds it except along one 

 side. The edges of the stigmata are beset with numerous strong 

 cilia, the action of which is to drive currents of water from the 

 pharynx into the atrial cavity. It is to the movements of these cilia 

 lining the stigmata that are due the currents of water already 

 mentioned as Howing into the oral and out of the atrial apertures, 

 the ciliary action drawing a current in through the oral aperture, 

 driving it through the stigmata into the atrial cavity, whence it 

 reaches the exterior through the atrial aperture. The stigmata 

 (Fig. 717) are all vertical in position; those of the same row are 

 placed close together, separated only by narrow vertical bars ; 

 neighbouring rows are separated by somewhat thicker horizontal 

 bars ; in all of these bars run blood-vessels. Extending across the 

 atrial cavity from the body- 

 wall to the wall of the l.z/ il 

 pharynx are a number of J^j^aX^vvu^aa/u^^'L^ 

 bands of vascular meso- p -<J^ ^ f\f\(\f\t\ i\ 't^ ^ f^^ 

 dermal tissue, the con- 

 nectives. 



It has been already men- 

 tioned that the atrial cavity 

 does not completely sur- 

 round the pharynx on one 

 side. This is owing to the 

 fact that on the side in 

 question, which is ventral 

 in position, the wall of the 

 pharynx is united with the 

 mantle along the middle 



line (Fig. 719). Along the line of adhesion the inner surface of 

 the pharynx presents a thickening in the form of a pair of longi- 

 tudinal folds separated by a groove : to this structure, consisting 

 of the two ventral longitudinal folds with the groove between 

 them, the term cndostyle end is applied. The cells covering the 

 endostyle are large cells of two kinds — ciliated cells and gland- 

 cells — the former beset at their free ends with cilia, the action ot 

 which is to drive floatino: -oarticles that come within their influence 

 outwards towards the oral aperture, the latter secretmg and dis- 

 charging a viscid and mucous matter. Anteriorly the endostyle 

 is continuous with a ciliated ridge which runs circularly round the 

 anterior end of the pharynx. In front of this circular ridge, and 



1 A distinction is sometimes made between the lateral parts of this space 

 {peribranchial cavities, right and left) and the median unpaired (dorsal) part, 

 {atrial cavity, or cloaca), in which the two peribranchial cavities coalesce, and 

 which leads to the exterior through the atrial aperture. 



VOL. II C 



Fig. 717.— Ascidia, a single mesli of the branchLal 

 sac, seen from the inside, i. I. internal longi- 

 tudinal bar; I. v. longitudinal vessel; p- p'. 

 papilte projecting inwards from the branchial 

 bar ; «f^ stigma ; tr. transverse vessel. (After 

 Herdman.) 



