14 



ZOOLOGY 



SECT. 



their margins with the margins of the apertures of the test, and 

 round the openings are the strong sphincter muscles by which 

 closure is effected. In the rest of the mantle the muscular fibres 

 are arranged in an irregular network, crossing one another in all 

 directions. Within the body- wall is a cavity, the atrial or peri- 

 branchial cavity (atr. cav.), communicating with the exterior through 

 the atrial aperture : this is not a ccelome, being formed by involu- 

 tion from the outer surface, and probably lined by a prolongation 

 of the ectoderm. 



Pharynx.- -The oral aperture leads by a short and wide oral 

 passage into a chamber of large dimensions, the pharynx or 

 branchial chamber (ph.). This is a highly characteristic organ of 

 the Urochorda. Its walls, which are thin and delicate, are pierced 

 by a number of slit-like apertures, the stigmata (Fig. 676, stigm.) 

 arranged in transverse rows. Through these the cavity of the 

 pharynx communicates with the atrial or peribranchial cavity, 



which completely surrounds 

 i.l 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 flowing 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. 675) 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. 



It has been already mentioned that the atrial cavity does not 

 completely surround 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. 677). Along the line of adhesion the inner surface of the 

 pharynx presents a thickening in the form of a pair of longitudinal 

 folds separated by a groove (end.). To this structure, consisting of 

 the two ventral longitudinal folds with the groove between them, the 

 term endostylc is applied. The cells covering the endostyle are large 



FIG. 675. Ascidia, a single mesh of the branchial 

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

 tudinal bar ; 1. r. fine longitudinal vessel ; p. p'. 

 papillae projecting inwards from the branchial bar ; 

 sg. stigma ; tr. transverse vessel. (After Herdman.) 



