316 MANUAL OF ZOOLOGY sect. 



when analysed, to consist largely of the substance cellulose, which has 

 already been referred to (p. 36) as a characteristic component of the 

 tissues of plants, and which is rare in its occurrence in the animal 

 kingdom. 



When the test is divided (Fig. 198), the soft wall of the body or 

 mantle (m ant), as it is termed, comes into view, and the body is 

 found to be freely suspended within the test, attached firmly to the lat- 

 ter only round the oral and atrial apertures. The mantle follows the 

 general shape of the test, and at the two apertures is produced into short 

 and wide tubular prolongations, which are known respectively as the oral 

 and atrial siphons (Fig. 199, atr. siph). These are continuous at 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. 

 Within the body-wall is a cavity, the atrial or peribronchial cavity 

 {atr. cav) communicating with the exterior through the atrial aperture. 



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 are 

 pierced by a number of slit-like apertures, the stigmata (Fig. 199, stig) 

 arranged in transverse rows. Through these the cavity of the pharynx 

 communicates 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 move- 

 ments 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 are 

 all vertical in position; those of the same row are placed close together, 

 separated only by narrow vertical bars; neighbouring rows are sepa- 

 rated by somewhat thicker horizontal bars ; in all of these bars run 

 blood-vessels. 



It has been already mentioned that the atrial cavity does not com- 

 pletely 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. 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. To 



