3 H MANUAL OF ZOOLOGY SECT. 



The former aperture is termed the oral, the latter the atrial. A strong 

 current of water will be noticed, by watching the movements of floating 

 particles, to be flowing steadily in at the former and out at the latter. 

 When the animal is removed from the water both apertures become 

 narrowed, so as to be almost completely closed, by the contraction of 

 sphincters of muscular fibres which surround them. At the same time 

 the walls of the body contract, streams of water are forced out through 

 the apertures, and the bulk becomes considerably reduced. 



The outer layer of the body-wall is composed of a tough translucent 

 substance forming a thick test or tunic (Fig. 185, test). This proves 

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

 a characteristic component of the tissues of plants, and is rare in its 

 occurrence in the animal kingdom. 



When the test is divided (Fig. 185), the soft wall of the body or 

 mantle (niant. ), as it is termed, comes into view ; and the body is found 

 to be freely suspended .within the test, attached firmly to the latter 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 at rial siphons (Fig. 1 86, or.siph., atr. siph.). These are con- 

 tinuous 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 

 peribranchial 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. 186, stigm.) 

 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 



