16 



ZOOLOGY 



SECT. 



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

 directions, but for the most part either longitudinal or transverse. 

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

 (atr. cav.), communicating with the exterior through the atrial 

 aperture : this is not a coelome, being formed to a great extent by 

 involution from the outer surface. 



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

 passage (stomodceum) into a chamber of large dimensions, the 

 pharynx or branchial chamber (Fig. 725, 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. 727, stigm.), arranged in transverse rows. Through 

 these the cavity of the pharynx communicates with the atrial or 

 peribranchial cavity, 1 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 



P ~^HW A 'f* */* ^ f\ i\ f&ir\ ~~* r c ^ a l mm g the stigmata that 



are due the currents of water 

 already mentioned as flowing 

 into the oral and out of the 

 atrial aperture, the ciliary ac- 

 tion 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 stig- 

 mata (Fig. 726) 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 pharynx are a 

 number of bands of vascular mesodermal tissue, the connectives. 



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. 728). 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 



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. 



FIG. 726. Ascidia, a single mesh of the 

 branchial sac, seen from the inside, i. I. 

 internal longitudinal bar ; /. v. longi- 

 tudinal vessel ; p. p'. papillae projecting 

 inwards from the branchial bar ; sg. 

 stigma ; tr. transverse vessel. (After 

 Herdman.) 



