A Guide to the Zoological Collections 



peribranchial cavity, through the atrial orifice, to the 

 exterior. 



In some of the dissections the whole of the left side 

 of the mantle, as well as of the tunic, has been removed, 

 to expose the whole of the large branchial sack. This 

 branchial sack is the pharynx : it almost fills the peri- 

 branchial or atrial cavity, and it opens directly to the 

 exterior by the mouth or incurrent orifice only as is 

 shown by the pointer in No. V : its walls, however, are 

 pierced with rows of pores, like a sieve, and through 

 these pores the branchial sack also communicates with 

 the peribranchial cavity. At its lower part, on the 

 dorsal side, (to the extreme right hand side of the ob- 

 server), it also leads into the alimentary canal: this 

 forms a double loop, the open end of which (anus) hangs 

 free in the peribranchial cavity at the bottom of the 

 atrial orifice. 



We can now understand how the Ascidian feeds 

 and breathes. Seawater, carrying Oxygen and minute 

 particles of food, is drawn into the capacious pharynx 

 or branchial sack, at the mouth or incurrent orifice. The 

 food finds its way into the intestinal tract, and the undi- 

 gested waste ultimately escapes into the peribranchial 

 cavity near the atrial orifice. The water passes through 

 the pores in the wall of the pharynx, also into the peri- 

 branchial cavity, whence it escapes through the atrial 

 orifice, carrying with it, as it sweeps through the atrial 

 siphon, the waste from the intestine. As the water runs 

 through the pores in the very vascular wall of the 

 pharynx, it gives up its oxygen to the blood. 



The other organs of the Ascidian can be seen in the 

 dissections and in the explanatory drawing beside 



them. 



It is necessary to explain how the particles of food 



