A SIMPLE ASCIDIAN 163 



called the subneural gland; it has a duct which communicates with 

 the pharynx. The function of this gland is probably excretory ; 

 it is supposed to be homologous to the hypophysis of vertebrates. 



Exercise 2, Make a drawing of the left side of the animal on a scale 

 of from 4 to 6, showing all the internal organs which appear in that 

 aspect. Label the dorsal and the ventral sides of the body and all 

 the organs. 



Exercise 3, Make a drawing of the right side of the animal showing 

 all the organs which appear in that aspect. 



Exercise 4. Make a drawing of the dorsal side showing the organs 

 observed there. 



The Peribranchial Chamber. Cut off the excurrent siphon at its 

 base and with a needle or bristle probe the opening. The probe 

 will pass into the large space between the mantle and the pharynx. 

 This is the peribranchial chamber ; it surrounds the pharynx on 

 all sides, except in the midventral line, and communicates with 

 the outside water through the excurrent siphon. It is not a part 

 of the body cavity, but has been formed by an infolding of the 

 outer surface of the body. Into it, near the base of the excurrent 

 siphon, the digestive and genital tracts discharge their products 

 for removal with the current of respiratory water which streams 

 out of that siphon. 



The Respiratory System. The principal respiratory organ is the 

 pharynx, which communicates with the incurrent siphon by an 

 opening fringed with a circular row of branched tentacles. Its 

 walls are pierced by numerous slitlike, ciliated openings, called 

 stigmata, through which the respiratory water streams from it 

 into the peribranchial chamber. A current of water is thus main- 

 tained, which passes through the incurrent siphon into the phar- 

 ynx, and thence through the stigmata into the peribranchial 

 chamber, and out again at the excurrent siphon. The stigmata are 

 vertical in position and are arranged in transverse rows, which 

 extend across the pharyngeal wall, and are separated from one 

 another by delicate vertical bars ; the transverse rows have be- 

 tween them large transverse bars, and running longitudinally 

 along the pharyngeal wall on each side are six large longitudinal 



