A SIMPLE ASCIDIAN i6i 



through the excurrent opening the water is ejected, charged with 

 fecal matter and reproductive products. 



Exercise 1. Make a sketch of the animal on a scale of 2 or 3 ; label 

 the dorsal and ventral aspects and the siphons. 



Beneath the tunic and in contact with it is the mantle, which is 

 the remainder of the body wall, the tunic being a highly modified 

 cuticula protecting its outer surface. Remove the entire tunic. 

 This may be easily done by snipping it with scissors and then pull- 

 ing it off with forceps ; it is not tightly joined with the mantle. 

 The mantle will be seen to be a transparent structure through 

 which the internal organs appear. Observe the white muscle 

 bands in the mantle, especially the transverse and longitudinal 

 muscles in the siphons by means of which they are extended and 

 contracted. Note also the short tentacles at the incurrent and 

 excurrent openings. Count those at each opening. 



The Digestive System. The most conspicuous internal organs 

 are the cream-colored genital glands near the center of the body 

 and the alimentary canal. The latter lies on the left side of the 

 body, where it appears as an S-shaped structure which incloses 

 the former. Place the body in water with the left side uppermost 

 and the siphons away from you, and study the arrangement of the 

 organs. The incurrent opening (at your left) will be seen to have 

 more prominent tentacles than the excurrent opening. From the 

 base of the incurrent siphon the large pharynx, the most volumi- 

 nous organ of the body and the principal organ of respiration, will 

 be seen extending to the lower side of the body. Note the six 

 longitudinal ridges which appear as light-colored bands in the 

 pharyngeal wall. Find and trace a white or cream-colored line 

 extending in the midventral line from the base of the incurrent 

 siphon to the opposite side of the body. This is the endostyle ; it 

 is a ciliated and glandular groove which lies between two folds in 

 the midventral wall of the pharynx ; it extends the length of that 

 structure and ends posteriorly near the opening of the pharynx 

 into the oesophagus. Find this point. The oesophagus is short 

 and communicates with the stomach, and these two divisions 

 form the lower and thicker limb of the S-shaped digestive tract. 



