126 THE FRESH-WATER MUSSEL 



Exercise 7. — The Excretory System. 



(c) In addition to the gills, by means of which carbon dioxide is 

 removed, a pair of special excretory organs, the nephridia, or kidneys, 

 are present. Carefully open the suprabranchial chamber of the right 

 inner gill from the side. The dark color of the nephridium can be 

 seen through the medial wall of this chamber. Toward the anterior 

 end of the chamber two small openings will be found; the dorsal one 

 of these is the excretory pore, or external opening, of the right 

 nephridium. Insert a tipped bristle into this opening and probe the 

 cavity. Cut across the auricles, posterior aorta, and intestine, and 

 turn these structures anteriorly, exposing the anterior wall of the 

 pericardial cavity. Careful search along this wall with a tipped 

 bristle should reveal two openings leading into the right and left 

 nephridia respectively. Pass the bristle through the one leading into 

 the right nephridium. Cut into this nephridium and observe that it is 

 a U-shaped tube, of which the lower arm is spongy and glandular, 

 and the upper arm is thin-walled. Note that fluid can pass from 

 the pericardial cavity to the outside by way of the nephridia; com- 

 pare with the relationship between ccelom and nephridia in the earth- 

 worm. Make a diagram (X 3), from a lateral view, to show the 

 structures disclosed by the foregoing dissection, and indicate the course 

 of the fluid by arrows. 



Exercise 8. — The Reproductive System. 



(d) Identify the genital pore as the lower of the two openings seen 

 previously on the median wall of the inner right suprabranchial cham- 

 ber. This is the external opening of the genital duct which leads from 

 the right gonad. The gonads, which are racemose glands located in 

 the upper part of the foot among the coils of the intestine, will be 

 observed when the digestive system is examined as directed in Exercise 

 9. The sexes are separate in most species of mussels. 



Exercise 9. — The Digestive System. 



(e) Look again into the excurrent siphon and recognize the in- 

 testine, lying on the posterior surface of the posterior adductor muscle 

 and opening into the excurrent chamber by the anus. Follow the in- 

 testine along the muscle to the place where it was cut across in making 

 the dissection of the nephridium. Using the handle of the scalpel, 

 scrape the nephridia away from the top of the foot. Recognize and 

 understand the function of the posterior retractor muscles of the foot 

 which are now conspicuous and whose attachment scars have been 

 noted on the shell. Leaving intact the parts of the intestine as cut, 



