THE FRESH-WATER MUSSEL 127 



free the foot and portions dorsal to it from the mantle of the left side. 

 In this manner remove the foot and the visceral mass, as the region 

 dorsal to the foot and containing the principal parts of the digestive 

 system is termed. Remove the palps and any remains of the mantle 

 and gills from each side of the visceral mass. Using a sharp scalpel, 

 split the visceral mass and the foot as nearly into right and left halves 

 as possible, leaving the free stump of the intestine attached to the 

 left half. Examine the cut surface of this half pinned down under 

 water in a dissecting pan. The visceral mass consists of a pasty 

 substance, composed mainly of the gonads and digestive glands, in 

 which parts of the digestive tract are embedded. Follow any of these 

 parts that are cut by the section. From the mouth the flattened 

 esophagus leads dorsally to an enlargement, the stomach, into which 

 open the right and left digestive glands. The complete course of the 

 tract should be noted as shown in Fig. 58. Make a diagram (X 3) 

 of the digestive system to show the essential relations of its parts. 



m. REPRODUCTION AND DEVELOPMENT 



Exercise 10. — The Glochidium. 



(a) During the breeding season spermatozoa are discharged from 

 the genital pores into the inner suprabranchial chambers and thence 

 pass to the outer water through the excurrent siphon. They may then 

 enter another mussel through its incurrent siphon. The ova are like- 

 wise discharged into the inner suprabranchial chambers where they 

 are fertilized by sperm that have been derived from another mussel 

 in the manner indicated. The zygotes then pass into the water tubes 

 of the gills which thus function as brood-pouches. Development en- 

 sues to a stage, known as the glochidium, which is eventually extruded 

 from the mussel through the excurrent siphon. 



(b) Examine preserved glochidia or living specimens just removed 

 from the brood-pouch. Note the valves of the shell and the single 

 adductor muscle. Projecting from the tissue lining the valves are 

 groups of sensory hairs. In one type of glochidium the valves are 

 triangular and armed with so-called hooks (Fig. 61) ; another type is 

 bookless (Fig. 60). With the exception of the shell, none of the adult 

 parts is visible in the glochidium. Watch living glochidia for any 

 movements. Are they capable of locomotion? When discharged from 

 the parent the glochidium must become attached to a fish and live 

 for some time as a parasite if it is to survive. Hooked glochidia nor- 

 mally become attached to the fish's fins; bookless glochidia become 

 attached to the fish's gills (Fig. 60). The glochidium leaves the fish 



