122 THE FRESH-WATER MUSSEL 



Exercise 3. — The Mantle and Mantle Cavity. 



(g) A specimen preserved in formalin or one just killed should be 

 used. Remove the right valve by inserting the point of a scalpel be- 

 tween the valves and cutting the attachments of the muscles whose 

 scars you have already identified. Being careful not to injure other 

 structures, separate the valve from the mantle along the mantle line. 

 Study the mussel from this right side as it lies in the other valve. 

 This places the specimen in the same orientation as the drawings of the 

 shell. Note the soft membrane, the mantle, which conforms to the 



ventricle 

 onter/or aorta auricle 



^tomac^y \ / pericoraial cavity 



enter/or oc/c/uctor >:^;^^S^ ^^^tC/ .posterior adductor muscle. 



musc/e 



cerebro-p/euro/ Aj^ 5ii^^^^!'**^!^^'^l^Si^^^^^£7/7^^ 



moath 



,post>erior aorta 



ejTcurrent sip^Kin 

 /ncurrent s/phcin 



pec/a/ pa'/Tg>//on 



foot --^^^T'~~f--^-'-~^^^[J^^^^^^^^'^^'y^'T^"^^ 



^nacy I nerve cord \ nephnd/um 

 intestine. gi// 



Fig. 58. — The internal structure of the mussel, Anodonta; semidiagrammatic. 



inner surface of the shell. There will be no breaks in the mantle unless 

 it has been mutilated. Find the ends of the followmg muscles, the 

 scars of which have already been seen upon the shell: anterior and 

 posterior adductors, anterior and posterior retractors, and the pro- 

 tractor. Find the line on the mantle which corresponds to the mantle 

 line on the shell. The following internal organs can be more or less 

 definitely recognized, according to the species or the method by 

 which the specimen has been prepared: the digestive gland, a greenish- 

 brown structure in the dorsal region near the anterior end, and the 

 pericardial cavity, or coBlom, posterior to the digestive gland and con- 

 taining the heart. The dark-colored nephridium may be visible 

 through the pericardium. The pericardial gland (Keber's organ), of 

 problematical function, is the darker region anterior and lateral to the 

 pericardial cavity. Consult a chart, or Fig. 58, and understand the 

 position of these structures even if you are unable to locate them at 

 present. 



(h) Determine the way in which the incurrent siphon communi- 

 cates with the mantle cavity, which is the space between the right and 

 left parts of the mantle. Lift the edge of the mantle on the right side 



