414 THE MUSSEL CHAP. 



%ills at the sides of the visceral mass ; and anteriorly c. the left pair of 

 small, triangular, labial palps. (Note that there is no distinct head.) 

 These parts are situated between the two mantle-folds in the large 

 ventral mantle-cavity. Note also the position of the pericardiiim on 

 the dorsal side of the gills, the pericardial gland (p. 406), and the left 

 excretory organ (nephridiuni], between the pericardium and gills, and 

 extending back to the posterior adductor muscle. 



5. Pass a seeker into the exhalant aperture and note that it enters a 

 short exhalant siphon or cloaca, separated from the mantle-cavity by 

 the bases of the gills. Slit this chamber open, and note that it extends 

 above the posterior adductor muscle, as well as above the gills. The 

 hinder end of the rectum will be seen just above the posterior adductor 

 muscle, opening by the amis into the cloaca. Insert a seeker into the 

 motith, between the anterior adductor muscle and the anterior edge of 

 the foot. 



II. I. Remove the left mantle-lobe, cutting very carefully along the 

 bases of the palps and gills. Then slit open the pericardium longi- 

 tudinally, a short distance dorsally to the gills, and note the rectum, 

 ventricle, and delicate left auricle. Cut away sufficient of the wall of 

 the pericardium to expose these parts, taking care not to injure the 

 auricle. Note 



2. The rectum, which runs straight through the pericardium, and in . 

 its middle is surrounded by the elongated muscular ventricle, which is 

 bilobed posteriorly. The thin-walled, triangular left auricle can be seen 

 opening into the ventricle by a valvular aperture : the base of the auricle 

 is attached along the dorsal border of the gills. (Note the right auricle 

 on the other side.) Inject French-blue (p. 90) into the left auriculo- 

 ventricular aperture, and note a. the anterior aorta above the rectum, 

 and b. \\\z posterior aorta below the rectum. In the middle line of the 

 floor of the pericardium the vena cava can be seen. 



3. Examine the gills. Note the left outer and inner gill, and that in 

 the female the former is often distended with eggs or larva. (Fig. 

 105) : save some of these for subsequent examination ( IV). 

 Each gill consists of an outer and an inner lamella, forming a kind of 

 trellis-work, with small meshes or ostia, separated by horizontal and 

 vertical bars. Cut away a piece of the outer lamella, noting that the two 

 lamellae of each gill are united at intervals by inlerlaineliar junctions. 



4. Make out the mode of attachment of the gills (see p. 404, and 

 Fig. 103). Sketch your dissection with the gills in their natural 

 position. 



