414 THE MUSSEL 



CHAP. 



to form the exhalant and inhalant apertures, the latter of 

 which is provided with short tentacles. 



4. Turn back the mantle-lobe and note the mantle-cavity 

 and its contents (Fig. 101) : a. the foot and visceral mass ; 

 b. the left pair of gill-lamina at the sides of the visceral 

 mass ; and anteriorly c. the left couple 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 

 pericardium on the dorsal side of the gills, the pericardial 

 gland (p. 406), and the left excretory organ (nephridium) , 

 between the pericardium and gill, 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 anus into the cloaca. Insert a 

 seeker into the mouth, 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 longitudinally, 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 peri- 

 cardium, 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. 99) 

 into the left auriculo-ventricular aperture, and note a. the 

 anterior aorta above the rectum, and b. the posterior aorta 

 below the rectum. In the middle line of the floor of th/ 

 pericardium the vena cava can be seen. 



3. Examine the gills. Note the left outer and inner 

 lamina, and that in the female the former is often distended 

 with eggs or larvce. (Fig. 105) : save some of these for 

 subsequent examination ( IV). Each lamina consists of 

 an outer and an inner lamella, forming a kind of trellis-work, 



