ANODONTA 55 



adductor impression is large and oval, close to the anterior 

 border ot the shell, ana nearer the dorsal that! the ventral 

 edge! Close behind it are two smaller impressions, the 

 anterior retractor, above and confluent with the adductor 

 impression, the protractor impression below and separate from 

 the other two. The posterior adductor impression lies some 

 way from the posterior end of the shell near the dorsal border, 

 and immediately in front of it, and rather above it is the 

 smaller posterior retractor impression. A curved line running 

 parallel" to the ventral border of the shell from the anterior to 

 the posterior adductor impression marks the attachment of the 

 mantle to the shell, and is called the pallial line. 



The two valves are united along the hinge line by a strong 

 elastic ligament, which is kept stretched when the valves are 

 closed by the action of the adductor muscles ; it is due to the 

 elasticity of the hinge ligament that the valves gape when the 

 adductors are relaxed. The shell can be opened by inserting 

 the blade of a knife between the margins of the shell, cutting 

 through the anterior and posterior adductor muscles, and after- 

 wards dividing the hinge ligament. It will then be seen that 

 each valve of the shell is lined by a sheet of soft muscular 

 tissue, which hangs down like a curtain from the dorsal region 

 of the animal. This curtain is the mantle, a very important 

 and characteristic mollusean organ. On lifting up the near 

 flap of the mantle it can be seen that it is really a fold of the 

 dorsal wall of the animal, and that with its, fellow it encloses. 

 a considerable space the sub-pallial space or mantle-cavity. 

 The line along which the mantle is attached to the body-wall is 

 shown in fig. n. It starts high up at the anterior end above 

 the anterior adductor muscle ; curving round the front of this 

 muscle, it runs nearly straight backward for a short distance, 

 bends sharply upwards, and then descends with a long slope to 

 the bottom of the posterior adductor. Turning round the 

 posterior adductor, it runs forward again towards the dorsal 

 surface, and ends near the posterior end of the hinge line. The 

 free margin of the mantle is thickened and muscular, its edge 

 forming a rather prominent grooved border. The edges of 

 both flaps are free i.e. they are not joined to one another 

 except at the extreme posterior end, where they are fused 

 together opposite the posterior adductor muscle for about half- 

 an-inch. The edges then separate and run forwards a little 



