XII 



PHYLUM MOLLUSCA 



665 



* ad. 



the shell the two valves are seen to be united, along a straight 

 hinge-line (Fig. 573, A, h.l.), by a tough, elastic substance, the 

 hinge-ligament (Figs. 572 and 578, Ig.) passing transversely from 

 valve to valve. It is by the elasticity of this ligament that the shell 

 is opened : it is closed, as we shall see, by muscular action : hence 

 the mere relaxation of the muscles opens the shell. In Anodonta 

 the only junction between the two valves is afforded by the liga- 

 ment, but in 

 Unio each is pro- 

 duced into strong 

 projections and 

 ridges, the hinge- 

 teeth, separated 

 by grooves or 

 sockets, and so 

 arranged that 

 the teeth of one 

 valve fit into 

 the sockets of 

 the other. 



The valves are 

 marked e x t e r- 

 nally by a series 

 of concentric 

 lines '(Fig. 572) 

 parallel with the 

 free edge or gape, 

 and starting 

 from a swollen 

 or eleva- 

 the umbo 

 situated 

 an- 

 of 



a. cui 



FIG. 573. Anodonta cygnea. A, interior of right valve ; B, the 

 animal removed from the shell, a. ad. anterior adductor or its 

 impression ; a. r. anterior retractor or its impression ; d. gl. 

 digestive gland, seen through mantle ; ex. sph. exhalant siphon ; 

 ft. foot ; gl. gills, seen through mantle ; h. I. hinge-line ; in. sph. 

 inhalant siphon ; M. kidney, seen through mantle ; k. o. Keber's 

 organ, seen through mantle ; m. mantle ; p. ad. posterior 

 adductor or its impression ; pc. pericardium, seen through 

 mantle ; pi. I. pallial line ; pi. m. pallial muscles ; p. r. posterior 

 retractor or its impression ; prr. jimti-.-ictor or its impression. 



knob 



tion, 



(urn.}, 



towards the 



terior edge 



the hinge-line. 



These lines are 



lines of growth. 



The shell is 



thickest at the umbo, which represents the part first formed 



in the young animal, and new layers are deposited under this 



original portion, as secretions from the mantle. As the animal 



grows each layer projects beyond its predecessor, and in this way 



successive outcrops are produced giving rise to the markings in 



question. In the region of the umbo the shell is usually more or 



less eroded by the action of the carbonic acid in the water. 



The inner surface of the shell also presents characteristic markings 

 (Fig. 573, A). Parallel with the gape and at a short distance from 



