300 



MOLLUSCA 



[CH. 



The edges of the mantle flaps are very much thickened and closely 

 adherent to the shell ; as stated above, it is by these edges alone 

 that the periostracum and the prismatic layer are formed. 



The hinge is strictly speaking part of the shell ; it is secreted 

 by the ectoderm of the back of the animal between the two mantle 

 lobes. When the valves of the shell are pressed closely together 

 the hinge is bent out of shape and by its elasticity it tends to 

 throw the valves apart; hence when a mussel is dead the valves 

 always gape. 



The two valves in Unio articulate with one another by means 

 of teeth. There are a pair of stout teeth a little in front of the 



13 10 



15 



FIG. 137. Eight side of Anodonta mutdbilis with the mantle cut away and the 

 right gills folded back x about 1. From Hatschek and Cori. 



1. Mouth. 2. Anus. 3. Cerebro-pleural ganglion. 4. Anterior adductor 

 muscle. 5. Anterior retractor muscle of the foot. 6. Protractor 

 muscle. 7. Dorsal siphon. 8. Inner labial palp. 9. Foot. 10. Ex- 

 ternal opening of kidney or organ of Bojanus. 11. Opening of 

 genital duct. 12. Outer right gill-plate. 13. Inner right gill-plate. 

 14. Ventral siphon. 15. Epibranchial chamber, the inner lamellae of 

 the right and left inner gills having been slit apart. 16. Posterior 

 protractor muscle. 



umbo, on the left valve, working on either side of one tooth on the 

 right valve; these are called the cardinal teeth. A long ridge 

 on the right valve, working between two ridges on the left valve, 

 is called the lateral tooth. Anodonta derives its name (Gr. dv-, 

 not ; oSovs, dSoVros, a tooth) from the circumstance that the shell is 

 devoid of teeth. 



When the shell is removed from the animal the cut ends of the 



fibres of two large muscles are seen. These muscles, which run 



transversely from the one valve to the other, are called 



TL he Body. . . 



the anterior and posterior adductors respectively, 



