384 



ESSENTIALS OF ZOOLOGY 



The shell which is largely composed of carbonate of lime is 

 secreted by the mantle and usually consists of three layers. Em- 

 bedded within the latter may be pigments that give the occasional 

 brilliant colors to certain species. The thickness of the layers is 

 dependent on the richness of lime salts in the environment; thus, 



Wrrnetus spiratms 

 (Worm shell) - 



Limacina australb 

 (Ptcropod) 



Murex tzmispina 

 (Venus's comb) 



, Urosalpinx. 

 (Oyster dnll) 



. Aeolis 

 (Sea slug) 



Teredo naval is. 

 (Ship worm) 



Fig. 151. — Marine mollusks. 



snails living in an acid bog have thin transparent shells, whereas 

 the same species inhabiting an area rich in lime salts have thicker, 

 perhaps opaque shells. Certain species, such as Polygijra roemeri, 

 P. albolahris and P. teooasiana are capable of repairing broken shells 

 if the damage is not too severe. 



