Chap. 31 MOLLUSKS — specialists in security 637 



surface of the mantle. The other two layers are formed only by cells in the 

 border which at intervals add to the edge of the shell and thus produce the 

 lines of growth. The main function of the shell is protection but it also neutral- 

 izes acid. Clams flourish where the mud abounds in organic matter, much of 

 it decayed. Oxygen is scarce; carbon dioxide and sulfur abound and acidity is 

 high. Under these conditions the calcareous shell is an important source of 

 neutralizer of the acid. In clams (Venus) kept out of water experimentally, 

 oxygen is depleted and carbon dioxide accumulates. Under these conditions 



Fig. 3 1 .6. In the innermost, pearly or 

 nacreous layer of the shell the crystals 

 of lime are irregularly parallel and rays 

 of light are broken upon them. This is 

 the cause of iridescence. 



If a sand grain or minute animal gets 

 between the shell and the mantle the lat- 

 ter forms a pocket around it and then 

 a pearly cover. Many a natural pearl is 

 the casket of a worm. (Courtesy, Fasten: 

 Introduction to General Zoology. Bos- 

 ton, Ginn and Co., 1941.) 



t^%^mwx\av-5}« *!«s,\<s; 



■Nacreous layer 



Epithelium of mantle 



Parasite or foreign 

 particle 



Mantle tissue 



Pearl 



some of the shell is dissolved by the mantle and the calcium content of the 

 fluid in the mantle cavity is this increased with the necessary neutrahzer. 



Respiration. If one shell and flap of the mantle are removed the gills are 

 conspicuously displayed hanging into the mantle cavity with their ventral edges 

 free. The dorsal edges of each pair are so attached that a chamber above the 

 gills (suprabranchial) is shut off from the large rnantle cavity below (Fig. 

 31.8). The incurrent siphon opens into the chamber containing the gills; the 

 excurrent siphon opens out of the chamber above them. The fold of each gill 

 is divided by partitions into narrow water tubes. Minute holes open into these 

 from the mantle cavity and the tubes extending from these open into the supra- 

 branchial chamber. Urged on by cilia, water continually enters the holes in 

 the gills and passes through the water tubes close to blood vessels comparable 

 to arteries and veins (Fig. 31.8). When breathing, a clam always extends the 

 siphons. It gets little or no oxygen when its shells are closed. This is the time 

 when it draws on the calcium carbonate of the shell to neutralize the acidity 

 produced by the excess carbon dioxide. 



Circulation. Oxygen diffused from water in the gills, and digested food ab- 

 sorbed from the stomach and intestine are distributed over the body by the 



