208 HALF AN HOUR WITH SHELL-FISH (BIVALVES). 



marine organism, although the adult animal is 

 stationary, the embryo is a free-swimming creature, 

 moving about by means of cilia. 



The true bivalve mollusca are distinguished by 

 having three pairs of nerve-cords, or ganglia, and a 

 distinct heart, which has two chambers. The 

 scientific name given to this class of Lamelli- 

 Iranchiata, or " plate-gilled," is in allusion to the 

 gills, or organs of respiration, being in the form of 

 leaves, or thin plates. These useful organs are" 

 perhaps better known to us in the so-called " beard " 

 of the oyster. The body of all bivalves that is to 

 say, the internal organs is wrapped up in a tough 

 integument, technically called the " mantle." This 

 mantle is a most useful apparatus, for it has the 

 power of secreting the carbonate of lime held in 

 solution by sea-water, and out of it forming the 

 shells. Whenever the latter get damaged, also, it 

 has the means of repairing them. The lobes of this 

 mantle generally unite, and leave two openings, 1 

 through which certain tubes or siphons pass, as in 

 the sand-mussel. The foot of bivalves is not the 

 important member it is with the gasteropods, where 

 it is used as a means of locomotion. Occasionally, it 

 is employed in digging in the soft sands, or in 

 making short leaps, as with the common cockle. 

 In the mussel, the foot has the power of secreting a 

 kind of mucus, which soon hardens and forms the 

 tyssus (better known to us as the " moss "), by means 

 of which the mussel is enabled to anchor its shell to 



