Ill] THE PEARL OYSTER 27 



secreted by the whole face of the mantle in contact 

 with the shell. 



The mantle itself is formed of connective tissue 

 with numerous blood spaces — it has an extensive 

 blood supply. It is bounded by a layer of cells, the 

 ectoderm, and the ectodermal cells, next to the shell, 

 are secretory, as we have seen. The ectoderm facing 

 the pallial cavity is ciliated, that is, the cells possess 

 delicate vibratile processes or " ciHa." 



The Foot is an organ capable of very consider- 

 able expansion and contraction. It arises from the 

 visceral mass just below the mouth. 



The variation in size is brought about in the 

 following manner. The foot is highly muscular, but 

 it is also full of cavities which communicate with the 

 blood system. If blood is forced into the spaces the 

 foot increases in size and becomes several times as 

 large as in the contracted condition. Contraction 

 takes place by muscular action. It is by alternate 

 expansion and contraction in this way that the 

 common razor shell of our coasts bores its way down 

 into the sand. 



On the ventral surface of the foot and at the end 

 nearest the body is a gland, situated in a pit. From 

 this a groove runs to the end of the foot to another 

 pit. The gland secretes the fibres known as byssus, 

 easily seen in the common edible mussel, and in both 

 cases the byssus is used for attaching the animal to 



