38 PEARLS [CH. 



The Nervous System and Sense Organs. 



Since bivalves like the pearl oyster and the mussel 

 do not make a very active search for food, but lead 

 a sluggish existence, partaking of such nutriment as 

 is wafted to them, they miss that active struggle for 

 existence which is bound up with a search for food 

 and a combat with enemies. Not that they take no 

 part in a struggle for existence^ That seems to be 

 part of the ordained law of life for every organism. 

 It is here, however, more of a passive nature. We 

 are not surprised then to find that both nervous 

 system and sense organs are but simple. 



Here in fact is the explanation of the absence of 

 a definite and obvious head with eyes and other 

 organs. To the man in the street an animal lacks all 

 individuality if it does not possess such a structure, 

 on which he may interpret some expression. One 

 cannot point to the animal's head, but merely to the 

 head region. Specialised sense organs are few, and 

 they cannot be compared to any possessed by well- 

 known vertebrate animals. They are of use to the 

 animal for orientating itself and for testing the water 

 which continually streams into the shell. The edges 

 of the mantle are sensitive to light, and a shadow 

 thrown on to them causes an immediate closure of 

 the shell. This is the sole means of defence pos- 

 sessed by the pearl oyster. As a matter of fact, 



