132 SCIENCE FROM AN EASY CHAIR 



heart 



with its muscle relaxed and its shell " gaping," the 

 nourishing stream is kept going. If poisonous matter, 

 bad water, or some violent disturbance make themselves 

 apparent, the shell-muscle acts, and the oyster tightly 

 closes his shell. Such things make themselves " apparent " 

 to the oyster, for it has a nervous system, and though it 

 has no eyes (the nearly allied " scallop " has a number of 

 eyes) it has a delicate sense of smell and touch, and also 

 what is usually considered to be an organ of hearing. 

 The oyster has also a heart and blood-vessels (Fig. 30) 



and blood ; in some 

 few bivalves and 

 snails the blood is 

 red like our own. 

 I The beating of the 

 heart may be seen 

 by careful examina- 

 tion of a freshly 

 opened specimen. 

 The oyster has also 

 a " liver," or diges- 

 tive gland, and a 

 kidney and a soft, 

 branched, tubular 

 structure embedded 

 in the body, within 

 which the egg-cells 

 and sperm-cells grow 

 by means of which 

 the oyster propagates 



itself in the summer. Our north European oyster pro- 

 duces in the same individual both egg-cells, and the male 

 fertilising sperm-cells or spermatozoa. The eggs are just 

 visible to the unaided eye (Fig. 31), and as many as a 

 million are produced in the warm breeding season by a 



FIG. 30. The animal of an oyster removed 

 from the shell : a, the thick edge of the 

 left side mantle-flap or skirt ; b, same of 

 the right side ; /, position of the mouth ; 

 m, shell-muscle or adductor-muscle, bring- 

 ing the two shells tightly together when it 

 contracts. 



