150 PRACTICAL ZOOLOGY 



easy to understand what produces the current of water entering 

 and passing out through the siphons. These cilia always pro- 

 vide a fresh supply of water from which oxygen and food are ob- 

 tained, thus enabling the sluggish mussel to live successfully 

 without moving about for its food and oxygen. In one respect 

 the mussel and crayfish are similar; both create currents of water 

 which allow them to breathe when resting quietly in one place, 

 but the crayfish must go out after its food and is therefore active, 

 whereas the mussel draws the food to itself and may therefore 

 be as lazy as it pleases. 



Sensitiveness to Surroundings. — The mussel has no distinct 

 head, although it possesses near the mouth the nervous ganglia 

 called the brain (Fig. 84, b). Its sense organs are also poorly 

 developed. These are all indications that the animal is de- 

 generate. Nevertheless it copes successfully with its enemies 

 and its physical surroundings, which is about as much as can be 

 said of any of the animals, not excluding man. 



If the water in which the mussel is living is charged with an 

 injurious chemical substance or if the edges of the siphons are 

 touched, the siphons are drawn in and the shell closed. The 

 animal thus protects itself from injurious substances in the water 

 and from mechanical injury, and the results of the experiments 

 indicate that the edges of the siphons bear sense organs of touch. 

 The sense organs which detect chemical changes in the water are 

 supposed to be two yellowish patches, called osphradia, situated 

 just beneath the posterior adductor muscle and hence near where 

 the incoming stream of water enters. No eyes are present, al- 

 though casting a shadow upon an individual lying in the sun 

 causes a retraction of the siphons and proves it to be sensitive 

 to different light intensities. 



Digestion. — The mouth lies near the anterior adductor 

 muscle (Fig. 84, m) and is provided with a pair of leaf-like pro- 

 cesses on either side, the labial palps (Fig. 84, Ip). The cilia 

 covering these palps drive food particles into the mouth and 

 down the oesophagus. The digestive apparatus is not very dif- 



