100 



BOYS AND GIRLS IN BIOLOGY. 



gets into the house, but how does it get out ? It can- 

 not go back by the large, lower opening, or ventral 

 siphon, because there is such a strong current of fresh 

 water coming in ; if it did start to go that way, the 

 lashes, or cilia, would throw it back, because they are 

 all sweeping the water inward. There must be some 

 place to get it out, else our poor mussel would soon die. 

 The water from the gills passes through the door which 

 opens from the front or ventral chamber, into the 

 dorsal or upper back-chamber, and then out of the 

 house by the dorsal siphon — the small upper tube. 

 This back siphon, or tube, is also fringed with strong 

 hairs (Fig. 91), and these sweep the current outward. 

 These cilia, or lashes, around the openings, are known 

 as the mussel's beard. But all the water which enters 

 the house does not go tb the gills, the rest of it enters 

 the mouth. Now, where is the mouth ? It is quite a 

 puzzle to find it, for the mussel has no head, and hence 

 has been called acephalous, which means headless. If 

 you go into the big room, or ventral chamber, and 

 look about carefully, you will find, just in front of the 

 gills, a small Y-shaped flap (Figs. 92-103), which is 

 joined to another one of the same shape on the other 

 side, and between these two flaps you will discover a 

 w T ide aperture, and this is the mouth (Fig. 103). The two 

 flaps are called labial palps, or lip-feelers. The mussel 



