INVERTEBRATA. 



329 



Wo can now more fully realize the method of life of 

 Anodonta. The cilia which abound on the gills and inner 

 surface of the mantle produce a constant current inwards 

 through the inhalent aperture. This aperture is fringed 

 with sensory tentacles, so that if there is any obnoxious 

 substance in the water the shell can be tightly closed and 

 no water allowed to enter. Normally, however, the water 

 brings in oxygen and food (microscopic plants and animals). 

 Most of the water is strained through the gills, exchanging 

 its oxygen for the carbon dioxide of the blood as it does so, 

 and then as it is swept outwards along the supra-branchial 



Fig. 169. SECTION IN A HORIZONTAL PLANE ACROSS TUB Two GILLS or ONE 

 SIDE or ANODONTA. 



chambers it carries off to the exhalent aperture the products 

 of excretion, faeces, and (in the mature male) the sperma- 

 tozoa. Meanwhile the rest of the water, with the solid 

 particles that could not pass through the gills, is carried 

 forwards to the mouth. This is effected by the cilia on the 

 labial palps. These are two pairs of lamellae, not unlike 

 the gills, but smaller and simpler in character. They lie in 

 front of the gills on either side, and the corresponding 

 right and loft ones unite anteriorly, the mouth lying 

 between the two. 



5. Alimentary Canal (fig. 170). The mouth is a 

 simple opening, without lips or jaws. It leads by a short 

 oesophagus to an expanded stomach. Into this there open, 



