VJ^Ari 





I I I I I 1 



Microns ^50 



Figure 205. — Cross section of the midgut. Bouin, hematoxylin-eosin. 



epithelium. There is no peristaltic motion since 

 the muscular layer of the intestines is either 

 absent or poorly developed, and the feces are 

 discharged in a continuous ribbon which is carried 

 away by the cloacal cm'rent and eventually settles. 

 The time required for food to pass through the 

 entire intestinal tract can be measured by record- 

 ing the time between addition of a suspension of 

 carmine or yeast to the gills and the appearance 

 of the red or white particles in the feces. The 

 rate of passage naturally depends on the length 

 of the intestinal tract and the rate of feeding. 



In large oysters (about 10 by 6 cm.) kept in run- 

 ning sea water of about 15° to 16° C. the time 

 required for food to pass through the entire intes- 

 tinal tract varied from 90 to 150 minutes. The 

 length of the intestinal tracts of the oysters used 

 in these tests was measm-ed on latex casts which 

 were left in situ and exposed by dissecting the 

 tissues above them. The lengths of the alimentary 

 tracts were as follows: 



In an oyster measuring 1 1 by 6 cm 14.5 cm. 



In an oyster measuring 10.0 by 7.5 cm 11.1 cm. 



In an oyster measuring 1 1 by 6 cm 12.9 cm. 



In an oyster measuring 11.5 by 5.5 cm 12.6 cm. 



ORGANS OF DIGESTION AND FOOD OF THE OYSTER 



227 



