NATURAI^ HISTORY OF AMERICAN EOBSTER. 
251 
the cardio-pyloric valve to the mill to be ground. The contraction of the anterior and 
posterior gastric muscles reacts upon the articulated plates of the elastic frame in such 
a way as to bring the lateral grinders together and to draw the median tooth forward 
with great force. This upper middle tooth, or prepyloric ossicle, is shaped like a bird’s 
beak and has brown indurated surfaces, while the lateral teeth, or surfaces of the zygo- 
cardiac ossicles, the principal grinders, are divided by parallel transverse furrows into a 
series of yellowish-brown hardened tubercles. According to Williams the forward and 
downward movements of the median tooth tend to drive much of the food back into 
the cardiac sac, so that it is reground again and again. Some of it, however, enters the 
pyloric division of the stomach, and filters back and forth in its chambers and canals. 
Here it is sorted and strained ; the finer parts, suspended in fluids, are delivered by the 
canals to the intestine in four streams, while the coarser elements are swept up by 
bristles of the cardio-pyloric valve and sent to mill again. Two streams from the 
dorsal pyloric canal pass into the intestinal caecum; a stream from the middle pyloric 
canal also delivers food to the intestine, while finally a current from the lower pyloric 
canal conducts food particles to the lateral pouch, where a final sifting occurs, the 
finest parts, suspended in fluids, entering the liver by the “bile ducts,’’ and the coarser 
by way of the middle pyloric canal reaching the intestine. 
When the muscles of the gastric mill relax, the elasticity of the framework is 
sufficient to separate the parts. While it is not possible to see these movements in 
the living animal, they can be roughly imitated by concerted pulls upon the anterior 
and posterior gastric muscles. Undoubtedly the clashing movements of the teeth go 
on for hours after a full meal until all of the food has been thoroughly stirred up, 
brought to mill, ground, and reground. After the soft and semiliquid parts have been 
filtered and delivered to the intestine and gastric glands, the indigestible residue is 
regurgitated through the mouth, as is the habit with many birds. 
The intestine is a delicate tube of small caliber, and since there are no coils it is 
quite short. This suggests the need of a gastric mill, and the absorptive function of 
the glands, for the area of the intestinal surface being limited, the digestive process 
must be conducted as rapidly and efficiently as possible. As already seen, there is a 
caecal enlargement on the dorsal side of the pyloric sac of the stomach. The intestine 
suddenly enlarges at the beginning of the sixth segment of the tail, where it gives off 
from its dorsal side another slender blind pouch or caecum, which is apparently a rudi- 
mentary structure. (PI. xxxiii.) From this point to the vent, which is closed by a 
sphincter muscle, and from the mouth to the beginning of the intestine, the canal is 
lined with cuticle which is continuous with that over the body and is accordingly 
renewed at each molt. The embryology of the animal shows that the inner wall of the 
intestine is primarily due to an ingrowth from the outside skin and in the early larvae 
an intestinal cuticle can be detected, but if the latter is present in the adult it is 
reduced to a layer of extreme thinness. 
