NATURAL HISTORY OF AMERICAN LOBSTER. 25 1 



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. 



