18 DIGESTIVE SYSTEM. 



distinct as we ascend toward the oesophagus, and totally disappear from this tube and from 

 the rectum. The fundus of the stomach appears to differ from the rest of the alimentary 

 canal in structure and function ; the well-defined longitudinal rugse and deep brown colour of 

 the internal layer of the stomach nearly disappear in it, and, during the process of digestion, 

 we may perceive that the peculiar peristaltic action of the walls is more marked in it than in 

 any other part of the gastric cavity, while it is every now and then separated from the rest of 

 this cavity by a momentary hour-glass constriction. 



In the oesophagus there are only two layers (PI. Ill, fig. 7, X', /x'). These correspond 

 to the middle and external layers of the stomach, the former being here largely developed (A'), 

 while the internal or hepatic layer of the stomach is entirely absent, and there are no longitudinal 

 iiigse. The mouth and upper part of the oesophagus are in all the genera clothed with vibratile 

 cilia, but I could detect no appearance of cilia further than a short distance down this tube. 



The structure of the intestine closely resembles that of the oesophagus ; vibratile cilia 

 however, are altogether absent. In Cristatella, the cells of the internal layer corresponding 

 to the middle layer of the stomach are large, and filled, in the well-fed animal, with a clear 

 greenish-blue fluid. 



With the exception of the mouth and upper portion of the oesophagus, no part of the 

 alimentary canal is ciliated in the phylactolaematous fresh-water Polyzoa. In Faludicella 

 however, the stomach in the immediate neighbourhood of the pyloric orifice is lined with 

 long vibratile cilia (PL X, figs. 3, 4), by which portions of alimentary matter, pushed onwards 

 by the peristaltic contractions of the stomach, are kept in a constant state of active rotation 

 previously to their being delivered into the intestine. The entire tract in all the genera 

 examined is highly irritable, the presence of alimentary matter stimulating it to rapid and 

 vigorous contraction. 



The whole course of the alimentary matter, from the moment of its prehension to its final 

 ejection, may be easily witnessed in many of the fresh-water Polyzoa. If a polypide of 

 Pluniatella repens be watched while in an exserted state, diff'erent kinds of Infusoria and other 

 minute organic bodies may be observed to be whirled along in the vortices caused by the 

 action of the tentacular cilia, and conveyed to the mouth, where many of them are at once 

 seized and swallowed, and others rejected. The food having once entered the oesophagus, 

 experiences in this tube no delay, but is rapidly conveyed downwards by a kind of peri- 

 staltic action, and delivered to the stomach ; and at the moment of the passage of the 

 alimentary matter from the oesophagus into the stomach the cardia may be observed to 

 become more prominent. In the stomach the food is destined to experience considerable 

 delay ; it is here rapidly moved up and down by a strong peristaltic action, which first takes 

 place from above downwards, and then inverting itself, propels the contents in an opposite 

 direction. Every now and then the fundus of the stomach, which, as has already been said, 

 seems to perform some function distinct from that of the rest of the organ, seizes a portion of 

 the alimentary mass, and retains it for a moment by an hour-glass restriction separate from 

 the remainder, and then powerfully contracting on it, forces it back among the other contents 

 of the stomach. All this time the food is becoming imbued with the peculiar secretion of the 

 gastric walls, and soon assumes a rich brown colour. After having thus undergone for some 

 time the action of the stomach, the alimentary matter is delivered by degrees into the intes- 

 tine, where it accumulates in the wide pyloric extremity of this tube. After continuing here 

 for a while in a state of rest, and probably yielding to the absorbent tissues its remaining 



