a. 
ON FRESHWATER POLYZOA. 311 
appearance, when viewed under a high power of the microscope ; external to 
this, another layer, possibly muscular, seems also to be present. The mouth 
and upper part of the cesophagus are clothed with a ciliated epithelium ; but 
I could detect no appearance of cilia further than a short distance down the 
tube. 
The structure of the intestine closely resembles that of the cesophagus ; 
vibratile cilia, however, are altogether.absent. In Cristatella the cellules of 
the internal layer 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 cesophagus, no 
part of the alimentary canal is ciliated in the species with bilateral lopho- 
phores. The whole tract is highly irritable, the presence of alimentary matter 
stimulating it to rapid and vigorous contraction. 
2. Species with orbicular Lophophore.—In Paludicella articulata, the only 
freshwater representative of the species with the lophophore orbicular, the 
mouth is a perfectly circular orifice, with slightly projectile margin, and is 
totally destitute of the valve-like appendage which is found in all the other 
freshwater species, The upper part of the cesophagus is wide, and may 
perhaps here, more decidedly than in the other species, be distinguished as 
pharynx. It soon contracts into a long narrow tube, which leads to an oval 
sac corresponding to the cardiac cavity of the stomach in the other fresh- 
water Polyzoa, and to the gizzard in certain marine species. This sac is 
much more distinct from the great cavity of the stomach than in the other 
Polyzoa of fresh water. It enters this cavity near its upper extremity, and 
presents here a well-marked constriction; in extreme retraction of the polypide 
it is bent back upon the rest of the stomach. The great cavity of the stomach 
‘is of a nearly cylindrical figure ; from its upper extremity arises the intestine. 
This tube presents, just after its origin, a wide dilatron, and then suddenly 
contracting, continues as a narrow cylindrical tube to its termination just 
below the mouth. The stomach is furnished with an internal layer of coloured 
cells, as in the other species, but is destitute of longitudinal ruge. The 
pylorus is clothed with long vibratile cilia, which extend for a short distance 
into the cavity of the stomach. The mouth and upper part of the pharynx are 
also clothed with a ciliated epithelium. 
The whole course of the alimentary matter, from the moment of its pre- 
hension to its final ejection, may be easily witnessed in many of the fresh- 
water Polyzoa. If a polypide of Plumatella repens be watched while in an 
exserted state, different 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 cesophagus, experiences in this tube no delay, but is rapidly conveyed 
downwards by a kind of peristaltic action, and delivered to the stomach ; and 
at the moment of the passage of the alimentary matter from the cesophagus 
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 mement by an hour-glass restriction separate from the re- 
mainder, and then powerfully contracting on it, forces it back among the 
other contents of the stomach. All this time the food is becoming imbued 
