204 NATURAL HISTORY OP 



a circular or crescentic stage, the lopliopliore, in tlie 

 centre of which opens the mouth. Each tentacle is 

 richly ciliated, and the cilia, which are vibratile, aid in 

 forming currents by which the food is carried into the 

 mouth. The number of the tentacles varies from 8 to 

 80. In addition to the tactile use of the tentacles, for 

 which from their mobility and their muscular struc- 

 ture they are well adapted, they perform the functions 

 of gills, and act as respiratory organs. On invagina- 

 tion tbe tentacles are protected by the sheath before 

 referred to, and the protrusion and retraction of this 

 sheath and the tentacles are the only movements of 

 which the polypide is capable. In all the fresh-water 

 genera (exce'pt Pahidicella) the tentacles are surrounded 

 at the base by a delicate cup or calyx, which is pro- 

 longed more or less on to the back of each tentacle so 

 as to form a series of little triangles connected by a 

 broad base. This festooned appearance is co-extensive 

 with the presence of the epistome. In the fresh-water 

 family Plumatellidce, the margin of the lophophore is 

 extended into two triangular lobes or arms, causing 

 the lophophore to assume a crescentic appearance. 

 This crescentic appearance is not found in any marine 

 species. 



The oesophagus leads directly from the mouth to the 

 stomach and is thickly ciliated, and in some species 

 the upper portion forms a funnel-shaped pharynx 

 (Plate XVI., fig. 6 ph), which is often dotted with 

 spots. The oesophagus communicates with the stomach 

 by means of the cnrdia (Plate XVI., fig. 2), which 

 is a conical projection so placed as to prevent the 

 return of the food into the oesophagus during the con- 

 traction of the stomach in the process of digestion. 



