OBSERVATIONS ON FRESH-WATER POLYZOA. 109 



There are two ciliiited tnhes just l^enentli the outer layer, on tlie 

 anal side of the body, l-jetween the aims and the bases of the median 

 tentacles of the inner row. The walls of these tubes are continuations 

 of the epithelial lining- of the invaginable portion of the endoeyst. 

 They open below into the body-cavity by fnnnel-shaped opening's. 

 They measure 0.15-0.19 mm. in length, though the portion where 

 the wall is entire is much shorter (fig. 26 bis. PI. XVIII). The 

 shape of the funnel-like openings may be compared most appropriately 

 with the obliquely cut end of a hollow tube. 



The exact form of these tubes and their relation with other 

 organs will be best undestood by referring to figs. 21-26, PI. XVIII, 

 which show their cross sections with the neighboring parts at 

 various levels. 



In a cross section passing through the middle part of the 

 tubes, we see them as two oval sections lying side by side (figs. 24 

 and 2-1 A). The ciliated epithelial wall consists of cells which are 

 cul)ical near the median plane of the polypide and flat on the opposite 

 side. Consequently both the nuclei and cilia are densely set in the 

 portion nearest the median plane of the polypide and scattered at 

 some distance from one another on the outer side. The tubes are 

 closely enveloped on the anal side by the outer layer of the invagina- 

 ble tube (Out. ki)/.), and on the oral side by the lining epithelium 

 (^Liii. cpit]!.) of the body-cavity. The diameter of the tubes measures 

 about 0.03 mm. 



Tracirig these tubes downward, that part of the wall farther 

 removed from the median line soon disapjtears, i, e., the tubes 

 open into the body-cavity on that side (fig. 26). As the two tubes 

 deviate from each other below, a part of the perigastric s})ace appears 

 between them (fig. 26, rpistom. car.) This is the passage by which 

 the cavity of the epistome commu.nicates with the perigastric. 



