OBSERVATIONS OX FRESH-WATER POLYZOA. 99 



takes |)]:ice, and is thinnest in the tentacles with nuclei scattered 

 widely apart (%s. 16 and 17, PI. XVIIL). The cells of this layer 

 are fused, hence cell boundaries cannot he distinguished. The 

 nucleus is oval, but I am unable to detect distinct nucleoli. The 

 size of the nucleus is nearly the same everywhere, and is about 

 0.00(S X 0.004 mm. This layer is furnished with short cilia, which 

 set the perigastic fluid in motion. Average thickness of the layei" in 

 the cœnœcium is 0.008 mm. 



2. Digestive Sj^stem. 



Minute algae and infusoria that pass by are caught in the whirl- 

 pool caused by the vibrating cilia of the tentacles, and sent into the 

 œsophagus. The e[)isti)me that guards the mouth is ftu'ni.shed with 

 special muscles which enable it to shut the oral aperture now and 

 then. Perhaps the entrance of non-nutritive matters is prevented by 

 this contrivance. The food, after staying ibr a short time in the 

 oesophagus, pushes open ihe funneldike valve (fig. 4, PI. XVI I. 

 fniuicl-like v.) that intercepts free conununication between the oeso- 

 phagus and the stomach, and enters the gastric cavity where it is 

 moved about by the peristaltic contraction of the wall of that organ. 

 After l)eing fully digested, the residue mainly composed of the cell- 

 wall of diatoms and other algae, passes through the pyloric valve little 

 by little, and accumulates in the intestine. Here, the refuse matter, 

 usually of a dark-grayish color, is cemented together into a mass by a 

 transparent gelatinous secretion of the intestinal wall. AYhen the 

 intestine is full, the contents are pushed out of the anus by the 

 agency of the muscles of that part. The form of the excremental 

 mass, characteristic of each genus, is the same in form as the lumen 

 of the intestine which in our species is an elongated oval tapering 

 toward the anus. 



