OBSERVATIOXS OX FRESH- WATER POLYZOA. 1^9 



the exterior <jf tlic bottom of the cy.stogenou.s cup, aiid when the cup 

 closes into ÎI complete capsule, tis it does later, the same condition is 

 also seen on the opposite side, so that on a mature statoblast the polar 

 surfaces show only a network of very low ridges. However, on both 

 sides of the belt plate and of the capsular surface immediately adjoining 

 them, the chitinous wall of cells attains considerable height, but never 

 reaches the surface of the cell-layer. The open ends of chitinous tubes 

 thus formed are finally closed by the formation »jf what is called the 

 lid-plates. This process proceeds on the one hand centrifugally from 

 the outer layer of the chitinous cup, at a line which circumscribes the 

 reticulated polar area, and on the other in the opposite direction starting 

 from the marginal edge of the belt-plate, so that the tubes on the 

 midway are closed last. A glance on tigs. 48-J:6 will make the 

 matter clear at once. j\Ioreover, the lid-plates divide the prisniîitic 

 cells on either side of the belt-plate into an outer and an inner \)uv- 

 tion. The latter is completely enclosed in chitinous caskets, while 

 tiie former conjointly with the epithelium covering the polar area 

 invests the entire outer surface of the young statoblast. This invest- 

 ment is to be seen as long as the statoblast remains at the place of 

 its development, but decays when the latter is set free by the dissolu- 

 tion of mother-polypides. As the lid-plates are developing, the 

 marginal spines appear. Also at about this stage, the closure of the 

 mouth of the cystogenous cup takes place. It thus completely 

 encloses the granular cell-mass, followed by closure of the two layers 

 of the chitinous cu^), which then is turned into a perfect capsule. 

 After this, the two polar areas present no point of structural 

 difference. 



The portion of the prismatic cells that are enclosed within the 

 chitinous wall soon undergoes decomp(jsition and gives place to a gas 

 filling up the caskets. Thus the formation of the swimming- belt is 



