OBtJEHVATlOXS OX FRESH-WATEE POLYZOA. 93 



portion of each branch is turned nearJy vertical to the plane of the 

 colony and is capped by another short tube (polypidal endoeyst\ 

 through tlie pellucid wall of which is seen the alimentary canal con- 

 tained within. This terminal tube, with the tentaculate lophophore 

 at its free end, and several delicate organs in its cavity, is called the 

 polypide (fig. 2, PI. XYIL). 



Besides this division of the colony into the cœnœcinm and 

 the polypides, we may divide it into a number of equal parts, each 

 consisting of a polypide and a portion of the cœnœcium. For the sake 

 of convenience I shall call such a part ^' pulijzouid" and the portion 

 of the cœnœcium belonging to it ^^ cystid.'' AVe thus consider a 

 colony as being made up of as many polyzoöids, all structurally alike, 

 as there are polypides. 



In all genera with chitinous ectocyst. the ca'nœ^ciuni is dixided 

 by more or less de\eloped septa into a number c)f compartments or 

 cells, each destined to receive a polypide when the latter is retracted. 

 Such septa are not found in forms wdth gelatinous ectocyst, and the 

 cystidal cavities stand in open connection with one another. 



When a polypide is retracted by the contraction of the muscles that 

 connect it with the bottom of the cystid, its tubular wall invaginates 

 and 1)6 G(3mes a sort of sheath for the tentacles, known as the tentacular 

 sheatli. In the process of evagination, the tentacular sheath begins 

 to reflect upon itself from the lower end. The evagination generally 

 stops when the lower end of the polypide is still within the 

 cystid. In other words, the evagination is incomplete, thus leaving 

 a permanent fold at the boundary between the cystidal and the 

 polypidal endocyst. In this genus, howevei-, the polypides are often 

 stretched out their whole lengtli, and then no such fold is to be seen. 



The shape of the polyzoan colony is ditterent in dilferent genera 

 and species, but it is characteristic for each species. The manner of 



