CYSTS AND CYSTIPHRAGMS 355 



tliickeii tliL'ir walls, secrete diaphragms and cystipliragins (in species in 

 which the latter are a normal feature), and show all the various char- 

 acteristics of maturity. In other words, a mature zone is carried across 

 the tip of the branch. Such zones are often seen in longitudinal sections 

 of zoaria. Again, in erect ramose or frondescent zoaria, in which the 

 axes of individual zocecia lie in a more or less horizontal plane, the 

 cystiphragms are on the upper side of the zooecia. This fact probably 

 indicates that the endosarc tends to sag away from the zooccial wall in 

 response to the weight of the polypide and other zooocial contents. 

 Where the zooecia stand in a more nearly vertical direction, as in Praso- 

 pora, the cystiphragms are about equally developed on all sides of tlie 

 zooecium. A further argument in favor of the view that the cystiphragms 

 are concerned solely with the restriction of intrazooecial space, especially 

 in mature and senile stages of growth, is furnished by the peculiar relation 

 of cystiphragms to secondaiy deposits on the zooecial wall, as shown in 

 figure 17. Here the greatly thickened cystiphragms are seen to pass 

 directly into the secondary thickening (cingulum) of the wall, and in- 

 deed the cingulum of the distal portion of the zooecium figured consists 

 (luite evidently of thickened cystiphragms laid on flush — that is, without 

 any voids between them. This fact is shown at cy' in the figure. Sim- 

 ilar relations of the cingulum to cystiphragms are shown in figures 1, 6, 

 and 9. Lee (IS) suggests that the thickening of the walls in the mature 

 region is for the pui^pose of filling up the extra interzooecial space due to 

 the extension of the zooecial tubes radially outward from the axial region. 

 That it has this effect is obvious; but it is somewdiat doubtful whether 

 this is the primary reason for the thickening, and especially for the 

 secondary deposits so common in thick-walled species. This subject will 

 be furthof discussed under tlx' head of wall structure. 



It is \\v\\ known tliat a'singlc zoo'cium may produce a succession of 

 polypides. each in iis turn dcgiuici at ing into a brown body. It has also 

 been ])oinlcd out hy l.c\iiiscii ( 1!») that the zoccciuni may be rcgiMiei-ated 

 /'// Idh). ('al\ct ( ■") ) holds. \\v hclicxc witli \ci'V good reason, thai these 

 successive degciici-alious ^A the polypide are eoiiiieeled wiih successive 

 ovulations. Such liguics as 1, "i. ;!, I, (!. !), K). and II \ci-v I'orclblv 

 suggest such a succession of reproductive efforts, witli a progressive re- 

 duction of the size and vigor ol the successive polypi(h's, reaching a cli- 

 max in the final extinction of the individual, with the sealing up of its 

 zooecial cavity ami the retained pro(lucts of the successivelv formed 

 brown bodies. 'I'lieic is ph-nty of analogy among recent Bryozoa for such 

 a life history. After the culmination of the process, we may suppose that 



