422 STUDIES IN EVOLUTION 



features, and therefore the species M. lenticularis is here 

 referred to this genus. The large calices and their constant 

 origin at the basal epitheca are not, however, essential 

 characters of Pleurodictyum. The structure and growth 

 of this species indicate that it represents one of the simpler 

 types of poriferous corals. For this reason its develop- 

 ment is without the numerous modifications necessary in 

 more complex forms, and its laws of growth are not 

 complicated. 



Development of PleurodActyum lenticulare. 



The nepionic stage is well marked. It comprises the 

 growth of the corallum to the completion of a simple initial 

 cell. This primitive cell, or nepionic stage (Plate XXVII, 

 figures 1, 7, 8), has the form of an oblique inverted cone, 

 flattened on one side. The flattened area represents the 

 lower or attached side, and the oblique base of the cone is 

 occupied by the aperture of the corallite. The apical portion 

 is smooth for about one-fourth the length of the cell. Then 

 the concentric lines of growth become apparent, and over the 

 distal half radiating ribs are also developed. The interior of 

 the apex is granulose. At about the middle of the cell the 

 granules are arranged in rows, forming the beginnings of the 

 septal lines. 



The simple growth of the initial cell continues until the 

 entire procumbent portion is completed. A thickening of 

 the margin then takes place, and an upward growth of the 

 corallite is initiated. At the commencement of this upward 

 growth the first bud starts out from the lateral edge of the 

 initial calyx, either to the right or left of the axis. This 

 condition represents the first neanic stage. The bud resem- 

 bles the parent cell in all particulars, and reaches considerable 

 size before the second appears, as shown in Plate XXVII, 

 figures 9, 10. The visceral cavities are confluent, as tlic 

 initial apex of the bud opens into the calyx of the first cell. 



The succeeding neanic stages, to the completion of the 



