X INTRODUCTION. 



especially for the determination of fossil forms ; and no 

 doubt they may be available to some extent^ chiefly in 

 cases in which the salient characteristics of the cell have 

 been more or less obliterated. Otherwise the difficulty of 

 examining them must always limit their use in ordinary 

 diagnosis. 



Nitsche, in his interesting monograph on Membranipora 

 membranacea, has pointed out a modification in the struc- 

 ture of the ectocyst in this species by which it is adapted 

 to its usual habitat. The coenoecium is not a continuous 

 stony framework, but in every zocecium certain sections 

 only of the wall are calcareous, and between these are 

 interposed uncalcified and flexible plates. In this way the 

 whole cell is rendered flexible, and consequently the whole 

 colony, composed of thousands of cells ; and instead of 

 breaking up as the great Laminarian fronds in which it 

 delights sway backwards and forwards in the water, the 

 living lacework adapts itself to their undulations and 

 escapes destruction. So far as function is concerned, these 

 flexible plates may be compared, as Nitsche has already 

 remarked, with the corneous joints in the stems of many 

 of the erect and branching Polyzoa. 



The ectocyst, then, or external layer, is a mere struc- 

 tureless excretion from the endocyst, charged with a pro- 

 tective function, but with no special physiological signi- 

 ficance. 



We pass on to a much more important element of the 

 structure, the endocyst itself, or internal layer of the cell- 

 wall. 



Endocyst. — This presents itself under somewhat diffe- 

 rent forms in the Gymnolesmata and PliyUictolamata (or 



