326 



THE FORMS OF TISSUES 



[CH. 



Where larger and smaller cells, corresponding to two different 

 kinds of zooids, are mixed together, we may get various results. 

 If the larger cells are numerous enough to be more or less in 

 contact with one another (e.g. various Monticuliporae) they will 

 be irregular hexagons, while the smaller cells between them will 

 be crushed into all manner of irregular angular forms. If on the 

 other hand the large cells are comparatively few and are large 

 and strong-walled compared with their smaller neighbours, then 

 the latter alone will be squeezed into hexagons, while the larger 

 ones will tend to retain their circular outline undisturbed (e.g. 

 Heliopora, Heliolites, etc.). 



When, as happens in certain corals,, the peripheral walls or 



Fig. 129. Amchnophylhnii pentagoyium. 

 (After Nicholson.) 



"thecae" of the individual polypes remain undevelope'd but 

 the radiating septa are formed and calcified, then we obtain new 

 and beautiful mathematical configurations (Fig. 131). For the 

 radiating septa are no longer confined to the circular or hexagonal 

 bounds of a polypite, but tend to meet and become confluent 

 with their neighbours on every side; and, tending to assume 

 positions of equilibrium, or of minimal area, under the restraints 

 to which they are subject, they fall into congruent curves; and 

 these correspond, in a striking manner, to the lines of force running, 

 in a common field of force, between a number of secondary centres. 

 Similar patterns may be produced in various ways, by the play 

 of osmotic or magnetic forces ; and a particular and very curious 

 case is to be found in those complicated forms of nuclear division 



