VII] OF HEXAGONAL SYMMETRY 327 



known as triasters, poly asters, etc., whose relation to a field of 

 force Hartog has explained*. It is obvious that, in our corals, 

 these curving septa are all orthogonal to the non-existent hexagonal 

 boundaries. As the phenomenon is wholly due to the imperfect 

 development or non-existence of a thecal wall, it is not surprising 

 that we find identical configurations among various corals, or 

 families of corals, not otherwise related to one another; we find 

 the same or very similar patterns displayed, for instance^ in 

 Synhelia (Oculinidae), in Phillipsastraea {Rugosa), in Thamnas- 

 traea {Fungida), and in many more. 



The most famous of all hexagonal conformations and perhaps 

 the most beautiful is that of the bee's cell. Here we have, as in 



Fig. 131. Surface-views of Corals with undeveloped thecae and confluent septa. 

 A, Thamnastraea ; B, Comoseris. (From Nicholson, after Zittel.) 



our last examples, a series of equal cylinders, compressed by 

 symmetrical forces into regular hexagonal prisms. But in this 

 case we have two rows of such cylinders, set opposite to one 

 another, end to end; and we have accordingly to consider also 

 the conformation of their ends. We may suppose our original 

 cylindrical cells to have spherical ends, which is their normal and 

 symmetrical mode of termination ; and, for closest packing, it is 

 obvious that the end of any one cylinder will touch, and fit in 

 between, the ends of three cylinders in the opposite row. It is 

 just as when we pile round-shot in a heap; each sphere that we 



* Cf. Hartog, The Dual Force of the Dividing Cell, Science Progress (n.s.), i, 

 Oct. 1907, and other papers. Also Baltzer, Ueber mehrpolige Mitosen bei Seeigel- 

 eiern, Inaug. Diss. 1908. 



