620 THE FORMS OF TISSUES [ch. 



aphorism that whatsoever is possible, that Nature does, all in her 

 own time and way; what things Nature does not do are the things 

 which are mathematically impossible or are barred by physical 

 conditions. It is possible for our little Medusa to develop nine 

 radial canals instead of eight, and so at times she does ; again they 

 may be simple or branched, and trifurcations as well as bifurcations 

 may appear. So we may extend our Hst of possible permutations 

 and combinations, and find as before that a fair proportion of these 

 possible arrangements have been observed already. There are 

 many other Medusae ' (e.g. Willsia), where the number of radial 



III ^<^JU^ IV 



Fig. 268. A medusa {Willsia ornnta) 

 shewing, diagrammatically, the order 

 of development of the numerous radial 

 canals. After Mayor. 



Fig. 269. Section of Alcyonarian 

 polyp. 



canals much exceeds the simple symmetry of four or eight ; and in 

 these we may sometimes see, very beautifully, how the successive 

 canals arrange themselves according to the same principles which 

 we have now studied in so many diverse cases of partitioning 

 (Fig. 268)*. 



In a diagrammatic section of an Alcyonarian polyp (Fig. 269), 

 we have eight chambers set, symmetrically, about a ninth, which 

 constitutes the "stomach." In this arrangement there is no diffi- 

 culty, for it is obvious that, throughout the system, three boundaries 

 meet (in plane section) in a point. In many corals we have as 



* Such branching canals are characteristic of the Dendrostaurinae, a subfamily 

 of the Oceanidae, a family of Anthomedusae ; and very much the same occur in 

 a certain subfamily of Leptomedusae. See Mayor's Medusae of the World, i, 

 p. 190. 



