VIIl] 



OF RADIATE PATTERNS 



619 



among our diatoms; but when it does so, it is looked upon as the 

 mark and characterisation of the allied genus Arachnoidiscus. 



A simple case, introductory to others of a more complex kind, 

 is that of the radial canals of the Medusae. Here, in certain cases 

 (e.g. Eleutheria), the usual arrangement of eight radial canals is not 

 seldom modified, as for example, when two or more of them arise 



14 -< ^ 15 --<.^-^l6 



Fig. 207, Variations observed in the catial-system of a raedusoid {Eleutheria); after 

 Hans Lengerich. 1-8, the eight possible arrangements of eight radial canals; 

 9-10, some observed instances of nine radial canals. 



not separately but by bifurcation*. We then have just eight 

 possible arrangements, as shewn in Fig. 267, 1-8, and of these eight 

 no less than six have been actually observed. The other two are 

 just as hkely to occur, and we may take it that they also will in 

 due time be recorded. It is yet another simple illustration of the 



* Hans Lengerich. Verzweigungsarten der Radialkanale bei Eleutheria, Zool. 

 Jahrbuch, 1922, p. 325. 



