METHODS. 13 



4. Graphic representation. 



All the figures on the plates in tliis Report are photographs. These photo- 

 graphs were taken partly with ordinary, and partly with ultraviolet (wave 

 length 280 ^i) light ; with the same apparatus and in the same way as those illus- 

 trating my Report on the Geodidae (Mem. M. C. Z., 1910, 41, p. 12 ff.) where 

 the photographic methods employed are described. I found it very difficult 

 to obtain good photographs of fioricomes and other small microhexaster-f orms ; 

 chiefly because it is hardly possible to get good clean preparations of intact 

 spicules of this kind, either in balsam (for photography with ordinary light), 

 or in chloral-hydrate glycerine (for photography with ultraviolet light). My 

 hexaster-photographs are consequently not nearly so attractive as the drawings 

 of them in the papers by other authors — but they accurately represent what 

 one actually sees. 



To facihtate comparison the figures representing the systematically most 

 important spicules of the same kind in the different species are given in the same 

 magnification throughout. To these commensurate figures others, in other mag- 

 nifications, are added where necessary. The uniform magnifications selected 

 for the commensurate figures are such that the smallest forms observed come 

 out just large enough to allow their main characters to be distinctly made out. 

 They are : — for the pinules .300 ; for the microhexactines, the hexasters and 

 their derivates, and the amjihidiscs .500. The photographs of parts of the 

 spictfles and of whole small spicules showing minute details were all taken with 

 ultraviolet light and are magnified 2000. 



5. Measuring. 



Every exact description m\ist be based on measured dimensions. The 

 dimensions of organisms and their parts are inconstant and vary in various ways. 

 To obtain dimensional data sufficient for use as premises for a systematic or 

 any other biological conclusion it is therefore necessary to ascertain the range 

 and biometrical character of the variation in the extension in space of the parts. 

 In the case of such organisms as the Hexactinellida the smaller spicules at 

 least should be studied biometrically. They can be most easily and accurately 

 measured and are considered by all authors as the most important part from a 

 systematic (phylogenetic) point of view. It would have been quite impossible, 



