METHODS. 13 
4. GRAPHIC REPRESENTATION. 
All the figures on the plates in this Report are photographs. These photo- 
graphs were taken partly with ordinary, and partly with ultraviolet (wave 
length 280 yu) 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 floricomes and other small microhexaster-forms; 
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 facilitate 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 amphidises 500. The photographs of parts of the 
spicules and of whole small spicules showing minute details were all taken with 
ultraviolet light and are magnified 2000. 
5. MEASURING. 
Every exact description must 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, 
