936 
THE VOYAGE OF H.M.S. CHALLENGER. 
regions, where the temperature is over 68° or 70°, and the specific gravity of the water is 
not lowered by the presence of coast and river water. 
“ The shape is spherical, from 0'6 to 0'8 mm. in diameter, the colour of the protoplasmic 
matter is brownish, like that of Diatoms and Ceratium. The external capsule is composed 
of a resistant organic matter containing cellulose. The 
internal ball or protoplasmic body (cytode) is sometimes 
separated by a considerable interval from the external 
capsule, as shown in figs. 336-337 ; sometimes, however, 
it is closely applied, and protoplasmic threads ramify over 
the inner surface of the capsule, or through the internal 
cavity (fig. 335). In the fresh state the internal ball 
colours slightly with osmic acid. The nucleus stains easily 
with carmine solution. Very frequently a number of small 
pellucid dots are observed over the surface of the external 
capsule, generally aggregated toward one portion of the 
sphere. If a little magenta solution be added to the sea 
water these dots instantly colour, and the organism then 
presents a very beautiful appearance. The cytode is very often observed in simple 
division beginning with the nucleus. Sometimes the internal ball divides into four ; the 
outer capsule eventually bursts. The external capsule will persist for days in cold strong 
nitric acid and caustic potash, but disappears by boiling in the former. When treated 
with sulphuric acid and iodine solution it becomes blue. 
Fig. 335 . — Pyroeystis noctiluca, Murray ; 
100 times the natural size. 
Fig. 326.— Pyrtfcystis noctiluca, Murray ; 100 times the natural size. 
“ Pyroeystis is strongly phosphorescent, the light proceeding from the nucleus, and 
it is the chief source of the diffused phosphorescence of the sea in equatorial regions. It 
is especially abundant in the warmest waters of the tropics, the most brilliant displays 
of phosphorescence observed during the cruise being due to its presence in great 
