,28 



DEPTHS OF THE OCEAN 



more expanded, and the two bent legs which issue from it do 

 not lie in quite the same plane, with the result that in sinking 

 the cell describes very long sweeps. Besides these we get other 

 genera, where the suspension-organs are not formed by the 



Oniithocercus. Cell itself, but by the membrane-curtains. In Ornithocei^cus 

 splendid2is the ring - borders are transformed into an un- 

 mistakable parachute, stiffened by a network of ribs (see Fig. 

 234, a), and in some species, such as O. steinii and O. qtiadratus, 

 the membrane-curtains are ventrally or posteriorly most highly 

 developed (see Fig. 234, b). The 

 majority of these more different- 

 iated forms are without chromato- 

 phores, but some of them by way 

 of compensation are in almost 

 constant symbiosis with small 

 brown naked cells that are prob- 

 ably immobile stages of brown 

 flagellates. In Oriiithocercus 

 7nagniJicuSy for instance, we find 

 these naked cells in the space 

 between the ring-borders, where 

 they are well protected against 

 harm (see Fig. 235) ; and in a 

 series of species of the remarkable 



Histioneis. tropical genus Histioneis this 

 home of theirs is expanded pos- 

 teriorly into a cavity which may 

 be of considerable dimensions 

 as compared with the cell. In 



Citharistes. Citkaristes the cavity takes up 



the whole of what should be the central portion of the cell, and 

 the cell-membranes are merely the outer skin like the shell of 

 a guitar (see Fig. 236). 



A remarkable subdivision of the peridinese is the genus 



Pyrocystis. Pyvocystis, which Sir John Murray discovered during the 

 "Challenger" Expedition. Pyrocystis noctihica (see Fig. 237) has 

 large globular cells with a thin layer of protoplasm along the 

 cell-wall, a denser mass round the nucleus, and brown pigment 

 granules. Murray stated that the genus was abundant in all 

 tropical and subtropical waters, where the temperature exceeds 

 68° F., and where the salinity at the surface is not lowered 

 by the presence of coast or river water. The cells have 

 no organs of motion, but belong to the most brilliantly phos- 



■Triposolenia b/cora'is m^). 

 (Kofoid.) 



