Proteids are organic compounds containing carbon, hydrogen, nitrogen, and oxygen, of which white of egg (albumen) is a familiar type. Nucleo-proteids are compounds of proteids with nucleinic acid, which in addition to the above elements contain phosphorus.
The specific gravity of living protoplasm has been estimated by determining the density of a solution of gum in which certain Infusoria float freely at any depth. It was found by the concurrent results of Julia B. Platt and Stephen R. Williams (see Amer. Natural. xxxiii. 1899, p. 31, xxxiv. 1900, p. 95) to be from 1.014 to 1.019, while the Metazoon Hydra was found to give a density of only 1.0095 to 1.0115. The difference of about 0.006, it is easy to show, is of the correct "order of magnitude," if we admit that the actual substance of the Hydra has about the same specific gravity as the Infusorian, while the density of the whole is lightened by the watery contents of the internal cavity, etc. Jensen obtained a much higher result for Paramecium, using a solution of the crystalloid substance, potassium carbonate; but it is almost certain that this would be readily absorbed by the organism, and so raise its density in the course of the experiment.
Energy may be derived from the mere splitting up of complex substances within the cell: when such a splitting involves the liberation of CO2 the process is (mis-)called "intramolecular respiration."
A similar organ, but with cellular walls, is the bladder of the Rotifers and certain Platyhelminthes, in connexion with their renal system (vol. ii. pp. 53, 199, and especially pp. 213-5).
In Rep. Brit. Ass. 1888, p. 714; Ann. Mag. Nat. Hist. (6), iii. 1889, p. 64. This view has been fully worked out, mainly on Ciliates, by Degen in Bot. Zeit. lxiii. Abt. 1. 1905.