Of the physiology of the pigments of sponges not much is yet known: a useful summary of facts will be found in Von Fürth's text-book.[280] SPONGIN.—Von Fürth[280] points out that this term is really a collective one, seeing that the identity of the organic skeletal substance of all sponge species is hardly to be assumed. Spongin is remarkable for containing iodine. The amount of iodine present in different sponges varies widely, reaching in certain tropical species of the Aplysinidae and Spongidae the high figure 8 to 14 per cent. Seaweeds which are specially rich in iodine contain only 1.5 to 1.6 per cent. In view of the fact that iodine is a specific for croup, it is of interest to observe that the old herb doctors for many centuries recognised the bath sponge as a cure for that disease. [Illustration: FIG. 122.—The ordinals measure (i.) the number of species, _a-f_, and (ii.) the number of stations, _a'-f'_, at which successful hauls were made. The abscissae measure the depth: thus at I. the depth is from 0 to 50 fathoms; at II. from 51 to 200; at III. from 201 to 1000; at IV. from 1001 upwards. _a_, _a'_, are the curves for Sponges generally; _b_, _b'_, for Monaxonida; _c_, _c'_, for Hexactinellida; _d_, _d'_, for Tetractinellida; _e_, _e'_, for Calcarea; _f_, _f'_, for Ceratosa.] DISTRIBUTION IN SPACE.—All the larger groups of Sponges are cosmopolitan. Each group has, however, its characteristic bathymetrical range: the facts are best displayed by means of curves, as in Fig. 122, which is based wholly on the results obtained by the "Challenger" Expedition. The information as to littoral species is consequently inadequate, and we have not the data requisite for their discussion. Sponges generally (_a_) and Monaxonida in particular (_b_) are more generally distributed in water of depths of 51 to 200 fathoms than in depths of less than 50 fathoms; but localities in shallow water are {240}richer, for the station curve (_a'_) rises abruptly from I. to II., while the species curve (_a_) in the same region is almost horizontal. The Hexactinellid curve (_c_) culminates on III., showing that the group is characteristically deep water. That for Tetractinellida (_d_) reaches its greatest height on II., _i.e._ between 51 and 200 fathoms. Even here, in their characteristic depths, the Tetractinellida fall below the Hexactinellida, and far below the Monaxonida in numbers. Again, the Monaxonida are commoner than Hexactinellida in deep water of 201 to 1000 fathoms, and it is not till depths of 1000 fathoms are passed that Hexactinellida prevail, finally preponderating over the Monaxonida in the ratio of 2:1.