EXTERNAL FORM AS A SPECIFIC CRITERION 375 



I cm. in diameter, in which the lower ( ?) half of the sponge consists only of collen- 

 chymatous (gelatinoid) tissue completely enclosing fragments of calcareous debris 

 (Fig. 56/)). The collenchymatous portion naturally suggests something in the nature of 

 a float, with the fragments of debris acting as balancers. 



Except for the species of Cladorhiza and Axoniderma, which may be habitually free- 

 living, Disyritiga and Motiosyringa , which may be occasionally free-living, it is probable 

 that the larvae first settle on small stones or pebbles, perhaps even large sand grains. 

 If these be sufliciently heavy the sponge becomes attached for life ; but if small, the 

 growing sponge will gradually lift the stone or pebble on which it has settled, as their 

 combined specific gravity decreases, and even, eventually, become grown round the 

 stone or pebble, entirely enclosing it. In this connection it must be remembered that the 

 specific gravity will vary from species to species, or from one individual to another of 

 a single species, accordingly as the skeleton is slightly built or densely constructed, the 

 relative quantities of spicules in individuals of a given species being subject to consider- 

 able fluctuation. It may, on the other hand, be argued that no organism with tissues 

 charged with silica and devoid of appendages or special organs of flotation could 

 possibly do anything else than remain permanently on the bottom. Yet, since the 

 tissues of many sponges are densely charged with fat globules, and the spicules are 

 not solid silex but contain an axial canal, it is possible that the specific gravity of 

 sponges generally is less than is usually assumed.^ The only real test worth applying 

 would be specific gravity readings on the (apparently) unattached forms while still 

 alive. One thing, at least, is clear, that many species of sponges are definitely free- 

 living, and there is every indication that some of these at least may not rest permanently 

 on the floor of the sea. 



THE VALUE OF EXTERNAL FORM IN THE IDENTIFICA- 

 TION OF SPONGES: WITH NOTES ON VARIATIONS IN 

 THE SIZE AND SHAPE OF SPICULES 



In the early days of the systematic study of sponges, external form was the only thing 

 considered worthy of description. It was, in fact, not until after i860, when Bowerbank 

 and Schmidt published their well-known monographs, that the spicules were seriously 

 considered as diagnostic features. In a very short time, the skeleton became the chief 

 object for study by the systematist. To-day, considerably more attention is paid to 

 the skeleton in the description of a new species than to the external form, and the 

 tendency is rather towards omitting illustrations of the external form. As a result of 

 my own experience, I am coming to the conclusion that, for the identification of species, 



1 I am indebted to members of the Discovery Staff, Dr Mackintosh and Mr Eraser, for the information 

 that many specimens of Tedania, notably T. massa, emit large quantities of mucus when taken from the 

 dredge. The presence within the tissues of large quantities of mucus would possibly result in lowering 

 totally the specific gravity of the sponge as a whole, and it is conceivable that the texture and volume of a 

 preserved specimen of T. massa differ appreciably from those of the living sponge. 



18-2 



