SPONGIN. 
12 Spong. 
regions, where the pressure does not vary, flexibility is not necessary, but 
the surface of the Sponge often has to bear a considerable load, since it 
may become covered by falling masses of mud. In moderate depths the 
pressure varies sensibly on account of wavos. In the littoral zone, which 
in the warmer seas is the chief homo of the ChaUnince and Keratosa, the 
skoloton is ofton subjected to uninterrupted pressure and strain. 
Among H exact inellida, characteristic deep-sea forms, the body is 
generally fragile, with little flexibility, but considerable power of 
bearing loads. They are continually covered by falling mud, which it is 
obviously the function of the oscular sieve to keep out. Perhaps 
also it is the task of the commensal crustaceans to clear out this mud. 
The Lithistida , occurring principally frpm 50-200 faths., resist differences 
of pressure and strain by a very strong siliceous skeleton. The Choristida 
reach their maximum from 0-50 faths. ; amongst them the Geodice resist 
, lateral strains by their broad bases, and pressure by their strong cortex. 
Other genera ( Tetilla , Tethya , &c.) protect themselves by a strong 
“ Turgor ” of the tissues, so considerable, that if a Tethya , for example, 
be cut, the surface of the incision bulges out. The Monactinellidce and 
Horny Sponges, numerous in shallow water, are subject to much 
pressure and strain, to resist which silica is not elastic enough ; hence, as 
a new skeletal material, spongin appears, , first as a cement to fasten 
spicules together, and then as strong fibres chiefly running longitudinally 
in the direction of the greatest strain. The tension which the tissues 
have to withstand being greater at the periphery than in the interior, the 
tissues with their supporting structures tend to be placed peripherally, 
and as a compromise between the necessity of nutrition and of support, 
arise the frequent tubular, funnel-shaped, or cup-like forms. Thus many 
morphological peculiarities of the Sponge body can be explained on 
mechanical principles. There is no doubt that the Sponges with spongin 
arose caenogenetically from siliceous Sponges without spongin as the result 
of life in shallow water. The mechanical cause which led to the for- 
mation and subsequent further development of the Monactinellidce and 
Horny Sponges was the agitated water with its resulting tensions. 
These conclusions are apparently contradicted by the Horny Sponges, 
e. g., Psammophyllum and Stannophyllum, described by Haeckel from great 
depth, but these genera appear to be nearest related to certain littoral 
forms, and hence may be descended from littoral ancestors. The for- 
mation of an oscular sphincter may heighten the turgor of the Sponge 
body, and the chones of Stellettidce perhaps have a similar function. 
Keller (2), after a short historical introduction, discusses the distri- 
bution of spongin, the structure of spongin secretions, the influence of 
life habits on the construction of the skeleton of Sponges, and the 
dependence of spongin formations upon vertical distribution. Spongin 
occurs in the three great orders of the Keratosa , the Monactinellida, and 
Tetractinellida. It is quite wanting in Ilexactinellida and in existing 
Calcarea , though amongst fossil forms the Pharetrones seem to have had 
their spicules cemented together with spongin. [Cf. Rauff (1) on this 
