AJfD PHYSIOLOGY OP THE SPONGIAD^. 
1125 
2. The Oscula. 
The oscula frequently afford good specific characters. Their peculiarities are, first, 
those of position ; and secondly, those of form. Thus it should always be noted whether 
they are dispersed or congregated, whether disposed on the exterior surface or on the 
parietes of internal cloacae. In form they are either simple orifices, or they assume a 
tubular shape to a greater or a less degree, and sometimes they are bounded by a 
slightly elevated marginal ring. All these characters are subject to a considerable 
amount of variation, which is sometimes dependent on peculiarities of locality, and at 
others on age or the amount of their development ; but a comparison of several speci- 
mens of the same species will generally lead the observer to a correct conclusion regard- 
ing their normal characters. 
In some species these organs are always more or less open; in others, especially 
littoral ones, they are entirely closed during exposure to the atmosphere, or while in a 
state of repose, during which condition they are frequently completely inconspicuous. 
3. The Pores. 
The pores afford but very few available characters. They are either dispersed or con- 
gregated, very rarely in the latter state. They are also either conspicuous or incon- 
spicuous : that is, in the former condition their presence, and the areas within which 
the groups of them are situated, may be readily detected by the aid of a hand-lens ; 
in the latter case they are perfectly undistinguishable without high microscopic power. 
4. The Dermal Membrane. 
The dermal membrane affords many important specific characters. In the greater 
number of the Spongiadse it is a simple pellucid membrane, which invests the whole of 
the mass of the sponge ; but in other cases it is of much more complex structure, some- 
times furnished abundantly with primitive fibrous tissue, or a network of spicula or 
kerato-fibrous tissue for its especial support ; and in the areas of such network there 
are frequently tension spicula differing in construction from those of the skeleton, and 
its interior surface is often supplied with anchorate retentive spicula of various forms. 
In its sarcodous lining there are occasionally an infinite number of stellate or sphero- 
stellate spicula to protect it from the ravages of minute enemies, and its surface is also 
often penetrated by large or small defensive spicula. Occasionally its external surface 
is profusely supplied with elongo-stellate defensive spicula. It has also frequently a 
thick stratum of cellular structure of various colours. 
These peculiarities of structure have no generic value. They are essentially specific 
differences ; and it is rarely the case that any two species, even in an extensive genus, 
are found to agree in the possession of the number, form, or mode of disposition of these 
peculiarities of the dermal tissues. They form therefore a constant and highly valuable 
series of characters, and claim the especial attention of the student in either the recog- 
nition or description of an unknown species. 
