216 ANATOMY AND PHYSIOLOGY 



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 shghtly elevated marginal ring. All 

 these characters are subject to a considerable amount of 

 variation, which are sometimes dependent on peculiarities 

 of locality, and at others on age or the amount of their 

 development; but a comparison of several specimens of 

 the same species will generally lead the observer to a correct 

 conclusion regarding 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 congregated; 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, or 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, sometimes 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 



