OP THE SPONGIAD^. 109 



might attempt to prey upon it. Fig. 298, Plate XVIII, 

 represents a small portion of the dermal membrane of this 

 sponge. This mode of defence is very general in the 

 genera Geodia, Tethea, and Pachymatisma, and it occasion- 

 ally occurs in other genera of Spongiadse, 



THE PORES. 



The pores in the Spongiadse are the orifices or mouths 

 through which the animals breathe and imbibe their nutri- 

 ment. They are situated in the dermal membrane, and are 

 exceedingly numerous when the imbibing powers are in 

 fuU^ operation. In Pachi/matisma and Geodia, and in some 

 other highly organized genera, there is good reason to 

 believe that they are permanent organs, opening and closing 

 repeatedly in the same situations. But in the greater part 

 of the Halichondroid types of sponges they are certainly 

 not permanent orifices, like the mouths of higher classes of 

 animals, and in these sponges, when they are in a state of 

 complete repose, there is not the slightest indication of 

 their existence. Their usual form is circular, but they fre- 

 quently assume the shape of an elongated oval, and within 

 a limited range they vary to a considerable extent in their 

 dimensions ; on the whole, they exhibit a very constant and 

 universal type of form and size ; however difierent may be 

 the internal structure of the sponges, or however great may 

 be the difference in size of the individuals, they always 

 appear to maintain their normal characters. No definite 

 law appears to prevail in their distribution over the surface 

 of the sponge, and they are liable to appear to a greater or 

 a less extent on every part of its external surface, wherever 

 there are intermarginal cavities beneath. The situations 

 where they may be expected to appear may in many in- 

 stances be readily recognised. Thus, in • Halichondria 

 panicea, wherever we see on the dermal membrane a well- 

 defined reticulation of spicula, with clear and distinct areas, 

 there, when the sponge is inhaUng, we may expect to find 

 the open pores, as represented in Fig. 303, Plate XIX, 



