AND PHYSIOLOGY OP THE SPONGIADJ3. 
793 
external defence, the dermal membrane is often supplied with special defences. Thus 
in Tethea muricata (figs. 14 & 15, Plate XXXI.) we find its outer surface abundantly sup- 
phed with elongo-stellate spicula, which project externally to a considerable extent ; and 
in Bictyocylindms stuposus, Boweebank, MS., beside the numerous defensive spicula 
projected through the surface, we find the membrane filled with minute sphero-stellate 
spicula, which would effectually protect it from the assaults of any minute enemies that 
might attempt to prey upon it. Fig. 6, Plate XXXII. represents a small portion of the 
dermal membrane of this sponge. This mode of defence is very general in the genera 
Geodia, Tethea^ and Pacliymatisma, and it occasionally occurs in many otner genera of 
Spongiadse. 
The Pores. 
The pores in the Spongiadse are the orifices or mouths through which the animals 
breathe and imbibe their nutriment. They are situated in the dermal membrane, and 
are exceedmgly numerous when the imbibing powers are in full operation. In Pachy- 
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 frequently assume 
the shape of an elongated oval, and within a hmited 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 different 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 theh normal characters. No definite law appears to prevail in their distribu- 
tion 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 instances 
be readily recognized. Thus in Halichondria ^anicea, wherever we see on the dermal 
membrane a well-defined reticulation of spicula with clear and distinct areas, there, 
when the sponge is inhaling, we may expect to find the open pores, as represented in 
Plate XXXII., fig. 5, while on spots perhaps immediately adjoining, where the dermal 
membrane is occupied by a thickly interwoven mass, a felting of spicula, the probabihty 
is that not a single pore can be detected. 
In some of the West India fistulose sponges we find the large or prim^vx^ area of the 
dermal surface composed of keratose fibre, and within these large areas the dermal 
membrane is strengthened and supported by a secondary reticulation of spicula, in the 
areas of which the pores are opened. In these secondary reticulations the spicula are 
abundant, while in other parts of the sponge the tension spicula are rather of rare 
occurrence. In Grantia, a sponge of a widely different construction from those of the 
Halichondroid type, they occupy the distal extremities of the large intermarginal cavi- 
