OF THE SPONGIAD^. 27 



HymerapMa, where the sponge is less in thickness than 

 the length of one skeleton spiculum, and where they pass 

 from the basal membrane of the sponge through the 

 dermal membrane, their apices acting as external defensive 

 organs, while their shafts form the essential skeleton of 

 the animal, there is an especial provision for their pre- 

 servation from injury. Their bases are expanded in the 

 form of large bulbs, so as not only to afford a greater 

 surface for attachment, but to allow them at the same time 

 to act on the principle of a ball-and-socket joint, giving 

 them a more than usual amount of attachment, and a 

 power of yielding in every direction to pressure on their 

 apices from without (Fig. 370, Plate XXXIV). The 

 defence of the surface of the Halichondroid sponges is 

 less apparent, but equally efficacious ; the abundantly 

 spiculous reticulations immediately beneath and supporting 

 the dermal membrane, would render attacks of annelids 

 or other small predaceous creatures exceedingly un- 

 palatable. 



In the calcareous sponges the spicular defences are 

 exceedingly interesting. In Grantia compressa, the distal 

 ends of the great interstitial cells are amply protected 

 by numerous flecto-attenuato-acuate spicula grouped 

 around their porous terminations, with their club-shaped 

 ends curving in every direction over them, but in no 

 degree interfering with the freedom of their inhalant action. 

 In Grantia ciliata they are grouped in circles around the 

 distal ends of the interstitial cells (Fig. 345, Plate XXVI), 

 but in this species they are acutely pointed ; and when the 

 inhalant system is in a state of repose, they are concen- 

 trated at their extreme points so as to form an elongate 

 cone, effectually enclosing and protecting the porous ends 

 of the cells within them ; but when the inhalant action is 

 in full activity, their apices recede from each other until 

 they assume the form of a cylinder, and then freely admit 

 the incxirrent streams of water, but effectually repel the 

 advances of any dangerous assailant that may attempt an 

 entrance. The distal termination of the cloaca in this 

 species is also abundantly protected by a marginal fringe 

 of long and very acute spicula, and is furnished with the 



