22 ANATOMY AND PHYSIOLOGY 



destroy it ; and thus it is that the external defensive spicula 

 are frequently of more than the usual length and strength 

 of these organs. They are projected for about half or two- 

 thirds of their length, at various angles from the surface of 

 the sponge, apparently with the object of meeting the 

 attacks of the larger class of depredators ; but as between 

 the large spicula the smaller tribes of annulate animals 

 would readily insinuate themselves, there is frequently a 

 secondary series of defences, consisting of innumerable 

 short, finely-pointed spicula, the apices of which are pro- 

 jected a short distance only beyond the surface of the 

 dermal membrane, thus rendering the progress of the 

 smaller and more insinuating enemies extremely difficult, 

 if not impossible. In young sponges, as in Spougilla 

 flitviatilis and others, the office of external defensive spicula 

 is frequently performed by the continued extension of the 

 radial lines of the skeleton, the terminal spicula of which 

 often project to more than the extreme length of a spiculum 

 beyond the surface of the dermal membrane. 



The arrangement of the spicula, in regard to their 

 especial office in the sponge, can only be approximately 

 correct as we frequently find them applied to what appears 

 to be abnormal offices ; thus the stellate forms, which are 

 especially applied to the protection of the sarcodous surfaces 

 of the interstitial membranes of the sponge, are occasionally 

 appropriated as external defences for the preservation of 

 the dermal membrane as in Tethea muricata, Bowerbank, 

 MS. (Fig. 35, Plate I). And the connecting spicula so 

 abundant within the crustular dermis of Geodia and Pliachy- 

 matisma are frequently, with various modifications of form, 

 applied as externally defensive and as tension spicula in the 

 dermal membrane, as in Dactyhcalyx Prattii, Bowerbank, 

 MS., as connecting and tension spicula and in the allied 

 form, with the addition of an external spicula ray and the 

 additional office of external defence, as represented by 

 Tig. 55, Plate II, from Geodia Barretti, Bowerbank, 

 MS. Similar spicula are found abundantly on the sur- 

 face of Dadylocalyx Bowerbankii, Johnson, in the British 

 Museum. The offensive is so frequently combined with the 

 defence office in the structure of some of these spicula, 



