BRITISH SPONGIAD^E. 137 



tissue becomes thicker, and the tension spicula numerous 

 and more approaching regularity of disposition, but there 

 is no appearance of the columnar skeleton and its peculiar 

 spicula ; so that a young and immature specimen might 

 readily be mistaken for a species of a different genus ; but 

 in the mature sponge this mistake cannot readily occur. 

 Although the keratose columns of the skeleton are but very 

 slightly produced, their fasciculi of large and characteristic 

 spicula render such an error very improbable. The skeleton 

 columns, generally speaking, are but very indistinctly pro- 

 duced ; the keratose portion frequently forming on the basal 

 membrane a slight elevation only, from amidst which one 

 or more of the large skeleton spicida sprung ; or it is repre- 

 sented by the cementing keratode that binds them together 

 for a short distance in their progress upward ; but the 

 peculiarities of their size, form, and mode of arrangement 

 will always indicate, unmistakeably, the genus to which this 

 species belongs. 



The dermal membrane is profusely furnished with its 

 peculiar spicula, which are more or less fasciculated, and 

 these bundles extend more especially from group to group 

 of the skeleton spicula, which they appear to serve, to 

 maintain, and support in their respective positions. One 

 third or half of the distal extremities of the skeleton spicula 

 are frequently projected through the dermal membrane, 

 thus performing the office of external defensive spicula as 

 well as of skeleton ones. 



The internal defensive spicula nearly all spring from the 

 basal membrane of the sponge. They are generally grouped 

 around the base of the skeleton columns, but they also 

 appear independent and isolated, and occasionally they are 

 based about midway on the side of the skeleton columns. 

 They rarely exceed a fourth of the length of a fully developed 

 skeleton spiculum, and are always entirely spined. 



The interstitial membranes are abundantly supplied with 

 tension spicula, especially so the basal membrane of the 

 sponge, on which they are so thickly dispersed as to simu- 

 late the appearance of a secondary skeleton ; they are rather 

 shorter than the internal defensive ones, or about a fifth of 



