66 BEITISH SPONGIAD^. 



fasciculated skeleton of a true Eymedesmia ; while tlie 

 dispersed, large, basally spined, attenuated acuate ones, 

 from their prostrate mode of disposition, form a strong 

 auxiliary skeleton destined to compensate the slender 

 and fragile structures of the true skeleton fasciculi, 

 and as well to aid and assist in the preservation of the 

 sponge, as defensive organs, in addition to the true 

 internal defensive spicules of the sponge. 



"From the extreme tenuity of the true skeleton 

 spicula they might readily, and especially the dispersed 

 ones, be mistaken for tension spicula, and especially so 

 when compared with the numerous and comparatively 

 gigantic auxiliary basally spined, attenuato-acuate ones, 

 which are distributed rather numerously amongst 

 them. 



" The disparity existing between the two portions of 

 the skeleton structtu-es is very remarkable. One of 

 the normal skeleton spicula of the fasciculi was yfe 

 inch in length, and its greatest diameter measured only 

 ■ ^5000 of ^^ iJich ; while the length of one of the large 

 attenuato-acuate, basally spined spicula was -^ inch, 

 and at its greatest diameter near the base it was sYoJ 

 inch. 



" The attenuate, entirely spined, acuate, internal 

 defensive spicula are all projected from the basal 

 membrane at about right angles to its plane, and their 

 apices frequently pass through the dermal membrane, 

 thus performing the double ofl&ce of internal, and to a 

 certain extent external defences. We may, therefore, 

 estimate the thickness of the sponge as about equivalent 

 to the average length of these organs, one of the largest 

 of which measured g-g-r inch in length. 



" The retentive spicula are also very characteristic of 



