with descriptions of new Species. 331 



below, which is almost entirely made up of tubes. The form of 

 these bodies, Pis. XII. & XIII. fig. 10 a. & fig. 2 b, is now ob- 

 served to be somewhat like that of the mulberry, and on closer 

 examination they are found to be composed of a stout central 

 axis, near each extremity of which is placed a whorl of six or 

 eight large, irregularly quadrate nodules ; the extremities of the 

 axis being each formed of a nodule similar to those of the whorls. 

 Tliese bodies measure yf jth of an inch long, are colourless, refract 

 light powerfully, and are as brilliant as the spicula, and in like 

 manner are unaffected by strong nitric acid, how long soever sub- 

 jected to its action. 



It is to the above-described peculiar siliceous bodies on the 

 surface of the excavating sponges that I attribute the power they 

 possess of burying themselves in calcareous substances. The 

 spicula may perhaps assist in Cliona ; but they seem ill-adapted 

 for the purpose in Thoosa, and indeed are not always present. In 

 the former they undoubtedly penetrate the surface, and originally 

 I was inclined to look upon them as the chief agents employed. 

 The discovery of the mulberry-like bodies on the surface of Thoosa 

 led me to examine more closely that of Cliontty and after finding 

 there those beautiful gem-like crystals, so well adapted for cut- 

 ting, their homology cannot be doubted ; and I am compelled to 

 adopt the view just expressed. 



The surface then of these animals will very much resemble 

 what I have elsewhere described the cutting surface to be in the 

 boring moUusks ; in the former as in the latter every portion of 

 it will cut with the keenness of glass-paper ; and as Cliona is ad- 

 mitted on all hands to be highly contractile, there can be no dif- 

 ficulty respecting the capabilities of the excavating apparatus as 

 just described. All that is necessary is, that each siliceous gra- 

 nule, or cluster of granules, should be put in motion. Action, — 

 very limited, — not more extensive than that of vibratile cilia, 

 would be sufficient ; and it would seem not at all improbable 

 that it may be of the same nature. From Ehrenberg^s inves- 

 tigations it would appear that the motion of these minute organs 

 is produced by a contractile tissue on which they are based, and 

 that in some of the animalcules they have a rotatory motion. 

 Now if we suppose these siliceous bodies of Cliona and Thoosa to 

 be in connexion with a similar contractile tissue, the whole sur- 

 face of the sponge would be composed of thousands of minute 

 drills quite able to cut into calcareous substances of the hardest 

 nature. 



Were the action of this character, the walls of the chambers 

 would be drilled full of little holes, and would present just the 

 appearance we have already seen they possess. And as the calca- 

 reous particles were removed they would be carried away by the 



