26 DB Thomson's an^ulysis of a mineral from the tweed. 



skin is marked vritli numerous fine transverse rings, which, require the 

 aid of the microscope to be made visible ; and when placed under 

 a high magnifier is seen to be beautifully and fijiely reticulated, 

 producing in the sun's rays a play of colours. This ringed and reticu- 

 lated appearance is well seen in parts which have become dry. The 

 skin is very thin, and, when cut transversely, shews that internally the 

 body is composed of a white medullary substance, in the centre of 

 which may be seen the intestinal canal. The whole organization 

 ajipears, as far as can be made out or seen, to be very simj)le ; so 

 simple, indeed, says honest Miiller, that " even by the aid of the 

 microscope, nothing more fully can be made out of it." The same 

 author says farther, no appearance of a mouth can be made out, 

 though the very celebrated Plancus represents the mouth fimbriated. 

 Plancus' work I have not been able to see to compare his representa- 

 tion with what I have seen mj^self, and no author that I have seen 

 describes it as being visible ; but though all appearance of a mouth 

 escaped certainly for a time my utmost efforts, at length, by means of 

 the powerful microscope belonging to Sir John Hall, Bai-t. of Dunglass, 

 and after repeated examinations, I discovered at the very extremity in 

 several specimens, a round aperture, having somewhat of a fim.briated 

 margin round it. In other specimens in which the round aperture 

 was not visible, I could distinctly see the white medullary part of 

 which the internal part of the animal is composed, filling up the aper- 

 ture, and in many a puckering was distinctly seen, as if the mouth 

 were closed. Once, and once only, I distinctly, as I thought, saw the 

 puckered appearance give way while under the microscope, and the 

 round aperatiu-e open to its full extent. This opening, therefore, I 

 have no doubt is the mouth of the animal ; and as upon repeated 

 examinations no other opening could be seen in any other part of the 

 body, I conckide that this, as in some other animals, may "serve the 

 purposes both of mouth and anus, or that this latter aperture remains 

 to be discovered. 



Analysis of a Mineral from the Tweed. By Dr Thomson. (Eead 

 December 21, 1831.) 



The mineral of which the analysis is subjoined, occurs on the banks 

 of the Tweed, near St Boswell's, in connexion probably with the sand- 

 stone of that district. It is extensively used as a slate pencil in the 

 neighboui-hood where it is found, and resembles indurated claj^stone. 



Colour milk white : opaque : lustre dull : sectile : hardness 2.5 : 

 specific gravity 2.558. Before the blowpipe ^cr se becomes blue and 

 brittle. Fuses with carbonate of soda into an opaque bead ; with 

 borax and salt of phosphorus into a transparent glass. 



