HARDWICKE'S SCIENCE-GOSSIP. 



121 



ON THE GENUS AEACHNOIDISCUS. 



Br ERED. KITTON, Norwich. 



HE following re- 

 marks on the above- 

 named genus may 

 assist the young 

 diatomist to dis- 

 criminate the few 

 species contained 

 in it. 

 It might naturally 

 be supposed that a genus 

 containing not more than 

 four species — these by no 

 meaus uncommon, and not 

 requiring any great amplifi- 

 cation,— could not offer much 

 difficulty in their identifica- 

 tion ; but such is not the 

 fact, as I frequently receive 

 specimens wrongly named. 



This beautiful genus was 

 originally detected in a perfect 

 state by Mr. Henry Deane, 

 and brought before the Micro- 

 scopical Society in 1S17. 

 Mr. Shadbolt read a paper before the same 

 society on the " Structure of the Siliceous Lorica 

 of the Genus Arachnoidiscus," afterwards pub- 

 lished in the third volume of the " Transactions of 

 the Microscopical Society," 1S19 (this was previous 

 to the amalgamation with the Quarterly Micro- 

 scopical Journal), and illustrated with some very 

 excellent figures from drawings made by Mr. Legg. 

 Single valves and fragments had previously been 

 detected in Ichaboe guano by Mr. Deane, Topping, 

 and others. This deposit seems to have been dis- 

 covered in 1843, and exhausted in 1S4±. This 

 form was therefore known to some microscopical 

 observers thirty years ago. 

 Ehrenberg, in 1848, describes* one of the species 



* In the " Monats-Berichten der Berliner Academie." 



No. 126. 



under the name Hemiptychus, but as there was a 

 Hemiptycha, a genus of Hemipterous insects, the 

 name was withdrawn. 



As Mr. Shadbolt's description of the structure, 

 &c, is very interesting, I shall give some portions 

 of it in his own words. 



" The following observations apply to the shells 

 alone, and not to the animals inhabiting them (if 

 animals they be) ; for although the specimens sub- 

 mitted to examination were recent, they were not 

 in my 'possession until they had been dead some 

 months." (The supposed animal nature of this 

 form may probably excite a smile in the present 

 generation of diatomists, but thirty years ago 

 diatoms were generally supposed to be animalcules, 

 and it was gravely stated that they possessed 

 mouths, were ciliated aud many-stomached (hence 

 called Polygastrica). That their animality was but 

 little doubted is evident from the following passage. 

 "In viewing these disks as transparent objects 

 mounted in balsam, it is by no means easy to de- 

 termine whether they have or have not a central 

 aperture ; but on viewing them as opaque objects it 

 becomes indisputably evident that a central open- 

 ing is present. When in situ these openings are 

 partially covered internally by a delicate cup-like 

 process, so as to form a species of valve. I consider 

 it highly probable that at these openings nutriment 

 is taken in, and from the peculiar radiating arrange- 

 ment of the siliceous parts immediately around them, 

 it seems not improbable that the animal may be 

 provided with organs somewhat resembling in 

 arrangement the tentacula of madrepores. This i s 

 supposing the objects to be of an animal nature, of 

 which I confess I think there is very little doubt." 

 I need scarcely remark that no central pore exists, 

 and that tentacula are never protruded. 



The author then proceeds to describe "two 

 annular valves lying between the two discoid ones. 

 These valves consist of a siliceous ring, within whicn 

 (extending a slight distance towards the centre) is 



G 



