SIAGONTIIEKIUM. 



[ 693 ] 



SILICA. 



very slender tubes (PI. 45. fig. 16), resem- 

 bling' those of tlentine. 



Shell of Ec/iiiioderinafa. — The perforated 

 structure of the homogreneous basis forming 

 this substance has been already noticed 

 (p. 274). In the spines of Echinus, Cidaris, 

 &c., the calcareous network consists of 

 slender tibres with large areolae at intervals, 

 arranged in a somewhat regular pattern, and 

 traversing a solid homogeneous substance, 

 which is thus divided into a number of ribs 

 or pillars. The transverse section of these 

 is seen in PL 45. figs. 6 & 6rt. 



Carpenter regards the c ileareous network 

 as corresponding to the fibrous structure of 

 the cutis of the higher animals, calcified. 

 This view does not, however, account for 

 the intervening substance. 



The method of procuring sections of shell 

 is noticed under Preparation. 



BiBL. Carpenter, Tr. Brit. Association, 

 1844 oc 1847 ; Ann. X. H. 1843, xii. 376 ; 

 Gray, Phil. Tr. 1833 ; Deshayes, Todd's 

 Cycl. An. ^c. iv. 556 j Bowerbank, Tr. 

 Micr. Soc. 1844, i. ; Lavalle, Ann. Sc. Nat. 

 3. vii. ; Siebold, Verql. An. ; Brewster, 

 Phil. Tr. 1814, and Optics, 1853; Wood- 

 ward, Shells ; Williamson, Qu. Mic. Jn. viii. 

 35 ; Carpenter, The Microscope, 1881, 666. 



SIAGONTHERIUM, Perty.— A genus 

 of Enchelia. Body ovate, very minute, 

 colourless, with an antero-lateral tuft of 

 reflexed ciha. S. tenue, in bog-water. 

 (Perty, Lif. 150.) 



SI'DA, Baird {Daphnia, anct.). — A genus 

 of Entomostra'ia, of the order Cladocera, 

 and family Daphuiidge. 



Char. Anterior branch of inferior an- 

 tennae two-jointed, posterior three-jointed 

 and with a row of spines at its .anterior 

 margin ; legs six pairs. 



S. crystallina (PI. 19. fig. 27). The only 

 species; freshwater. 



Daphnella belongs here. 

 BtBL. Baird, Brit. Entom. 107. 

 SIUEROLI'NA, Lamk.— Another name 

 for Caxcarina. (Carpenter, For. 199, 

 223.) 



SIDY'NUM, Sav.— A genus of Tunicate 

 MoUusca, of the family Botryllid^. 



S. turhinatmn. Amber or orange. On 

 the underside of shelving rocks. (Forbes 

 and Hanlev, Br. Moll. i. 13.) 



SIEVE-T LJBES.— These are thin-walled, 

 elongated cells, united end-to-end in rows, 

 the intervening transverse portions, or bases 

 of the cells, forming perforated sieve-like 

 plates ; the sides next other sieve-tubes are 



also sometimes perforated. The protoplasm 

 of tlie cells pass s through these perfora- 

 tions, forming minute fibres. 



Sieve-tubes are found in the liljer of 

 Dicotyledons, and in the fibro-vascular 

 bundles of Endogens. They may be most 

 easily obtained from the stems or leaf-stalka 

 of Cucurhita pepo, Calamus Rofamj, Pota- 

 mo()ef.on nutans, Bi(jnoniu speciosu, Vitis 

 vinifera, &c. Sachs recommends that their 

 longitudinal sections be treated with so- 

 lution of iodine until the cell-contents are 

 rendered brown, and the subsequent addition 

 of sulphuric acid; this dissolves the cell- 

 wall and sieve-plates, allowing the slender 

 protoplasmic filaments to be distinctly seen. 



BiBL. Sachs, Bot. 89 ; Ilenfrey-Masters, 

 Bot. 4S7; Wilhelm, Bot. Centralblatt, i. 

 908, Jn. Micr. Soc. 1881, i. 72. 



SILICA. — This inorganic substance is 

 most abundantly met with in the mineral 

 kingdom, in the form of earth, sand, or di- 

 stinct crystals. The crystals belong to the 

 rhombohedral system, usually forming six- 

 sided prisms, terminated hj six-sided pyra- 

 mids. The mineral fornix are noticed under 

 Agate, including chalcedony, and under 

 Rocks. 



In the animal kingdom, it is met with 

 especially in the Protozoa, forming the 

 spicula and elegant siliceous shells so 

 well-known as microscopic objects, and 

 which are noticed under the heads of the 

 classes, genera, &c. 



In the vegetable kingdom, silica is a con- 

 stant constituent of the epidermis of the 

 Graminaceae, the Equisetaceae, and certain 

 Algae, especially the valves of the Diato- 

 maceae. 



Chemically it is distinguished by its inso- 

 lubihty in boiling nitric acid, and its inde- 

 structibility by a red heat. 



If, as Schultze fir.-t pointed out, some 

 sulphuric acid be added to a mixture of 

 powdered fluor-spar and sand, in a wide- 

 mouthed flask, with a short tube of mois- 

 tened blotting-paper in the neck, the fluo- 

 ride of siJiciimi, which is evolved, is decom- 

 posed, and silica is deposited in Alms upon 

 the paper. If these are washed in distilled 

 water, dried, and examined under the mi- 

 croscope, they are found to consist of 

 minute crystalloid "hemispheres of silica, 

 often pointed, and grouped into plates. The 

 finer ones present considerable resemblance 

 to the markings of the finer valves of the 

 Diatomaceae, when so minute as to be only 

 visible by their lenticular foci or their dif- 



