SIDA. 



[ 577 ] 



SIPHONACE^. 



this substance has been ah'cady noticed 

 (p. 219). In the spines of Echinus, Cidaris, 

 &c., the calcareous network consists of 

 slender fibres 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 PI. 37. figs. 6 8z 6 a. 



Dr. Carpenter regards the calcareous net- 

 w^ork as corresponding to the fibrous struc- 

 ture 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 (p. 

 531). 



BiBL. Carpenter, Trans, of the British 

 Association, 1844 & 1847, Ann. Nat. Hist. 

 1843. xii. 376; Gray, Phil. Trans. 1833; 

 Deshayes, Todd's Cycl. of Anat., ^*c., iv. 

 556; Bowerbank, Trans. Micr. Soc. 1844. 

 i. ; Lavalle, Ann. d. Sc. nat. 3 ser. vii. ; 

 Siebold, Vergl. Anat. ; Brewster, Phil. Trans. 

 1814, and Optics, 1853 ; Woodward, On 

 Shells. 



SIDA, Baird {Daphnia, auct.). — A genus 

 of Entomostraca, of the order Cladocera, 

 and family Daphniadae. 



Char. Anterior branch of inferior antennag 

 tw^o-jointed, posterior three-jointed, and w ith 

 a row of spines at its anterior margin ; legs 

 six pairs. 



-S. crystallina (PI. 14. fig. 27). The only 

 species. Aquatic. 



Daphnella belongs here. 



BiBL. Baird, Brit. Entomostr. p. 107. 



SILK. — This valuable substance is se- 

 creted in Insects by two glandular organs, 

 described under Insects, Spinning Or- 

 gans. 



The fibres of which it is composed are 

 cylindrical or somewhat flattened, solid, 

 tolerablv highly refractive, and free from 

 structural markings of any kind. 



Chemically, silk consists of a proper silk- 

 cylinder, consisting of fibroine and forming 

 the principal part of the fibres, surrounded 

 by a coat of albumen, upon which is a layer 

 of gelatine. The fibres also contain a small 

 quantity of fat and colouring matter. 



Fibres of silk may easily be distinguished 

 from those of linen or cotton by the appli- 

 cation of Millon's or Schultze's test, both of 

 which colour the silk, but neither of them 

 the linen or cotton. The test for cellulose 

 is equally applicable to the same purpose. 



BxBL. That of Chemistry. 



SIPHONACE^.— Afamily of Confervoid 

 Algae, either marine, aquatic, or growing 

 on damp ground ; characterized by the indi- 

 vidual fronds being composed of large 

 branched cells, the contents of which, ex- 

 pelled in various forms, serve for the repro- 

 duction. The fronds mostly have a more or 

 less compound character, either from regular 

 ramification, or by a kind of stoloniferous 

 multiplication at the base of the cells ; and 

 in Hydrodictyon, which seems best placed in 

 this family, the cells are always connected 

 together by their extremities, so as to form 

 a net-like frond. In the majority of the 

 genera the cell-contents are green; mAchlya, 

 however, they are brownish or almost colom*- 

 less. The miodes of reproduction exhibit 

 considerable diversity, and are probably still 

 imperfectly known in most of the genera. 

 Codium,Bryopsis, andAchlya are reproduced 

 by the discharge of the contents of certain 

 cells in the form of numerous small ciliated 

 zoospores. Vaucheria is increased by large 

 elliptical, solitar}'^ zoospores, covered with 

 vibratile cilia; in Hydrodictyon, the cell- 

 contents are converted into a multitude of 

 ciliated zoospores, which unite to form a 

 new net or frond before leaving the parent- 

 cell; while in Botrydium the cell-contents 

 are said to be discharged in the condition of 

 motionless gonidia; but we imagine this 

 point is not quite certain. In addition to 

 the gonidial reproduction, spores have been 

 discovered in Achlya and Vaucheria, and 

 will probably be found in the rest. In Achlya 

 these occur in special lateral sporangial 

 branch-cells. In Vaucheria they also occur 

 in special branch-cells, here however accom- 

 panied by antheridiaL cells, which produce 

 spermatozoids, fertilizing the sporangial cell. 

 From the fact that orifices have been ob- 

 served in the wall of the sporange oi Achlya, 

 it is possible that an impregnation occurs 

 there also. Spores have not yet been ob- 

 served in the other genera, but it is to be 

 expected that they will be found in them 

 also. More particular details on the very 

 interesting genera of this somewhat hetero- 

 geneous family will be found under their 

 respective heads. 



Synopsis of British Genera. 



I. C ODIUM. Filaments green, branched, 

 closely interwoven into a spongiform frond, 

 producing biciliated zoospores in sporangial 

 cells borne on the sides of the erect clavate 

 branches. Marine. 



IL Bryopsis. Filaments green, free, 



2p 



