MELOSIRA. 



[ 422 ] 



MEMBRANES. 



MELOSIRA, Ag. (GalUonella, Ehr.).— 

 A genus of Diatomacese. 



Char. Frustules cylindrical, discoidal or 

 subspherica], united into jointed filaments. 



Hoopsoften very broad, to adapt themselves 

 to the breadth of the new frustules. In some 

 species a narrow projecting ridge or keel 

 encircles the valves near their ends. Valves 

 covered with depressions which are mostly 

 very minute and invisible under ordinary 

 illumination ; in the side view these some- 

 times have a radiate arrangement. In some 

 species the margins of the ends (side view) 

 of the frustules have coarse and distinct 

 radiating striae, their nature undetermined. 



This genus has been subdivided j by 

 Ehrenberg and Kiitzing into Lysigonium, in 

 which the keel is present ; and GalUonella 

 (proper), in which this is absent : again by 

 Thwaites into Aulacosira, in which the frus- 

 tules are cylindrical, surrounded transversely 

 by two furrows, with rounded (convex) ends, 

 but no line for division; Orthosira, in which 

 the frustules are exactly cylindrical (with 

 flat ends), exhibit the transverse line of divi- 

 sion, and have spherical or subspherical 

 internal cavities ; and Melosira (proper), 

 in which the frustules are convex at the ends, 

 and have the central line for division ; inclu- 

 ding also the varieties in the reproduction 

 (DlATOMACE^, p. 201). 



British species. 

 * Marine. 



M. nummuloides, Kg. (PI. 13. fig. 5 a; b, 

 a frustule more magnified). Prepared frus- 

 tules colourless, a distinct keel present; 

 valves not striated by ordinary illumination ; 

 breadth 1-1500 to 1-1200". 



This common species forms long, slightly 

 curved chains, and on account of the great 

 breadth of the frustules, shows well the 

 various stages of subdivision. The filaments 

 are sometimes stipitate. 



M. moniliformis, Ag. {M. Borreri, Grev.). 

 Prepared frustules dark brown, ends rounded, 

 entire surface punctate (ordin. ilium.), no 

 striae nor keel present; breadth 1-850 to 

 1-500". 



M. Dickiei {Orthosira Dichiei, Thw.) 

 (PI. 13. fig. 15 a, front view, b, side view). 

 Filaments short, frustules nearly colourless, 

 ends flat, no striae nor keel (ord. ilium.), 

 valves thickened, so as to render the cavity 

 of the frustules rounded ; breadth 1-1500 to 

 1-1200". 



The remarkable formation of sporangia in 

 this species (PI. 6. fig. 9) is noticed under 



DiATOMACEiE, p. 201. 



** Aquatic. 



M.varians {P\. 13. fig. 6, front view ; a, side 

 view, markings omitted). Frustules colour- 

 less, ends slightly convex and striated at the 

 margin (ord. ilium.), keel absent; breadth 

 1-1500 to 1-1200". The end view of the 

 frustules resembles that of CycloteUa. 



Formation of sporangia shown in PI. 6. 

 fig. 8 a, b, sporangial frustule. 



M. arenaria. Ends of frustules flat and 

 striated at the margin (ord. ilium.), the striae 

 appearing also in the front view ; keel 

 absent ; frustules broader than long ; breadth 

 1-660 to 1-260". 



M. crenulata. Kg. {Aulacosira crenulata, 

 Thw. ; M. orichalcea, Ralfs) (PI. 6. fig. 7 a 

 forming sporangia ; b, c, sporangial frustules). 

 Diff^ers from the last in its less diameter, and 

 the frustules being two or three times as 

 long as broad ; breadth 1-1400". 



See PoDosiRA. 



BiBL. Kiitzing, Bacill. p. 52; and Sp. Alg. 

 p. 27; Ralfs, Ann. Nat. Hist. 1843. xii. 

 p. 346; Thwaites, ibid. 1848. i. p. 168. 



MEMBRANES, Undulating.— These 

 are said to be simple membranous bands, one 

 margin only of which is attached, the other 

 being free, and exhibiting an undulatory mo- 

 tion. They are allied to and answer the same 

 purpose as cilia. They are described as occur- 

 ring upon the spermatozoa of salamanders 

 and tritons; as forming longitudinal pro- 

 cesses in the water-vessels of some Annelida, 

 as the Turbellaria; also as existing in 

 some Infusoria, as Trichodina, and some 

 Rotatoria. 



Some authors have regarded them as con- 

 sisting of rows of cilia or a spiral fibre, and 

 not membranes. They are most easily exa- 

 mined in the spermatozoa of the triton, in 

 which we believe the appearance of an un- 

 dulating membrane arises from the existence 

 of a fibre coiled around the spermatozoa 

 (PL 41. fig. 17), and undulating throughout 

 its length. This opinion is based upon the 

 circumstance, that if the coiled fibre be de- 

 tached from the proper filament of a s])er- 

 matozoon or spermatozoid, no margins of 

 the (lacerated) membrane can be detected, 

 other than that visible at first, and which 

 really represents the coiled fibre. 



This is, however, an interesting subject 

 for further investigation. Siebold, who has 

 paid most attention to it, remarks that Try- 

 panosoma, Gruby, a supposed entozoon 

 found in the blood of frogs and fishes, is not 

 an independent animal, but simply an undu- 

 lating membrane swimming freely. 



