PLAGIOCHILA. 



[ 511 



] 



PLATINUM. 



see Secondary deposits, Tissues, Ve- 

 getable, and Cells, Vegetable. 



BiBL. Works on Structural Botany; 

 Mohl, Vegetable Cell, London, 1852. p. 10, 

 and Vermischte Schrift. Tubingen, 1845. 

 pp. 268. 272 {Linncea, xvi. p. 1. 1842), 

 transl. in Ann. Nat. Hist. ix. p. 393, Abh. 

 d. Acad, zu Munchen, i. 445, and the Bibl. 

 of Spiral structures. 



PLAGIOCHILA, Nees and Montague.— 

 A genus of Jungermanniefe (Hepaticaceae), 

 containing a number of British species, viz. 

 P. {Jungermannia, Hook.) asplenoides, 

 spinulosa, decipiens, resupinata, undulata, 

 planifolia, nemorosa, and umbrosa, some of 

 which, especially P. asplenoides (fig. 590), 



Fig. 590. 



Plagiochila asplenoides. 

 Magnified 2 diameters. 



are among the most frequent and finest 

 plants of the family, its stems growing from 

 3 to 5" long. 



Bibl. Hook. Brit. Flor. ii. pt. 1. p. Ill, 

 &c., Brit. Jung. pi. 13, 14, &c.; Ekart, %- 

 nops. Jung. p. 6 et seq. pi. 1, &c.; Endlicher, 

 Gen. Plant. Supp. 1. No. 4/3-1. 



PLANARIA, Miill.— A genus of Annu- 

 lata, of the order Turbellaria, and family 

 Planarieae. 



Char. Body soft, flattened, oblong or oval, 

 not jointed, covered with vibratile cilia; 

 neither suckers, bristles, nor leg-hke ap- 

 pendages present. 



Some parts of the structure of these ani- 

 mals have been noticed under Ann u lata 

 in speaking of the Turbellaria. The mouth 

 is situated on the under surface of the mid- 

 dle of the body, at the end of a retractile 

 proboscis ; there is no anus ; the mouth 

 leads to a capacious stomach, giving oiF den- 

 driticallv branched caeca, somewhat as in one 



joint of a Tcsnia (PI. 16. fig. 14). Their 

 motion is continuous and gliding upon wa- 

 ter plants, or the sides of glass jars. The 

 anterior part of the body exhibits a curved 

 row or a single pair of eyes, and sometimes 

 ear-like projections. They multiply by divi- 

 sion, and the formation of ova, which are 

 enclosed in a coloured capsule. 



Some of the species are very common in 

 pools, and resemble, at first sight, minute 

 leeches. P. nigra, which is black, has a row 

 of marginal anterior eyes, and two lateral and 

 one mesial projections; length about 1-2". 

 P. brunnea, dusky-brown, wdth a dark me- 

 sial line ; eyes as above ; length rather less. 

 P. lactea, cream-coloured, tinged with pale 

 reddish brown, truncate in front, with two 

 slight lateral auricles; eyes two or fovu*; 

 length 1-2 to 3-4". P. torva, grey or black; 

 obtuse in front, angles rounded, centre pro- 

 jecting ; eyes two, with a white halo ; length 

 1-2". Of the other species some are marine. 



Bibl. Johnston, Non-parasitical Worms; 

 Duges, Ann. des Sc. nat. 2 ser. xv. and xxi. ; 

 OErsted, System. Eintheil. d. Plattvmrmer; 

 Diesing, Syst. Helminth.; Dalyell, Powers of 

 Creation, ii. ; Schultz, Naturg. Turbell. 



PLANARIOLA, Duj.— A genus of Infu- 

 soria. 



Char. Body lamelliform, oblong, variously 

 sinuous and folded at the margin, convex 

 and glabrous above, concave and ciliated 

 beneath. 



This genus is placed among the unsym- 

 metrical Infusoria, and has been provision- 

 ally founded to contain animals much 

 resembling Planarice in as])ect and consist- 

 ence, but without a mouth or any other 

 external orifice, and only cihated on the 

 under surface. 



P. rubra (PI. 24. fig. 65). Red, granular, 

 narrowed behind, enlarged in front, and with 

 two ear-like folds. Aquatic, in decomposing 

 vegetable matter ; length 1-250". 



Bibl. Dujardin, Infus. p. 568. 



PLANTAIN. See Musa. 



PLANULINA. See Foraminifera 



(p. 271). 



PLATINUM.— The sodio-chloride of pla- 

 tinum crystallizes in prisms and plates which 

 polarize light ; while the potassio-chloride 

 of platinum yields several forms, which do 

 not polarize light. This reaction of the 

 soda-salt has been been proposed as a means 

 of distinguishing soda from potash, or de- 

 tecting minute quantities of the former. 



Bibl. Andrews, Chem. Gaz. 1852. x. 

 378. 



