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 EM. 

 of SPIRAL STRUCTURES. 



PLAGIOCHILA, Nees and Montagne. 

 A genus of Jungermannieae (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.; Ekary Sy- 

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

 Gen. Plant. Supp. 1. No. 473-1. 



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-like ap- 

 pendages present. 



Some parts of the structure of these ani- 

 mals have been noticed under ANNULATA 

 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 off den- 

 dritically branched caeca, somewhat as in one 



joint of a Tania (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, with 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 four ; 

 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. ; 

 (Ersted, System. Eintheil. d. Plattwiirmer; 

 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 Planariee in aspect and consist- 

 ence, but without a mouth or any other 

 external orifice, and only ciliated on the 

 under surface. 



P. rubra (PL 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. 



