PLANORBITLINA. 



[ 605 ] 



PLATYZO^IA. 



decomposing vegetable matter ; length 

 1-250". 



BiBL. Du]. Inf. 568. 



PLANOKBULl'NA, D'Oib.— A genus 

 of Hyaline Foramiuifera. Shell spiral, 

 coarsely porous, subnautiloid or outspread, 

 often parasitic ; having from 15 to 200 

 chambers, with single septa and slight 

 rudiments of the canal-system. Aperture 

 sometimes produced and lipped. Compla- 

 nate (PI. mechterranensis, PI. 24. tig. 10) ; 

 plano-convex, Truncatulina (T. lobatula, 

 PI. 24. fig. 9) ; rotaliiform {PL Haidingerii, 

 PI. 24. tig. 6 ; PI. veneta, fig. 12) ; or sub- 

 nautiloid (AnomaUna and Planulina). 

 Smooth; limbate (Planulina); or granulate. 

 In all seas ; fossil in the Carboniferous, 

 Lias, and later formations. 



BiBL. Carpenter, For. 206 ; Parker and 

 Jones, Phil. Tr. civ. 379, 



PLANTA'GO. — The common plaiutaiu ; 

 its leaf and hairs furnish excellent exam- 

 ples of Cyclosis. See Carpenter, Microscope, 

 p. 431. 



PLANCJLA'RIA, Defi-ance.— A notice- 

 able gi'oup of delicate, elongate, flattened 

 Cristellarice, connecting the nautiloid with 

 the margiuuUne varieties, come under this 

 name. Recent and fossil. 



BiBL. Parker and Jones, Ann. N. H. 3. 

 V. 114 (crepidtda^ ; xii. 215 ; 4. viii. 166. 



PLANULI'NA, D'Orb.— a subgenus of 

 Planorhulina ; flat, discoidal, subsymmetri- 

 cal, and with raised margins and septal 

 lines (Umbate). Recent and fossil. 

 BiBL. Carpenter, Introd. For. 207. 

 PLASMODIUM.— A sponge-like net- 

 work, formed by the union of the pseudo- 

 podia of amoeboid bodies. 



PLAS'TIDS. — The simplest living forms, 

 and the most elementary parts of tissues, 

 consist of (1) small particles and masses of 

 protoplasm without a nucleus or any trace 

 of structure (our protoplasts, 1856), (2) of 

 similar portions of protoplasm in Avhich a 

 nucleus has been dift'erentia,ted. The first 

 group Hackel terms Cytodes, the second 

 cells ; and both are grouped under the head 

 of Plastids. 



The cytodes, or the protoplasmic masses 

 -s\dthout a nucleus, are : — 



GymnocytodtB, or naked cytodes. Such 

 are the freely moving Monera, the non- 

 nucleated Plasmodia of Myxomycetes, and 

 of several other Protista, the amoeboid 

 erms of the Greyariiice proceeding from 

 he pseudo-naviculse, &c. 

 Lepocytodce, or covered cytodes. These 



are plasma-masses without a nucleus, en- 

 closed in an entire or incomplete uiembrane 

 or shell. For example, the encapsuled 

 resting condition of vi\a.nyLepomonera,mimy 

 Siphoncca, and numerous other lower plants, 

 and the so-called non-nucleated cells of 

 many higher plants and animal tissues. 



The cells or cyta are plasma-masses with 

 a nucleus, and are divided into : — 



(xymnocyta, naked cells. Such are the 

 naked plasma-masses with a nucleus, but 

 without a membrane or shell ; for example, 

 the true Amoeba, the naked zoospores of 

 Alga;, the eggs of Siphonophora, and other 

 animals, the colomiess blood-cells, many 

 nerve-ceUs, &c. 



Lepocyta, or covered cells. Such are the 

 cells of animals, plants, and tissues with 

 nuclei and cell-walls. See Pkotista, 

 Cytode. 



BiBL. Hackel, Biol. Studien, H. i. ; Qu. 

 Mic. Jn. 1869, 331 ; Hallier, Plastids, 1878. 



PLATINUM.— The sodio-chloride of 

 platinum 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 proposed as a 

 means of distinguishing soda from potash, 

 or detecting minute quantities of the 

 former. 



BiBL. Andrews, Chem. Gaz. 1852, x. 378, 



PLATYCE'RIUM, Desv.— A geims of 

 Acrosticheee (Polypodiaceous Ferns), o 

 species ; tropical. (Hooker, Syn. 425.) 



PLATYCHE'LIPUS, Br.— A genus of 

 Copepodous Entomostraca. P. littoralis ; 

 brackish water. (Bradv, Copep., Ray Soc, 

 ii. 102.) 



PLAT YC 'OLA, Kt.— Like Vaginicola, 

 but the sheath decumbent, and attached by 

 one side. Freshwater ; several species. 

 (Kent, Inf.7?A.) 



PLATYG'RAPHA, XyL— A genus of 

 Graphidei (Lichenaceous Lichens). P.ri- 

 mata, on ash trees, rare. (Leighton, Lich. 

 Flor. 388.) 



PLATYS'MA, Hoffin. — A genus of 

 Ramalodei (Lichenaceous Lichens). 10 spe- 

 cies, on rocky mountains &c. (Leighton, 

 Lich. Fl. 93.) 



PLATYTHE'CA, St.— A genus of Fla- 

 gellate Infusoria. -Ovate, with a decumbent 

 rugose carapace ; flagellum single ; length 

 1-1200" ; on Lemna. (Kent, Inf. 262.) 



PLATYZO'MA, Br.— A genus of Glei- 

 cheniaceous Ferns. P. tnicrophyllum, Aus- 

 tralia, (Hook. Syn. 11.) 



