POL 



[361 ] 



POL 



millions have been calculated as 

 being contained in one drop of 

 water, they avoid infringing on 

 one another while swimming. All 

 their movements appear to be as 

 well directed, regular, methodical, 

 and spontaneous, as those of the 

 higher classes of swimming animals. 

 These movements are effected by 

 means of very minute, hair- like, 

 tapering, transparent, vibratile fila- 

 ments disposed frequently around 

 the mouth, where they are gene- 

 rally largest and longest. There is 

 no proper skeleton in the whole 

 order polygastrica, nor any secre- 

 tion of shell on the surface, yet 

 there are parts destined to give 

 support : no nervous system has 

 hitherto been detected in the poly- 

 gastric animals. Some of the 

 poly gastric animals exude on their 

 surface a secretion which aggluti- 

 nates foreign particles floating in 

 the waters which surround them, 

 and thus form for themselves a 

 partial covering. In the majority 

 of polygastric animals there is an 

 alimentary canal, with an oral and 

 an anal orifice, which traverses the 

 body ; in the simplest forms of 

 animalcules, however, there is but 

 one general orifice to the alimentary 

 cavities, which is placed at the 

 anterior extremity of the body, and 

 is surrounded with long vibratile 

 cilia, which serve both as organs of 

 motion and tentacula. No teeth 

 for mastication, nor any glandular 

 organs to assist in digestion, have 

 been discovered in them. Not- 

 withstanding their extreme minute- 

 ness, they appear to be the most 

 numerous, the most prolific, the 

 most active, and the most voracious 

 of all living beings. Some natu- 

 ralists, like Lamarck, believed that 

 these animals were without a 

 mouth or any internal organs, and 

 were nourished by superficial ab- 

 sorption, like marine plants. 

 Lewenhoeck and Ellis, however, 



observed that they possessed an in- 

 ternal cavity, and devoured each 

 other. Spallanzani perceived them 

 swallowing each other so avari- 

 ciously that their bodies became 

 distended with their prey. Lec- 

 tures on Comp. Anatomy, passim. 



POLYHA'LLITE. A mineral found at 

 Ischel, in Austria. It occurs in 

 masses of a fibrous texture. Lustre 

 pearly. Specific gravity 2'76. 



POLYME'KIC ISOMORPHISM. The re- 

 placement of an equivalent by a 

 multiple of another, is termed poly- 

 meric isomorphism. Jukes. 



POLYMI'GNITE. (from TTO\VS, many, 

 and fijywjii, to mix, Gr.) A min- 

 eral, thus named in consequence of 

 the variety of its constituent parts. 

 It consists of titanic acid, zirconia, 

 lime, yttria, the oxides of iron, 

 cerium, and manganese, with mi- 

 nute portions of magnesia, potash, 

 silica, and oxide of tin. It is of a 

 black colour ; crystallized in small 

 prisms ; scratches glass ; specific 

 gravity 4-8. Fracture conchoidal. 

 Lustre almost metallic. 



PO'LYPE. The name given to each 

 tube, surrounded with its tentacula, 

 of the Polypus. 



POLYPA'KIA. } The fourth class of Ea- 



PO'LYPI. ) diata or Zoophytes; 

 thus named from a supposed re- 

 semblance to an Octopus, called 

 Polypus by the ancients, this re- 

 semblance arising from the arrange- 

 ment of the tentacula around the 

 mouth. 



The polypi constitute the second 

 class (the Infusoria forming the 

 first) of Lamarck, and form a large 

 family of animals, in which we 

 trace a gradual process to a more 

 complicated organization. They, 

 according to Lamarck, have a gela- 

 tinous, contractile body, a distinct 

 mouth, surrounded by tentacula, or 

 branching arms, and a simple ali- 

 mentary canal or stomach, showing 

 no vents; they increase by separa- 

 tion, or internal spontaneous pro- 

 2 A 



