PTERIDOPHYTA 



6383 



PTOLEMAIC SYSTEM 



near him with a trailing wing as if 

 wounded. The ptarmigans seen in 

 the markets are mainly willow 

 grouse imported from Scandinavia. 

 See Bird ; Grouse. 



Pteridophyta (Gr. pteris, fern ; 

 phyton, plant). Botanical group 

 which includes the ordinary ferns, 

 the water ferns (Marsileaceae), the 

 club mosBes (Lycopodiaceae and 

 Selaginellaceae), and the horsetails 

 (Equisetaceae). Anatomically the 

 pteridophyta are more complex 

 than the mosses ; the specialisa- 

 tion of tissues for distinct functions 

 has reached a higher stage of effi- 

 ciency. In the same way as the 

 mosses they exhibit a regular alter- 

 nation of generations, but with a 

 difference. The fern-plant produces 

 spores and is the asexual genera- 

 tion ; from the spore is produced a 

 small green body, not as a rule 

 exceeding an eighth of an inch hi 

 length, known as the prothallus, 

 and this bears the male and female 

 organs. From the fertilised egg the 

 new fern-plant arises. It is there- 

 fore the fern - prothallus which 

 corresponds to the moss-plant. In 

 some pteridophytes, e.g.' in Selagi- 

 ndla, two kinds of spore are pro- 

 duced, larger (megaspores) and 

 smaller (microspores); each spore 

 on germination forms a prothallus. 

 The prothallus developed in the 

 megaspores bears female organs, 

 while the microspore furnishes the 

 male gametes. See Fern. 



Pteris (Gr., fern). Genus of ferns 

 of the natural order Polypodiaceae. 

 Natives of all regions, they have 

 usually creeping rootstocks. The 

 leaves or fronds are of very varied 

 form, from the simple undivided 

 arrow-shaped leaf of P. sagittifolia 

 (S. America) to the three or four 

 times divided one of P. aquilina, 

 the bracken. The distinguishing 

 feature is the massing of the spore- 

 capsules in a continuous line around 

 the margin of the frond or its 

 divisions. Many of the species are 

 in cultivation as ornamental 

 plants. 



Pterobranchia (Gr., wing- 

 gilled). Small group of minute 

 organisms dredged from the depths 

 of the ocean. They were formerly 

 supposed to be polyzoa, but are 

 now regarded as hemichorda (q.v. ). 



Pterodactyl (Gr. pteron, wing ; 

 daktylos, finger). Name given to 

 the flying lizard, the fossil remains 

 of which are found in rocks of 

 mesozoic age. Varying in' size from 

 only a foot in the spread of their 

 wings up to some 20 ft., these rep- 

 tiles were remarkable for their 

 bird-like habits. The wings, how- 

 ever, resembled more those of 

 the present-day bat, being a mem- 

 brane attached to the body and 

 the long, jointed fingers of the fore 

 limbs. The hind legs of the ptero- 

 dactyls bore a strong resemblance 

 to those of reptiles, but the heads 

 of many were more bird-like, 

 possessing jaws covered with a 

 horny beak. In some members the 

 jaws were provided with strong 

 conical teeth. The bones were 



Pteris. Arrow-shaped leaves oi P. 



sagittifolia. Inset, left, leal bud; 



right, srore-capsulcs 



Pterodactyl. Skeleton of P. specta- 

 bilis, a short-tailed flying reptile 



Bycuurlesyof the trutttetof the British 

 Museum 



hollow, in this respect resembling 

 birds, the skulls pointed, the eyes 

 large, and hi some of the earlier 

 forms the tail was very long and 

 slender. The majority were in- 

 capable of long-sustained flight. 

 Remains of the reptile have been 

 found in England, Europe, and N. 

 America. See Lizard. 



Pteropoda (Gr. pteron, wing; 

 pous, foot). Group of marine 

 molluscs. They were formerly 

 ranked as a separate order, but are 

 now usually included in the 

 Gastropoda (q.v). The foot has 

 been modified into a pair of wing- 

 like processes, by the aid of which 

 the animal swims. Hence they 

 are popularly called sea butterflies. 

 Some of them bear small, trans- 

 parent, glassy shells, and some 

 species form the chief food of the 



Pterobranchia. Cephalodiscus dode- 

 calophus, from the Straits of Magel- 

 lan. A. Colony. B. Highly magni- 

 fied individual specimen, indicated 

 by a in A 



baleen whales. The majority are 

 tropical. See Mollusca. 



Pterosauria (Gr. pteron, whig; 

 sauros, lizard). Name given to ex- 

 tinct flying reptiles that are known 

 also as Ornithosauria and Ptero- 

 dactyls (q.v.). 



P.T.O. Abbrev. for please 

 turn over. 



Ptolemaic System. Theory 

 expounded by Ptolemy to account 

 for the movements of heavenly 

 bodies. He supposed that the 

 moon, sun, and stars revolved in 

 circles about the earth. Beyond 

 the latter, and beyond the fire and 

 water which it supported, was the 

 ether. The zones of the heavens 

 were in and beyond the ether, each 

 zone a transparent spherical shell. 

 Each shell or sphere had its own 

 heavenly body which, revolving 

 with it, moved round the earth. 



The innermost sphere was that 

 of the moon, and the others in 

 order were those of Mercury, 

 Venus, the sun, Mars, and Jupiter, 

 with Saturn and the fixed stars 

 sharing the eighth outer sphere. 

 Afterwards a ninth sphere was 

 added to explain the slow move- 

 ment of the precession of the 

 equinoxes, and beyond that a 

 tenth sphere to account for day and 

 night. The tenth sphere or primum 

 mobile there- 

 fore revolved 

 from E. to W. 

 in twenty-four 

 hours and car- 

 ried along with 

 it all the inner 

 spheres. The 

 apparent pro- 

 gression and 

 regression of 

 planets were accounted for on the 

 supposition that each moved in a 

 circle on its own sphere, while the 

 latter moved round the earth. The 

 theory dominated all astronomi- 

 cal thought till the time of Coper- 

 nicus (q.v.). See Astronomy. 



Pteropoda, shell- 

 bearing Cavoliuia 

 tridentata 



