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Rh (by rail) N.N.E. of Albany. Pop. (1890), 7010; (1900), 8434, of whom 1053 were foreign-born; (1910, census), 11,138. It is served by the Delaware & Hudson railway, and has steamer connexions with lake ports. Its situation in the region of lakes and mountains and its delightful climate have made it a summer resort. Among its institutions are the Samuel F. Vilas Home (for aged and infirm women); the Home for the Friendless of Northern New York (1874), for the care of homeless children; the Plattsburg State Normal and Training School, the D'Youville Academy for girls (founded in 1860, chartered in 1871), under the direction of the Grey Nuns; the College St Pierre (Roman Catholic, 1903), and the Champlain Valley Hospital. The barracks, about a mile away, is an important military post. Cliff Haven, 2 m. south, is the seat of the Catholic summer school Plattsburg has a fine harbour and is the port of entry of the Champlain customs district; in 1909 its exports were valued at $15,169,502 and its imports at $8,167,527. Among the city's manufactures are lumber, wood pulp, paper, shirts, sewing-machines and automobiles. The total value of the factory products in 1905 was $1,056,702.

Plattsburg was incorporated as a village in 1795, and derived its name from Zephaniah Platt (1740–1807), who had led a colony of settlers to this place from Long Island; it became a city in 1902 About Valcour Island (5 m. south-east of Plattsburg), on the 11th of October 1776, a British fleet under Captain Thomas Pringle and an American flotilla under Benedict Arnold engaged in the first conflict between American and British fleets, the British being victorious. On the outbreak of the War of 1812 the village became the headquarters of the American army on the northern frontier. On the 11th of September 1814, in Plattsburg (or Cumberland) Bay, Captain George Downie, commanding a British flotilla, was defeated by an American flotilla commanded by Commodore Thomas Macdonough, losing his life in the engagement (see ).  PLATTSMOUTH, a city and the county-seat of Cass county, Nebraska, U.S.A., situated in the valley and on the bluffs of the Missouri river near the mouth of the Platte. Pop. (1900), 4964 (979 foreign-born), (1910) 4287. It is served by the Chicago, Burlington & Quincy, and the Missouri Pacific railway systems. There are railway car-shops, and a considerable trade is done in grain and cattle. A trading-post licensed by the United States government was opened here in 1853, and a town platted in 1854. Plattsmouth was first incorporated as a city in 1855, being one of the oldest settlements and cities of the state.  PLATYELMIA, a phylum of the animal kingdom which comprises three classes, the, (q.v.) and Cestodes. It is the group of animals in which the act of creeping has first become habitual. In association with this movement in a definite direction the body has become vermiform and bilaterally symmetrical. One end of the body, through contact, during locomotion, with fresh tracts of medium and other forms of stimuli, has become more specialized than the rest, and here the nervous system and sense-organs are more densely aggregated than elsewhere, forming a means of controlling locomotion and of correlating the activities of the inner organs with the varying stimuli that impinge upon the body. The form and habits of the group vary widely. The Planarians are free-living animals, the Trematodes are parasitic upon and within animals, and the Cestodes are wholly endoparasitic.

Structure.—The chief features which Platyelmia possess in common are the following. The body is not metamerically segmented and is composed of a muscular tunic covered externally by a more or less modified cellular layer. Within this muscular tube lies a parenchymatous tissue which may be uniform (Cestodes) or differentiated into a central or digestive, and a peripheral portion (some Turbellaria), or finally the central portion becomes tubular and forms the digestive sac (Trematodes), while the peripheral portion is separated from it by a space lined in some forms by a flattened epithelium (most Planarians). It is characteristic of the group that the mouth should be the only means of ingress to and egress from the digestive sac and that no true perivisceral space or coelom exists in the sense in which these terms are used in higher Invertebrates. The peripheral parenchyma gives rise to protonephridia, that is to coiled tubes commencing in pyriform cells containing a flame-like bundle of cilia and provided with branched outgrowths, and communicating with the exterior by long convoluted canals which open at the surface of the body. These protonephridia are the excretory organs. The nervous system, though centralized at one end of the body, contains diffused nerve-cells in the course of its tracts, which are disposed in two or more longitudinal bundles interconnected by transverse bands. The Platyelmia are hermaphrodite and the reproductive organs are complex. The male organs consist of paired testes communicating by delicate canals with a protrusile penis. This organ is generally single but sometimes paired and occasionally multiple. It is frequently armed with spines, hooks or stylets, and is further complicated by the addition of a nutritive secretion (the prostate gland) which may open at its base or pass separately by a special duct to the exterior. There is some reason to believe that this complicated and variable apparatus is used for stabbing the body of another animal and that beginning as a weapon for catching prey it has become modified for hypodermic impregnation and only gradually adapted for insertion into the bursa copulatrix. The female organs are no less complex. They consist of solid or tubular ovaries which may be single, double or multiple. In the majority of Platyelmia the primitive ovary becomes divided into fertile and sterile portions, i.e. into distinct ovarian and vitellarian regions. The yolk prepared by the latter is conducted by one or more specialized ducts to the oviduct and the point of union is distinguished by the opening of a “shell-gland” which secretes a membrane around the conjoined mass of ovum and yolk. From this junction there proceeds an oviduct or “uterus” (paired or single) which before opening to the exterior expands to form a muscular protrusile pouch—the bursa copulatrix. Frequently also from this junction of the ovaria and the vitellaria a median tube is given off which either opens to the exterior or into the intestine, in the latter case it appears to serve as means of conveying superfluous yolk to the gut, where it may serve as food.

Inter-relationships.—The inter-relationships of the three members of the Platyelmia are of a more doubtful nature than is the unity of the phylum. The Turbellaria undoubtedly form the most primitive division, as is shown by their free-living habits, ciliation and sense-organs. The Trematodes are somewhat modified in accordance with their ecto- or endoparasitic life, but they exhibit such a close similarity of structure with the Turbellaria that their origin from Planarians can hardly be doubted, and indeed the Temnocelphaloidea (see ) form an almost ideal annectant group linking the ectoparasitic Trematodes and Rhabdocoel Planarians. The Cestodes, however, are connected by no such intermediate forms with the other Platyelmia. Their adaptations to parasitic life in vertebrate animals appear to have involved such modifications of structure and development that their affinities are quite problematical. The entire absence of any trace of a distinct alimentary tract, the loss of true regenerative power, the peculiar gametic segmentation of the body into hundreds of “proglottides” budded off from