170 P L A P L A be chanted. And so intimate is the adaptation of these plain-chant melodies to the rhythm as well as to the sense of the sacred text, even after its translation into more modern languages, so strongly do they swing with the one and emphasize the other, that it is difficult to believe that the composition of the music was not coeval with that of the poetry. Xext in antiquity to the psalm-tones are the melodies adapted to the antiphons, the offertoria, the graduals, and the introits, sung at high mass. Those proper to the Ordinarium Mizsse are probably of later date. Those belonging to hymns and sequences are of all ages. Among the latest we possess perhaps the very latest of any great importance is that of Lauda Sion, a very fine one, in modes 7 and 8, adapted to the celebrated sequence written by Thomas Aquinas about 1261. To the melodies adapted to the Lamentationes and the E.rtdtet, as sung in the Church of Rome during Holy Week, it is absolutely impossible to assign any date at all. All we know is that they are of extreme antiquity, and beautiful beyond all description. The melody of Exultet is, indeed, very frequently cited as the finest example of plain song in existence. To assert that melodies so old as these have been handed down to us in their original purity would be absurd. But the presence of corruption rarely passes undetected by the initiated ; and vigorous efforts have been made from time to time to purify the received text by reference to the oldest and most trustworthy MSS. attainable. Such an effort was made on a very extensive scale by the " Congregation of Rites," at the instigation of Pope Pius IX., in the year 1868; and the labours of that learned body, still in active progress, are doing all that can now be done towards the restoration of plain chant to the highest state of purity possible in the present stage of its existence. (w. s. E.) PLAXARIAXS. The name Planaria was first applied byO. F. Miiller in his Prodromus Zooloyiee Dcvnicae. (1776) to a group of worms, inhabitants of fresh and salt water, characterized, so far as was then known, by a flattened leaf -like form. Ehrenberg in 1831 changed this name to TtirMlaria on account of the cilia with which the body is furnished, by means of which the worms create a whirl pool in the surrounding water. The extent of this group was subsequently more restricted, and at present the name Turbdlmria is applied to all those (mainly free-swimming) Platyhelminths whose body is clothed externally with a ciliated epidermis (fig. 9), and which possess a mouth and (with the exception of one division) an alimentary canal, but are without an anus. The Turbellarians, exclud ing the NEMERTINES (q.v. which until recently were classed with them, form an order of the class Platyhelminthes, and the old name Planaria is now confined to a group of the fresh-water representatives of this order. Size and External CJuiracters. Many forms of the Turbellarians are so minute as to be hardly visible with the naked eye, while others attain to a length of several inches, and a land Planarian of no less than 9 inches in length has been described by Moseley. The freshwater forms are generally small, the largest representatives of the order being marine or terrestrial. The smaller species are mostly cylindrical, or convex dorsally and flat ventrally ; the anterior extremity is commonly trun cated and the posterior extremity pointed (fig. 1, , b). The larger aquatic forms are thinner in proportion to the increasing surface of the body, so that they come to resemble thin leaf-like lamelUe ((/), while the large land Planarians instead of increasing in superficies grow in length (> and/ ), so that they may be best compared to leeches. The larger aquatic forms are frequently provided with tentacles in the shape of paired finger-like processes or ear-like folds of the anterior part of the body (d and </) ; sometimes the tentacles arc papillary outgrowths of the dorsal surface ; the land Planarians are often to be distinguished by a crescent-shaped area at the fore end of the body, which is separated oil from the rest (/). In many cases the whole dor sal surface is beset with papilke (</). The aper ture of the mouth varies greatly in its position ; sometimes it is situated at the anterior extremity, sometimes in the middle of the ventral surface of the body, occasionally quite close to the posterior ex tremity; the single com mon or distinct male and female generative aper tures are also situated upon the ventral surface of the body, and the former , J . FIG. 1. a, Convoluta paradoxa, Oe.; b, Vor- m rare cases open in com- tex tii-uHs, M. sch. ; <, Mnnotus /.*, mnn with tlip month tlio Gff - ; d > Tl >y san oz on brochii, Gr., ith elevated anterior extremity (after Joli. fenital apertures always Schmidt); e, Ithynrhodenius terrestris, , 0. v i i i it, 4.1 1- - Miiller (after Kennel); f, Itipalium lie behind the niOUth. cereSt Mo?, (after Moseley); (i, 1 ohjcelis Many Turbellarians have >", o. sci,., attached by the pharynx J (#A) to a dead worm (after Johnson). All a Sucker Which Serves to the figures of natural size, and viewed attach the animal to sur- fr0111 tlie dors " ! BUrfacc - rounding objects, or to another individual during copulation. Tnteflitment. The integument is composed of a single layer of ciliated epithelium ; between the cilia there are often long flagella and stiff tactile hairs and even (in a single instance) chitinous spines ; these latter must be regarded as local thickenings of the firm cuticle which covers the epidermic cells. The epidermic cells are flat or columnar, and are united to each other by smooth opposed margins or by denticulate processes which fit into similar processes in the adjacent cells (fig. 2). Sometimes the epidermic cells are separated by an interstitial nucleated tissue. The structure and functions of the cells of the epidermis differ, and four varieties are to be found : (<i) indifferent ciliated cells; (/>) cells containing certain definite structures (rhabdites, nematocysts) ; (<) gland cells: and (d) glutinous cells (Klebzellen). The rhabdites are refracting homogeneous rod-like bodies, of a firm consistency, which are met with in most Turbellaria, and often fill all the cells of the epidermis ; they are not always found entirely within the cells, but the extremity often projects freely on to the exterior of the body. They are readily extruded from the cells by pressure, and are often found in great abundance in the mucus secreted by the glandular cells (many Turbellarians, like snails, deposit threads of mucus along their track) ; in this case the epidermic cells become perforated like a sieve. In many Turbellarians the rhabdites are chiefly massed in the anterior part of the body ; fre quently there are several varieties of rhabdites in one and the same species, some being pointed at both ends, others cylindrical with truncated extremities. These structures are either formed directly in the ordinary epidermis cells as a kind of secreted product of the cell, or in special formative cells which lie beneath the integument and are connected with the epidermis cells by protoplasmic fila ments, by means of which the rhabdites reach the surface
of the body. These cells must be regarded as epidermic