PLANARIANS , a well-defined See also:

• GROUP

Occurrence.—Most Turbellaria are aquatic. They abound on the seashore and in fresh water, amongst weeds or under cover of stones, shells and See also:

• SAND
• SAND, GEORGE (1804-1876)

The development of these eggs is rapid in warm water, slow in See also:

• COLD (in O. Eng. cald and ceald, a word coming ultimately from a root cognate with the Lat. gelu, gelidus, and common in the Teutonic languages, which usually have two distinct forms for the substantive and the adjective, cf. Ger. Kolte, kalt, Dutch koude

Only one See also:

• BRANCH (from the Fr. branche, late Lat. branca, an animal's paw)

The folded margins of other forms clasp the weeds on which they live. See also:

• ADHESION (from Lat. adhaerere, to adhere)

They function both as the nutritive and excretory organs of the Planarian, and the See also:

• YOUNG
• YOUNG, A
• YOUNG, ARTHUR (1741-1820)
• YOUNG, BRIGHAM (1801-1877)
• YOUNG, CHARLES MAYNE (1777–1856)
• YOUNG, EDWARD (1683–1765)
• YOUNG, JAMES (1811-1883)
• YOUNG, THOMAS (1773-1829)

Br... C A B (From See also:

• CAMBRIDGE
• CAMBRIDGE, EARLS AND DUKES OF
• CAMBRIDGE, RICHARD OWEN (1717-1802)

Between this aproctous gut and the integument the body consists of a jelly-like, vacuolated mesenchyme made up of branched gland-cells, excretory cells, pigment- and muscle-cells. A space may be secondarily hollowed out around part of the gut; but no coelomic or true perivisceral cavity exists in the sense in which these terms are used in higher animals. H nervous system is present and consists of an anterior "brain" and of ramifying ganglionic trunks that are developed in relation to the muscular integument and to the sense-organs for the See also:

• PERCEPTION (from Lat. percipere, to perceive)

f, Bipalium See also:

• CERES

Integument. The epidermis is ciliated and highly glandular. It consists of a single layer of cubical or oblong cells with the structure seen in fig. 3. The glandular secretion takes various forms, such as mucus, mucinoid granular blocks, or fusiform refringent homogeneous rods. These rods or "rhabdites " are G Rb R1ri CI I, I 1 1 11 `11~ ll l ~I~ i The epidermis (E) consists of cells divided into an See also:

• OUTER

Below the epidermis is a See also:

• FIRM

These are based on the type of a buccal invagination, which forms the pharyngeal sheath, and from the bottom of this there springs a muscular outwardly directed tube or fold. In the Alloeocoela and Tricladida the OT --pharynx is an elongate protrusible See also:

• CYLINDER (Gr. KvAwSpos, from KvXivaety, to roll)

4, Rmc). Such cells in embryonic life give rise to a process which perforates the soft basement-membrane and penetrates between the epidermal cells. The process becomes hollow, and the rhammites pass outwards along it on to the surface of ,the animal, forming in many Turbellarians thickly set rows of rods on the head. (2) Similar cells contain nematocysts in a few Planarians (Microstoma, See also:

• STENO, NICOLAUS (1631-1686)

Re, See also:

• ROD (O.E. rodd, probably related to Norw. rudda, stick, rodda, stake)
• ROD, EDOUARD (1857-1910)

In these characters the nervous system of Planarians shows an interesting transition from the scattered plexus of Coelenterates to the segmental ganglia and sympathetic nervous system of Annelids. Sense-organs.—These occur in the form of tactile organs, otocysts and eyes. The whole skin of many Planarians is sensitive, and amongst the See also:

• ORDINARY (med. Lat. ordinarius, Fr. ordinaire)

Two types are distinguishable—eyes with a See also:

• CUP (in O.E. cuppe; generally taken to be from Late Lat. cuppa, a variant of Lat. cupa, a cask, cf. Gr. KuaeXXov)

lies in the centre of the g, Germarium. body. i, Intestine. ot, Otolith. in, Longitudinal nerve See also:

• TRUNK (Fr. tronc, Lat. truncus, cut off, maimed)

increase during the summer by fission and during the winter by eggs. The body of the Microstomidae becomes constricted and partially subdivided into two, the posterior half regenerates a brain and pharynx. Subsequently each becomes again converted into two zooids, and the process is repeated until a See also:

• CHAIN (through the O. Fr. citable, chcene, &c., from Lat. catena)

These ducts are provided with accessory glands along their course and terminate in penial Anterior, and i2, i3, paired posterior branches of intestine. od, Oviduct. te, Tentacle. vd, Vas deferens. a', Male, and , female copulatory organ. Other letters as in fig.12. or vaginal structures, often of great complexity, which are surrounded by an " See also:

• ATRIUM (either from ater, black, referring to the blackening of the walls from the smoke of the hearth, or from the Greek aiOpeov, open to the sky, or from an Etruscan town, Atria, where the style of building is supposed to have originated)

When mature the eggs are transferred to the oviduct. At the point where the two oviducts join in order to open to the exterior they receive a conical sac (spermotheca) which contains spermatozoa. At this point the eggs are fertilized, and deposited in a mucilaginous mass which is attached to algae or buried in the sand. It is characteristic of the Acoela that the testes and ovaries should not be continuous with either the sperm-duct or the oviduct respectively. In one genus of the Acoelous Turbellaria—Polychaerus--this See also:

• PRIMITIVE

The testes are either follicular (Alloeocoela) or compact cn, Brain. i, Intestinal branches. Anterior unpaired intestinal branch. in, Longitudinal nerve See also:

• CORD (derived through the Fr. corde, from the Lat. chorda, Gr. xop(5rt, the string of a musical instrument)

The opening of the atrium into the oral cavity in Cylindrostominae and of the male organ into the mouth of Prorhynchus is possibly explained by this tact. From the Alloeocoela we pass readily to the Triclads. In both of these groups the reproductive organs are based on the same See also:

• PLAN
• PLAN (from Lat. planes, flat)

the uterus—which may either be a See also:

• DILATATION (from Lat. dis-, distributive, and lotus, wide)

In Cryptocelides two, four or six may be present, but in this genus they all lie in a common sac. In Polypostia twenty pores occur ranged about the female pore, but the most posteriorly placed of these structures are devoid of a seminal duct. This See also:

• CONDITION (Lat. condicio, from condicere, to agree upon, arrange; not connected with conditio, from condere, conditum, to put together)

Segmentation results in the formation of dislocated megacytes and microcytes. The latter give rise to the epidermis which is laid down in bilateral sheets, the former to the various See also:

• INTERNAL

The ovum by unequal segmentation gives rise to megameres and micromeres, and between the two, intermediate cells form one origin for the mesenchyma. The micromeres surround the intermediate and centrally placed macromeres. The latter undergo division into hypoblast cells and yolk-masses. The similarity of cell-lineage in Polyclads and Coelomate Invertebrates, together with the trochosphere-like Polyclad larval form (Muller's larva), have been the two chief arguments in support of the view that this group is a See also:

• LINK

Tribe I. Acoela (fig. 11).—Mesenchyma not differentiated into separate gut and parenchyma. No excretory organs of protonephridial type. A simple pharynx; A median otocyst (statocyst) over the brain. Small, often flattened forms. All marine and many infected by brown or green algal cells. One species parasitic in Echinoderms. Tribe II. Rhabdocoela (fig. 12).—Gut and parenchyma separate, the former a simple straight sac. Vitellaria usually present.

Testes compact. Penis and pharynx often complex, occasionally protruded through a common opening. Marine and fresh-water. Many fresh-water forms infected by algal cells. Typhloplana, Grala, Anoplodium, are respectively parasitic in See also:

• NEPHTHYS

Sub-order B. Dendrocoelida.—Large forms with flattened body, branched intestine, follicular testes and follicular ovaries or compact ovaries and yolk-glands. Tribe I. Tricladida.—Intestine with three See also:

• MAIN (from the Aryan root which appears in " may " and " might," and Lat. magnus, great)
• MAIN (Lat. Moenus)

Intestine composed of a median stomach with many branched or reticulate coeca; testes and ovaries follicular; genital openings usually separate, the male gonopore preceding the female one. Multiple male gonopores in some forms. All marine and widely distributed; some genera See also:

• COSMOPOLITAN (Gr. Kbvµos, world, and IroXirgs, citizen)

Two Brazilian forms are known, one from the pulmonary chamber of the Mollusc Ampullaria and the other from water tortoises. The genus Temnocephala is found in all the countries mentioned. The two others, Craspedella and Actinodactylella are only known from Australia. The epidermis offers an interesting transitional structure. It is still, as in Turbellaria, cellular, or rather syncytial without cell-boundaries, but in most species has lost its cilia and developed a (From Cambridge Natural History, vol. ii. " WORMS," &c., by permission of Macmillan & Co., Ltd.) d, Gut. ex.s, Excretory sac. in, Mouth. ov, Ovary. ovd, Oviduct. ph, Pharynx. rv, Receptaculum See also:

• VITELLI, VITELLOZZO ( ?-1502)

thick cuticle as in See also:

• TREMATODES

Cilia and rhabdites are present. Family I. : Temnocephalidae: 4–12 anterior tentacles. Farniiy II.: Actinodactylellidae. Lateral tentacular processes. See Haswell, - Macleay Memorial See also:

• VOLUME