the Week of Proper 28 / Ordinary 33
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Bible Encyclopedias
Planarians
1911 Encyclopedia Britannica
or a well-defined group of animals, characterized externally by their ovoid or vermiform shape, their gliding movement and their soft, unsegmented, ciliated bodies: internally by that combination of low somatic type of structure and complex gonidial organization which is characteristic of the Platyelmia. Their low type of bodily structure may be exemplified by the facts that the mouth is the only means of ingress to and egress from the blind alimentary sac, and that no vascular system is differentiated. Most Planarians are aquatic and the cilia that cover the body produce by their beating a stirring of the water. Hence the class is generally known by the name Turbellaria.
Planarians form one of the basal groups of the animal kingdom. They are the simplest of multicellular creeping things. In them the gliding movement has become habitual. The lowest Planarians are still largely free-swimming animalcule and we can trace within the limits of the group the development of the creeping habit and the consequences that flow from it. It has led to the differentiation of anterior and posterior extremities; to the formation of bilateral symmetry; and to the development of a mucilage protecting the body against friction. It entails the concentration of the scattered nervous system on the ventral surface and at the anterior end, and it has induced the segregation of the diffused sense-organs in the head. The Planarians occupy a position midway between the simple planula larva of Coelenterates and the segmented Annelids. They have probably sprung either from an early Coelomate stock, or represent an independent class descended from a two-layered parentage distinct from that of the Coelenterates; a view which is adopted in the present article.
Occurrence
Most Turbellaria are aquatic. They abound on the seashore and in fresh water, amongst weeds or under cover of stones, shells and sand. Few of them are pelagic or deepwater forms, and only some half-dozen Planarians are known to be parasitic. A large number of land Planarians are known, chiefly from tropical and south temperate countries.
The majority of marine Planarians are nocturnal or cryptozoic, hiding away during the period of low tide to avoid desiccation of their soft sticky bodies and coming out at night or during high tide to feed. They are mostly carnivorous, and their movements are correlated largely with the nature of their food. The smaller, more active species occur in companies amongst the finer seaweeds over which they creep or swim in pursuit of their food. The larger marine species occur singly or in pairs on Ascidians, Nullipores or Polyzoa, from whence as the tide rises they issue to feed. By the time the next low tide exposes them, these Planarians have so completely digested their meal that we know very little of its nature. The common fresh-water Planarians form either little companies of a dozen or more, usually of a single species, huddled together under a stone or in some cranny (see Pearl [81 1), or societies of several species that inhabit Sphagnum and other fresh-water vegetation. This fresh-water planarian fauna is of two kinds, the fauna of permanent and that of temporary sheets of water and both show a certain adaptation to their environment. The latter, being subject to greater extremes of temperature than the lacustrine Planarians, produce 1 These references are to the literature at the end of this article.
thick-shelled eggs only. The development of these eggs is rapid in warm water, slow in cold: so that a pool after a few days of early spring sunshine is soon populated and provision is made for the continuance of the race should a cold snap follow. The lacustrine Planarians exhibit a different form of adaptation. The eggs laid by many of these animals are either thin-shelled and rapidly hatched or thick-shelled and slowly hatched. The lake-water, however, is in spring, even after sunshine, of a much lower temperature than that of pool-water, but the masses of Sphagnum and other weeds that border lakes and marshes are often warmer than the open water and may be as much as 13° or 15° C. higher in temperature. Here the Planarians assemble to benefit by the warmth, and under such favourable conditions lay thin-shelled eggs which rapidly develop; whilst in colder surroundings or at the onset of winter thick-shelled resting eggs are laid. In this manner we can understand the abundance of Planarian life in cold meres and transitory pools in Great Britain, Scandinavia, Finland,. Denmark and North America.
In contrast to the general habit among Turbellaria of haunting dim or dark places, the station chosen by a few species is exposed and strongly illuminated. The marine Convoluta and Polychaerus and the fresh-water Vortex viridis may be taken as examples. Convoluta paradosa occurs among brown weeds which receive much light during neap tides and strong direct sun or light every fortnight. Polychaerus creeps about the 'New England shore without resorting habitually to cover, and is also strongly insolated. Vortex resembles the green Hydra of our ponds in choosing the lightest side of its surroundings; and finally, Convoluta roscoffensis paints the beach green in Brittany, part of Normandy and Natal. In every such case the Planarian is coloured brown or green by the presence of photosynthetically active cells and the singular heliotropic habit of these Turbellaria is associated with the illumination necessary for the activity of their coloured cells.
Only one branch of the Planarians has become terrestrial, but this has spread over almost all the whole globe. One species (Rhynchodemus terrestris, fig. i, e) is fairly common in Great Britain under stones, logs and occasionally on fungi, but the Holarctic countries (North America, Europe and North Africa, North Asia) are extremely poor in terrestrial species. In countries lying in the centre and in the south of the great continents and in the south temperate continental islands and archipelagoes these land Planarians become more abundant and varied; and being frequently transported with earth or plants they are often found in hothouses and botanical gardens far from their native country. Their distribution offers some points of special interest showing a close relationship between the South American fauna and that of Australia and New Zealand: between the land Planarians of Madagascar, of Ceylon and of Indo-Malaya: and a marked contrast between Japan and the rest of the Palaearctic region (see Von Graff [i], 1899).
External Characters. - Planarians range from the minute forms no larger than Infusoria to ovate, marine species, 6 in. in diameter and to ribbon-like land forms 8 in. in length. The majority are small, somewhat cylindrical organisms with a flat creeping surface. Others, comprising the common freshwater and marine forms, are flattened and leaf-like, often provided with a pair of tentacles near the front end of the body, and in some cases the whole dorsal surface is beset with papillae. The land forms are elongate and smooth, and their anterior extremity is often modified into the arcuate shape of a cheese-cutter. Their movements are usually of a gliding character. The minuter forms perform short excursions into the water round their station, and in so doing recall Infusoria. The larger forms, in addition to gliding like pellicles, fold the expanded anterior part of their body into a couple of fins, with which they swim after the fashion of a skate. The folded margins of other forms clasp the weeds on which they live. Adhesion is effected by the mucous investment of the body and frequently by some specially developed local secretion of slime, or by a sucker. By these means, aided by their algal-frequenting and cryptic habits, the Turbellaria, though soft-bodied, are able to withstand the violence of the waves. The anterior end in all Turbellaria is the site of the chief sense-organs, and in some forms (Proboscida) becomes transformed into an invaginable proboscis of highly tactile nature. Such forms lead naturally to the Nemertina.
Coloration
The coloration of Planarians is of interest. The flattened marine forms are often brilliantly coloured on the dorsal surface, either uniformly or with some striking marginal band; or they may exhibit longitudinal bands of contrasting tints or a mottled appearance. The significance of these colours is not fully understood, but in some cases of sympathetic coloration the derivative function of the pigments is probably to aid cryptic resemblance. The terrestrial Planarians exhibit the most striking patterns in longitudinal striping and cross-bars which appear to have no relation to the environment of these essentially nocturnal animals. The fresh-water forms are colourless or dusky, often dark-brown, possibly in relation to the retention of heat; but in a number of both fresh-water and marine Planarians a green colour is present, constantly in 'some species, sporadically in others.
This green effect is due to the infection of the Planarian by a minute alga which multiplies in the tissues and may profoundly affect the habits and even the structure of its " host." The planarian so affected acquires a heliotropic habit; it becomes gregarious and in extreme cases ceases to ingest solid food. In Convoluta roscoffensis the green cells have become indispensable. They function both as the nutritive and excretory organs of the Planarian, and the young animal cannot develop until it is infected and has acquired a supply of these green cells which become incorporated into its tissues (Gamble and Keeble [7]). Brown algal cells (Zooxanthellae) are known in other species of Convoluta. Food. - The food of Turbellarians consists, in the smaller species, of diatoms, unicellular algae, microscopic animals and other Turbellarians; in the larger ones, of worms, mollusca and insects. The fine feeders capture their food chiefly at night by gulping down the minute organisms that settle or swim in their neighbourhood. The coarse feeders enclose their prey with a coating of slime and then proceed either to engulf it in their expansible mouth or to perforate it by their trumpet-like pharynx. The mouth is remarkably variable in position (fig. 2). In many flattened Planarians it is placed centrally on the ventral surface somewhat as in a jelly-fish. In the majority it is nearer the anterior end, but in a few remarkably elongate forms it occupies a position near the hinder end of the animal. In the cylindrical forms (Rhabdocoels) a similar variability in the position of the mouth is met with.
Anatomy
The structure of the Turbellaria though greatly varied in detail, conforms to a single type of somatic organization which is transitory in the higher invertebrates. The sexual organs, on the other hand, are founded on two or more types, and the astounding complications of these structures suggest that their evolution has been governed by quite other factors or combinations of factors than those that have guided the somatic evolution of the group.
C A B (From Cambridge Natural History, vol. ii. "Worms, &c.," by permission of Macmillan & Co., Ltd. After Lang.) FIG. 2. - A group of Polyclad Turbellaria, illustrating the various positions in which the mouth of Planarians may occur, and the concomitant changes in other organs.
A, Anonymus virilis: mouth central, male genital aperture (e) multiple and biradial.
B, Prosthiostomun siphunculus: mouth anterior, the pharynx protruded through it.
C, Cestoplana: mouth posterior (m); e, male; ?, female genital aperture; Br. brain; CG, eyes especially related to the brain; Ey, marginal eyes; m, mouth; MG, stomach; Ph, Pharynx; s, sucker.
The general structural characters are as follows. The body consists of a muscular envelope covered externally by a ciliated glandular epidermis and of an alimentary sac, cylindrical or branched, L BM PC EP U , O? uru?u ???? r ?? Uuoho 7(27" / r ? ?` O.o.:; ,?
areas ' '??t ' 'SC PC (After Biihmig.) FIG. 3. - To show the structure of the simplest Turbellaria.
The figure represents the left half of a transverse section across the body of the Acoelous planarian Haplodiscus. The mouth (M) is plugged up with a digestive polynuclear mass of cytoplasm and the transitions from this to the stellate scattered central parenchyma (SC) and again from the latter to a firmer peripteral zone (PC) are shown. The outermost layer (EP) is a ciliated epidermis resting on (BM), a basement membrane (dark line); the row of dots beneath this represents the longitudinal muscles (L).
for which the mouth serves both as ingress and egress. Between this aproctous gut and the integument the body consists of a jelly-like, vacuolated mesenchyme made up of branched glandcells, excretory cells, pigmentand 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. A 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 perception of light and pressure. No FIG. I.
a, Convoluta paradoxa, Oe.
b, Vortex viridis, M. Sch.
c, Monotus fuscus, Gff.
d, Thysanozoon brochii, Gr., with elevated anterior extremity (after Joh. Schmidt).
e, Rhynchodemus terrestris, 0. F. Muller (after Kennel).
f, Bipalium ceres, Mos. (after Moseley).
g, Polycelis cornuta, O. Sch., at tached by the pharynx (ph)to a dead worm (after Johnson). All the figures of natural size, and viewed from the dorsal surface. a, c and d are marine, b and g are fresh-water, e and f are terrestrial. All found in Great Britain except d. = ? r_ ? ?,, 4. ?._.:,,,...
4;?Cyrvy ii itaer„, respiratory organs are developed, probably in correlation with the absence of a blood-vascular system. On the other hand, the process of reproduction is elaborately organized. The Planarians are hermaphrodite and, as in so many other small animals, the body, after attaining maturity, becomes in many Planarians practically a genital sac and is soon exhausted by the repeated calls upon its reserves that are involved in the rapid production of eggs and spermatozoa. The intervals between successive clutches has been found in Convoluta roscoffensis to be a month, thus suggesting the influence of the lunar tides upon maturation.
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 Rm Ci IU11 .te, ?
%' 1 -Rmc (Partly after Luther: Zeitschri,'t fiir wissenschaft. Zoologie, by permission of Wilhelm Engelmann.) FIG. 4. - Portion of a transverse section of Mesostoma ehrenbergii (X 800).
The epidermis (E) consists of cells divided into an outer and inner zone, the latter containing rhabdites (Rb); the cilia (Ci) are thickened about the middle of their length. Below the epidermis is the basement-membrane (BM), a layer of circular muscles (C) and of longitudinal ones (L). Below this again is the mesenchyma (M), made up of branched cells and dorso-ventral muscle-fibres (DM). The mesenchymatous glands (Rmc) are producing rhammites (Rm) which pass outwards.
frequently coloured red or yellow, and are highly characteristic of the Turbellaria. Their real use is unknown. In only two genera does the epidermis produce cuticular spines (Acanthozoon, Enantia) on the surface, but chitinoid hooks, spines and spirals occur frequently on the lining membrane of the male and female copulatory ducts.
Below the epidermis is a firm basement membrane into which the subjacent muscles are inserted. They are divided into outer circular and inner longitudinal groups and subdivided in the larger forms by diagonal fibres, and in the most highly differentiated Planarians there are six muscular layers, two of each kind. In a number of Turbellaria the musculature is modified to form a sucker either single or double and anterior or posterior, and it undergoes further modification in connexion with the pharynx and reproductive organs.
FIG. 5. - Integument of Mesostoma lingua, O. Sch.
On the right hand is the epidermis (z) with perforations (1) through which the rhabdites (st) project. Beneath this the basement membrane (bm), and beneath this again the muscular layers consisting of circular (rm), diagonal (sm), and longitudinal (lm) fibres.
Alimentary Sac
The alimentary sac consists of a muscular pharynx opening outwards through the mouth and inwards into a median digestive organ which may be solid or hollow, and in the latter case straight, lobate or branched. These characters are correlated with such a number of distinctive features that the classification of the Planarian is based on them. Thus we have the Rhabdocoelida with straight gut and the Tricladida and the Polycladida with triple and multiple branches to the gut. The Rhabdocoelida are further divided into three groups: the Acoela (From Lankester's Treatise on Zoology, Part IV.) FIG. 7. - Flame-cell from the Excretory System.
a, nucleus; b, excretory granules; c, " flame "; d, branches of cell; e, beginning of excretory tube.
with a simple syncytial gut not sharply separated from the surrounding mesenchyma; the Rhabdocoela, with a hollow gut and a perivisceral schizocoelic span; and the Alloeocoela with a lobate gut and reduced schizocoele. The last group leads one naturally to the Tricladida; the Polyclads being an independent group.
