Planarian

“ That regeneration of this kind could have been arranged for if it had been useful we learn from the Planarians among the flat worms, in which every piece cut out of the body, large or very small, from the middle, from the left side, or from the right side of the animal, grows into a complete Planarian. ”

“ As many as seven different heads have thus been made to grow on a single planarian—some on the middle of the back, some underneath the body, some even on the sides of the tail, and all, no doubt, greatly to the embarrassment of their owner.
As for cut off heads and tails, one might expect them to grow new bodies and become whole animals again. They do not, however, unless the pieces are pretty large. A head, cut off close behind the eyes, grows out, not a body, but another head. ”

“ It seems that in some measure this bodily immortality of the Protozoa is shared by some simple many-celled animals like the freshwater Hydra and Planarian worms. Here is an interesting chapter in evolution, the evolution of means of evading or staving off natural death. Thus there is the well-known case of the Paloloworm of the coral-reefs where the body breaks up in liberating the germ-cells, but the head-end remains fixed in a crevice of the coral, and buds out a new body at leisure. ”

“ 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. ”

“ 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. ”

“ In a few species, however, from two to four of these black dots nearest to the brain seem somewhat more complicated than the others. As the faculty of vision evolves among animals these will become actual eyes and all the other light-sensitive spots will be discarded. In most planarians, however, the number of eyes increases with the age of the animal.
Nearly all are predatory meat eaters. They are both hunters and scavengers. Some pursue, kill, and eat living animals, such as earthworms and small mollusks, as big as themselves. ”

“ The colours may be a warning signal, as some Geoplana at least are disagreeable to the taste of man and some birds; but since Land Planarians are largely nocturnal animals, living by day under logs, banana-leaves, and in other moist and dark situations, this explanation is clearly insufficient. ”

“ Some species are two-eyed. Many varieties are eyeless. Hundred-eyed worms are quite common. The black dots are light-sensitive. Presumably they represent the beginning of vision. By means of them the worms can distinguish between light and darkness. They also tell the direction from which light comes. Actually, however, planarians without eyes have the same ability, but they are slower to react. This is demonstrably true for fresh-water forms. For most of the land forms at least exposure to strong sunlight would be fatal. ”

“ 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. ”

“ They are found in great numbers on water-weeds, and being very sluggish are [Pg 102] readily caught or picked off the plants they frequent with the hand. Do you notice those small, flat, brown or black dabs so common on almost any water-weed you pluck up? These are planarian worms, and though not of prepossessing appearance generally, are extremely interesting animals to study. These large, reddish, oval or round cocoons are the eggs of the planariæ. Here is one of the largest of the family. It is of a milk-white colour, beautifully marked with delicate tree-like branches ”

“ About forty species of Planarians have been discovered on the Australian continent, thirty-five of which belong to the predominant genus Geoplana, distinguished by the presence of numerous eyes along the border of the simple anterior extremity. Of the remaining five, four belong to the genus Rhynchodemus, with, lastly, the introduced Bipalium kewense. The distribution of any one species, however, is so limited that only three forms are common to the two former colonies ”

“ They build their houses from the slime of their own bodies.
This class is that of the ribbon worms or nemertina. There are about five hundred known species—perhaps as many more are unknown. Still near the bottom of animal life, they represent revolutionary advances from the lowest of worms, the planarians, with which they share many characteristics. They have evolved integrated brains and nervous systems. They have, for the most part, taken on a true worm shape. They have acquired weapons and, in some cases, arsenals of weapons. They have eyes that see. ”

“ Most planarians have both male and female reproductive systems. Then come the muscles of the body wall. Never however, do the worms eat their own brains or nervous systems, although the brain may be reduced greatly in size. The I Am of the worm can devour its vestments of protoplasm; it cannot eat itself. When food is available again the organs are regenerated and return to normal size. ”

“ With other forms, for instance, with planarians, a sudden decrease of the intensity of the light causes a decrease in movement. Such animals gather chiefly in parts of the space where the light intensity is relatively small. ”

“ 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. ”

“ 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 deep-water forms, and only some half-dozen Planarians are known to be parasitic. ”

“ In this country Montagu, G. Johnston, and in Ireland, William Thompson, discovered several marine species, one of which, Planocera folium (from Berwick) , has not again been met with on British shores. Dalyell [7] conducted classical researches on the habits of Planarians, and Faraday [8] made interesting experiments on their power of regenerating lost parts. The credit of assigning the correct interpretation to most of the various organs of fresh-water Planarians belongs to von Baer [9] and Dugès, [10] while Mertens [11] effected a similar service for the marine forms, or Polyclads. ”

“ Walter Voigt has shown, by a series of most interesting experiments, that it is possible not only to cause the development of a new head in Planarians by cutting them, in which case a tail may grow from the anterior portion and a head from the posterior portion, but it is also possible in an intact animal, that is, one with both head and tail, to cause the production of a second head, or a second tail, or both at once, at any part of the body margin at will, according to the direction of the cut. ”

“ Pangolins are of small or moderate size, terrestrial and burrowing, and feed mainly on termites or white ants; some of the species being more or less arboreal. They can roll themselves up in a ball when in danger. Their peculiar elongated form, short limbs, long tapering tail, and scaly covering give them on a superficial inspection more the appearance of reptiles than of mammals. ”

“ It might be objected here that the tail-piece is too small to give rise to the whole body, but in Planaria it is only very diminutive heads and tails which grow from the artificial incisions, and the same is true of starfishes when only a single arm and a small piece of the disk have been left. ”

“ The Phalangistidæ, or phalangers, are one of the most varied and interesting groups of Marsupials, being modified in a variety of ways for an arboreal life. We have the clumsy-looking tail-less koala, or native sloth; the prehensile-tailed opossum-like phalangers; the beautiful flying oppossums, so closely resembling in form the flying squirrels of North America and India, but often no larger than a mouse; the beautiful dormouse-like Dromiciæ, one species of which is only 21⁄4 inches long or less than the harvest-mouse ”

“ Still more would the Pangolins flourish, for though they are toothless and walk very clumsily, because their front feet are bent under so that they tread on the upper part, yet they have two means of protection.204 First, like the ant-eater, they live chiefly underground and come out at night; and secondly, their back is covered with sharp-edged scales, which grow from the skin as hairs do, and can be raised into a complete cheval-de-frise as they roll themselves up, or tuck their tail and head between their legs when they are attacked. ”

“ The preceding Campanian stage, in contrast, has yielded an exceptionally diverse assemblage of dinosaur taxa that span a far greater latitudinal range (Alberta to Mexico) and are much better constrained both geographically and stratigraphically [7] , [9] , [27] , [25] . With a dense and relatively well sampled fossil record, the potential for high-precision geochronology, and faunas occupying a peninsular continent, the Campanian WIB represents arguably the best time and place to investigate major questions surrounding the ecology and evolution of Mesozoic terrestrial ecosystems. ”

“ The genus Palæoniscus has been made to include an immense number of species, which are, indeed, referable to more than one generic type, some of them actually not being Palæoniscidæ at all. Authors have, in point of fact, been only too apt to refer nearly every small rhombic-scaled fusiform-shaped ganoid fish from Upper Palæozoic rocks to Palæoniscus, without inquiring too narrowly into how far its structure agrees with that of the original type of the genus, and have even sometimes overlooked distinctions of more than generic importance. ”

“ Subsequent investigations have brought to light a variety of facts of the same order, the most curious and striking of which are those which prove the existence, in the mesozoic epoch, of a series of forms intermediate between birds and reptiles—two classes of vertebrate animals which at present appear to be more widely separated than any others. Yet the interval between them is completely filled, in the mesozoic fauna, by birds which have reptilian characters on the one side, and reptiles which have ornithic characters on the other. ”

“ Yet these groups are fundamentally the same, from the beginning of the palæozoic until now; and modern species seem scarcely at all to differ from specimens procured from rocks at least half-way back to the beginning of our geological record. If we suppose that the present sponges and foraminifera are the descendants of those of the Silurian period, we can affirm that, in all that vast lapse of time, they have, on the whole, made little greater change than that which may be observed in variable forms at present. ”

“ PHALANGER, a book-name applied to the more typical representatives of the group of diprotodont marsupial mammals, including the cuscuses of the Moluccas and Celebes, and the so-called opossums of Australia, and thus collectively the whole family Phalangeridae. (See Marsupialia.)
Phalangers generally are small or medium-sized woolly-coated marsupials, with long, powerful, and often prehensile tails, large claws, and opposable nailless first hind toes. They seem in the day to be dull and sleepy, but are alert at night. ”

“ Falconer and Sir Proby Cautley, continued for fifteen years, for the discovery in these marls and sandstones of a great variety of fossil mammalia and reptiles, together with many fresh-water shells. Out of fifteen species of shells of the genera Paludina, Melania, Ampullaria, and Unio, all are extinct or unknown species with the exception of four, which are still inhabitants of Indian rivers. Such a proportion of living to extinct mollusca agrees well with the usual character of an Upper Miocene or Falunian fauna, as observed in Touraine, or in the basin of Vienna and elsewhere. ”