From a study of marsupials, both in the wild and in captivity, it seems that they are able to breed at any time of the year, though strict breeding seasons are normally observed. A virginia opossum female usually has two litters a year and becomes pregnant agnin after the first litter has been weaned. Not all marsupials have a pouch or marsupium as well developed as that of the kangaroo. Some merely have two flaps of skin whilst others, for example the woolly opossum, have no pouch. In this case the youngsters hang on the nipples of the female as she moves around. Animals with a well developed pouch have special processes epipubic bones-on the pelvic girdle. These help to support the pouch.

Mandible - The lower jaw of vertebrates. Also the paired biting and crushing appendage near the mouth of insects and crustaceans.

Mantle - Thick fold of skin covering all or part of the mollusc body.The mantle cavity between mantle and body contains the gills while the outer edge of the mantle secretes the outer edge of the mantle secretes the shell.

Mastigophora - (=Flagellata).

Maxilla - A hone of the upper jaw in vertebrates, carrying most of the upper teeth. (See Skull). Also one of the pair of appendages behind the jaws (mandibles) of insects and crustaceans.

Mayfly - (See Ephemeroptera). .

Meckel's Cartilage - Piece of cartilage forming the lower jaw of elasmobranch fishes also of other vertebrate embryos. Becomes articular bone in adults.

Medulla. Central part of an organ where it differs from outer part (cortex). E.g. of kidney. adrenal gland.

Medusa Oblongota - Hind part of brain merging into spinal cord. A centre for the senses of taste and hearing. it also unconsciously controls the rate of breathing.

Medusa - Free swimming saucershaped stage of coelenterate lifehistory. Larger ones are the typical jellyfish. They almost alway reproduce by producing sex cells although they are themselves formed by budding.

Meiosis - A type of nuclear division during which the number of chromosomes per nucleus is halved: This division occurs at some time during the life cycle of any organism that reproduces sexually normally during the formation of sex cells. Without meiosis. the number of chromosomes in a cell would double with each new generation. giving rise to an impossible state of affairs. In meiosis the pairs of homologous chromosomes. come together. They then separate, one of each pair going to each end of the cell. A new nuclear membrane forms around the chromosomes and the cell divides into two. The two new cells then divide again but this time the chromosomes themselves divide into two identical halves so that the number is not reduced further in the four new cells. Thus. one original cell gives rise to four sex cells.

Meissner Corpuscle - Pressure receptor, hundreds of which are found in the skin of the hands and feet. especially on the tips of fingers and toes. Consists of a capsule of connective tissue surrounding a coiled nerve fibre.

Melanin - Dark brown pigment of many animals. Brown and yellows are produced by different concentrations of melanin.

Melanism - Excess of melanin in the tissues. Several moth species are known to produce occasional black individuals. The gene or genes controlling colour occasionally change so that the black (melanic) form appears. Under abormal conditions the black form was easily seen by enemies and was eaten but in the last hundred years or so melanic forms have increased in industrial areas. Smoke pollution has blackened buildings and so the occasional mutation was valuable: the black moth was protected by camouflage and gradually increased is numbers as the black gene was passed on to the offspring. The normal form then declined in these areas. This is a good example of Darwin's idea of natural selection-the changing environment leading to the increase of new forms. The new form was not, however, caused by the changing environment.

Melanophore - Pigment cell containing melaning.

Membrane Bone - (=Dermal Bone).

Mendelism - (See Heredity)

Meninges - The membranes covering the brain and spinal cord.

Menstrual Cycle - Cycle of changes in the female reproductive system found only in apes, man, and the old world monkeys, and which ends in the sloughing of the Uterine lining with some loss of blood. The uterus lining thickens as the egg cell develops in the ovary. It becomes more muscular and richer in glands and blood vessels. If fertilisation does not occur when the egg-cell is released, the growth of tissue continues for a time and then breaks down with the loss of blood characteristic of menstruation. Growth of the uterus lining and subsequent menstruation takes place regularly during approximatley 28-day cycles in Man.

Mesentery - Fine tissues suspending stomach and other organs in the body cavity. Also the vertical partition found in some numbers in the gut cavity of corals and sea-anemones.

Mesoderm - Middle layer of triploblastic animal embryo and all the tissues to which it gives rise-muscle, blood, cartilage, and connective tissue.

Mesogloea - Jelly like substance in body wall of Coelenterata.

Mesozoic - Geological era.

Matabolism - The sum total of the chemical processes going on witlin an organism

Metacarpal Bones - Bones of the paim region of hand or front foot.

Metameric Segmentation -

Metamorphosis - A transformation during the life history of an organism especially from larval to adult form e.g. the change from caterpillar to butterfly or from tadpole to frog.

Metatarsal Bones - Bones of the sole of the hind foot between ankle and toes.

Metatheria - (See Marsupiatia).

Metazoa - Term used to cover all animals other than protozoans i.e. the many celled animals. Sponges, however, are sometimes excluded from the metazoa because their structure is so very different from that of other animals.

Micro - (=Small).

Microphagous - Feeding on particles that are relatively tiny when compared to the size of the animal's body. Filter feeders such as bivalve molluscs and whalebone whales are of this type. In contrast to macrophagous animals they tend to feed continuously.

Migration - Fairly regular. instinctive movement of animals. There is normally a seasonal basis and the animals move one way in winter. the other in summer. Fishes going to the spawning grounds. swallows moving north in summer, reindeer moving south into the forests in winter, are all examples. Random mass movements with no later return are called emigrations and are normally brought about by overcrowding. The arctic lemmings are famed for their emigrations, Buttert1y and locust swarms are also emigrations.

Milk Teeth - The first, deciduous teeth of mammals. Later replaced by permanent teeth.

Mimicry - Phenomenon whereby an animal derives benefit from resembling another species. It can be explained by the Theory of Natural Selection. Among so many species of insect it is not unreasonable to assume that a number of them will look alike, and if one species is protected by evil smell, sting or warning colours other similarlooking ones will also derive benefit. The resemblance will then be continued and improved by natural selection over many generations.

In 1861 a naturalist named H. W. Bates was travelling along the Amazon and observed that large numbers of black and brown butterflies were protected by having a distasteful flavour but occasinally there appeared specimens of a very different kind. They looked like the common ones but lacked the distasteful flavour realised that the edible species were protected from enemies by virtue of their resemblance to the other butterflies and that here was an example of mimicry. This type of example, where a harmless species imitates a harmful one, is known as Batesian mimicry.

The animals that are 'copied' are called the models and the others, the mimics. Predators soon learn that certain types of insect or certain colour patterns are associated with stings or vile taste and they leave all such insects alone. The mimic thus gains protection. Even if only one per cent the mimics are saved, there is a great advantage. The model and mimic must obviously live in the same areas and mix freely. They must also behave in a similar fashion. For example, many spiders mimic ants. The spiders have dark marks on their sides that give that appearance of a narrow thorax while the front legs are held out rigidly in front as if they were antennae. All this would be useless if it were not accompanied by the correct behaviour, and so they dash to and fro in the urgent manner of ants, mingling with them so well that even trained entomologists have captured them thinking they were ants.

Malpighian Layer - Basal layer of epidermis where new cells are made.

Malpighian Tubules - Excretory organs of insects.

Obviously, for this type of mimicry to be effective, the models must be much commoner than the mimics. If this were not so, the predators would be quite likely to associate good food with the colour pattern and both mimic and model would decline.

There are, however, many instances, notably among tropical insects, in which both models and mimics are common, and both arc distasteful to predators. This type of mimicry is called Mullerian, after the Brazilian naturalist Muller who described it in 1879. There may be two or more similar species. The advantage of the system is seen in the training of would-be predators. Suppose a bird requires 150 attempts before it realises that certain insects can sting. If a second species is also distasteful, three hundred insects will die be ore one bird learns to avoid these two species.

However, if the two insect species share a common colour pattern, only seventy-five of each will perish. Thus the mimicry is effective and the more species sharing the pattern the better. But mimicry helps some predators too. The most remarkably examples are the assassin bugs which feed upon other insects. They are remarkably like their prey, even to the smallest detail so that some species resemble stick insects while others look like mosquitoes or even, in one case, the Praying Mantis. The Assassin bugs can deceive even human collectors.