Two major hormones produced by the anterior lobe act on the sex glands. One stimulates the production of eggs in the ovaries of the female or sperm in the testes of the male. The other causes the development of a special tissue in the spaces left after the eggs have been discharged from the ovaries. This tissue-corpusluteum is particularly important in producing progesterone, a hormone that stimulates the development of the womb lining and, if a fertilised egg is implanted there, in the later development of the placenta through which the growing embryo is nourished.

Our present knowledge indicates at the posterior part of the pituitary has three main activities produced ny two hormones. One, oxytocin (or pitocin), causes the womb (uterus) to contract. It is thought that this action is extremely important at birth in assisting the passage of the young out of the uterus.

The second hormone produced by the posterior pituitary is vasopressin. Its effect on the blood system is slight, injections causing the smooth muscle of the blood vessel walls to contract. Its normal role in man is not certain. However. another effect is well established, that of controlling the loss of water and salt in the urine. Water loss is known as diruesis. Vasopressin acts on the kidneys causing them to reabsorb more water from the urine it reduces water loss and it is known therefore, as the antidiuretic hormone (ADH).

It promotes the retention of water by the kidney tubules but at the same time it causes a greater loss of salt. Cells in the part of the brian above the pituitary (the hypothalamus) are sensitive to the salt concentration in the blood. Special nerve cells of the hypothalamus probably produce the ADH and it passes to the posterior pituitary along the nerve fibres, being released when required into the blood capillaries there. Such a transport of stimulating material along nerve fibres is called neurosecretion.

Failure of the pituitary to produce ADH in sufficient quantities causes the disease diabetes insipidus when as much as thirty five pints of water may be lost per day in the urine. It is interesting that the level of ADH in the blood of the kangaroo rat, that lives in deserts, is eight times as high as that of a dog. Consequently the former produces very much more concentrated urine.

Placenta - A structure, found only in the higher mammals not monotremes and marsupials that is concerned with the nourishment of the embryo. The placenta is formed from the lining of the mother's uterus and certain of the embryonic tissues the allantois. For this reason it is known as an allantois placenta. The placenta. The placenta is rich in blood vessels, partly supplied by the mother and partly by the embryo or foetus. Food materials and oxygen pass from the blood vessels of the mother into those of the foetus, and waste substances (e.g. urea and carbondioxide) in the reverse direction. The placenta persists throughout the development of the young.

Placoid Scale - (Denticle). Characteristic tooth like scale of sharks and other elasmobranchs. The skin over the jaws carries modified scales which act as teeth. The structure of placoid scales, with dentine, enamel and pulp cavity, is very like that of the teeth of higher vertebrates.

Planarian - One of the free living flatworms.

Plankton - The fre floating organisms near the surface of the sea or lake. Includes many crustaceans, the young stages of fishes, jellyfishes, and the host of tiny organisms plant and animal on which they feed.

Plantigrade - Walking on the soles of the feet, as man and bears.

Plasma - The fluid of blood.

Plasma Membrane - The very thin membrane covering the surfaces of all cells.

Platyhelminthes - Phylum of worm like animals, many of which are important parasites. The flatworms arc bilaterally symmetrical, slightly flattened animals with no body cavity the space between body wall and gut is full of cells. If the gut is present it has only one opening the mouth. The reproductive system is usually hermaphrodite Excretion is by flame cells.

Pleistocene Epoch - Division of Quaternary period.

Pleural Sac - Space surrounding the lungs of a mammal.

Pliocene Epoch - Division of Tertiary Period.

Poikilothermic - (See Homoiothermic).

Polychaeta - A large order of annelid worms (See Annelida) which differ from earthworms (Oligochaeta q.v.) mainly in the possession of many bristles and in the presence of a distinct head. The bristles arise in bunches from leaflike projections of the body wall called parapodia (sidefeet). Each segment possesses a pair, one on each side of the body. Parapodia are well shown in one of the commonest shore dweIling, polychaetes, the rag worm (Nereis), Each individual parapodium consists of an upper half, the notopodium and a lower half, the neuropodium; both support a bunch of chaetae. Above and below each parapodium are flaps of skin, the dorsal and the ventral cirri.

Another feature in which poly chaetes differ from oligochaetes is the formatibn of a distinct head. Earth worms have a segment orprostomium in front of the mouth but there are no features suggesting a concentration of sensory organs. In Nereis, however, the prostomium is equipped with a pair of sensitive tentacles above and a pair of palps below. The next two segments have become fused together. Of all the segments of Nereis, they alone lack parapodia, but the cirri are still there modified as sensitive tentacles.

Nereis is an example of an errant polychaete, a worm which moves about using its rows of parapodia as padsles for swimming or just undulating its body. Actually, most of its time is spent in shallow burrows. The sharp bristles which at other times probably protect the creature now enable the animals to grip the smooth walls of its burrow, while gentle undulatory movements of the body cause water to flow in and out; carrying oxygen and possible indications of food in the vicinity. When small animals do pass the burrow, the worm's pharynx is turned inside out so that it extends from the burrow. On the end are two horny jaws for gripping the prey.

Another group of wandering polychaetes are the scale worms. The upper surface of these creatures appears to he covered with armour plates, causing them to resemble the molluscan chitons. The plates are called elytra and are adaptations of the dorsal cirri; these structures have curved upwards and backwards on either side of the body and overlap on the top surface. Scale worms are usually small rarely more than an inch or so in length.

A notable exception, however, is the 'seamouse' (Aphrodite). Not at all like a worm in appearance, this creatrue, which burrows in muds in off shore waters, is six inches or so in length and two inches or so in length and two inches across. Its scales are not visible, for a dense coat of matted hair covers the upper surface. The hairs are really modified chaetae formed from the notopodium. Short iridescent chaetae, projecting laterally, are used for locomotion. In the burrow, water is continually pumped through the space between the back and the covering of hairs. The scales probably absorb oxygen.

Nereis has been described as primarily a burrower. But for reproductive purposes, a remarkable change takes place both in its habits and its structures. As eggs and sperm develop in the bodies of the male and female worms (unlike the earthworm Nereis has separate sexes), the parapodia become larger and the spiny chaetae become flattened almost oar shaped.

The colours become richer and the males eyes become enlarged. With the change complete, the worms take to the open waters where eggs and sperm are liberated into the sea. this sexual stage is known as the heteronereis; formely heteronereids were thought to be different animals altogether and not surprisingly) so.

In the paddleworms, parapodia and dorsal cirri are paddle-shaped all the time and the animals swim as well as crawl. A remarkable pelagic worm is Tomopteris. Its parapodia have developed into enormous lobes, On the head are two very large 'antennae'. In fact these are really parapodia much modified and supported by very long chaetae.

Distinct from the wanderers and shallow burrowers are the tubemaking polychaetes they actually construct their own homes. Appendages on the head are usually far more numerous than in Nereis, and greatly modified and increased in size. Tube living worms are filter feeders. The tentacles projecting from the head have ciliated grooves down which food particles are washed into the mouth. They do not have the eversible pharynx or jaw of Nereis and the parapodia are usually reduced in size.

The serpulid tube dwellers make their shells of calcium carbonate which they secrete themseleves. Some tubes are relatively straight but others are so tightly coiled that they resemble the spiral shells of snails. Another feature of serpulids is that the peristomium the fused segments forming the head is produced into a collar which folds back over the outside of the tube and secretes reinforcing, hoop shaped rings. One of the tentacles from the head has a swollen branch; when the animal withdraws into its tube the structure fits exactly over the mouth of the tube.

The sabellids make tubes by cementing sand and mud particles together. The tubes are either buried in the sand or attached to rocks Sabella pavonina the peacock worm is a magnificent sight. Its tube is a foot long and brightly coloured tentacles project form the opening.

Terebellids make mud or sand tubes. In addition food collecting tentacles, they have three pairs of gills behind the head, for respiration. The serpulids and sabellids have no special structure but respire over the whole body surface.

Of all polychaetes, the worm which most closely resembles the earthworm in appearance is Arenicola, the lugworm. The lugworm is a burrower and like the earthworm consumes vast quantities of mud from which organic food is obtained. Evidence of its activities are the conspicuous coiled worm casts on any sandy beach. Chaetae are small in the front and middle of the body, but absent altogether at the rear; the middle region, however carries pairs of feathery gills on the dorsal surface. Sensory tentacles are absent; they would only be a hindrance to burrowing. But the pharynx can be extended. Instead of heavy jaws, the surface is coated with minute papillae which by adhesion enable the worm to pull itself ! forwards through the sand.

Polymorphism - Occurring in several forms, usually with a fairly constant ratio between them in anyone region. Human blood groups, and the occurrence of certain varieties of insects are examples. The occurrence of worker castes as well as sexual forms in many social insects is also a special case of polymorphism.

Polyp - The fixed, tube like form of the Coelenterata.

Polyhpyletic - Having a mixed ancestry. A group of animals that is polyphyletic is not therefore a true grouping be cause they are not all related through a common anceestor.

Polyzoa - (=Bryozoa). Old name for the animals now divided into Ectoprocta and Endoprocta. Polyzoa is a polyphyletic grouping.