Mosquitoes and various other blood sucking insects although they give nothing in return for their food, are not normally classed as parasites because they spend only a relatively short time on the so called host. Ichneumon flies show a special form of parasitism in which the host the larva of another insect is killed when the parasites have finished with it.

The female ichneumon lays her eggs in the larva of the host insect and the young ichneumons grow inside the host., feeding on its food stores and various non vital organs. The vital organs are not touched at first and the host can go on providing food for the parasites.

When the young parasites are almost fully grown they start to destroy the vital parts of the host and then they bore through the body and pupate outside what is now little more than an empty skin. Social parasitism is displayed by the cuckoo and various cuckoo bees; instead of raising their own families they lay their eggs in the nests of others and the young are reared by foster pareats.

Parasympathetic System - Part of Autonomic Nervous System.

Parathyroid Glands - Endocrine glands of vertebrates, usually associated with, though not connected with, the thyroid gland. There may be several pairs of parathyroids. They are particularly concerned with the metabolism of calcium, possibly also with that of phosphorus. Their removal leads rapidly to the continued contraction of the muscles (tetany), as a result of the disturbance of muscle and nerve fibres, and death ensues. Disruption of the normal working of muscles and nerves is largely due to the drop in the blood calcium level which follows parathyroid removal.

The parathyroid hormone (PTH) has not been purified yet and an accurate analysis of its role in the body is not possible as present, though many experimental data have been obtained. Injection of parathyroid extract or of calcium relieves the symptoms that follow the glands' removal. There is a relationship between the size of the parathyroids and the amount of calcium in the diet. A low dietary calcium level causes their enlargement and conversely an increase in the level of calcium produces a reduction in their size. Milk is rich in calcium, and so it is not surprising that the parathyroid gland of a female mammal increases in size whilst she is suckling her young.

PTH increases the absorption of calcium from the food through the wall of the intestine. It also has profound effects on bone. The injection of PTH causes calcium to be removed from the bone and there is a consequent in crease in the blood calcium level. A large part of the body's calcium is stored in the skeleton. Besides its structural role there and in teeth, and its part in the working of muscles and nerves, it is a1so necessary for the proper clotting of the blood. Calcium affects the passage of ions from one cell to another, particularly through the walls of the blood capillaries. Since calcium has so many functions it follows that the effects of the parathyroid are multiple, and hence abnormalities of them produce such far-reaching results.

Parazoa - Sub kingdom of animals in cluding. Sponges (Phylum Porifera) differing considerably from other many celled animals (Metazoa).

Parietal Bone - Bone of skuff.

Parthenogenesis - The production of fertile eggs and young without fertilisation. It is common in the aphids. stick insects, and also in water fleas (Daphnia). The parthenogenetic eggs develop quickly and large populations are rapidly built up. Normally the young are genetically identical with the other and. therefore, all female. Males are rare and may be produced only at certain times of the year. Normal sexual reproduction then jakes, place. The drones of bees are produced from unfertilised eggs; when laying eggs in drone cells, the queen cuts off the flow of sperm from her spermathecae.

Passerine Bird - Member of the order Passceriformes the perching birds whose first toe (Hallux) is directed backwards and adapted for gripping branches. About half the living birds are passerines, including all the common garden birds thrushes, tits, robin etc. The crows are the largest passerines.

Patella - Knee cap, a small bone in front of knee joint.

Pathogen - A disease causing organism, e.g. many bacteria and protozoans.

Pectoral Girdle - The shoulder girdle connecting the front limbs to the trunk. The primitive pectoral girdle consisted of a simple 'U' shaped structure to the angles of which the limb bones were joined. The upper part of each side is the scapula (shoulder blade) and the lower part the coracoid. The coracoids normally join the breast bone in tetrapods but; unlike the hip girdle, there is no connection to the backbone. There is also another bone in most tetrapods the clavicle. It forms the collar bone of man and the two clavicles join to form the 'wishbone' of birds. The coracoid is very reduced in mammals and appears as a small knob on the scapula.

Pelagic - Inhabiting the open sea but not fixed to the bottom. Divided into free floating plankton and actively swimming nekton.

Pelvic Girdle - Hip girdle connecting hind legs to trunk. The dorsal region the ilium unites with one or more sacral vertebrae in tetrapods. The ventral region is the ischio pubis and normally consists of two bones the ischium and the pubis. The acetabulum the socket taking the head of the femur lies between the ilium and the ischio pubis. In those animals where the bones all join up to form a single structure, each half of the pelvic girdle is called the innominate bone.

Penis- Male organ used for transferring sperm to female.

Pentadactyl Limb - Basic limb structure of tetrapods named from the presence of five digits. Both front and hind limbs are of the same basic structure.

Pepsin - Protein-splitting enzyme secreted in verebrate stomach.

Pericardium- Wall of pericardial cavity surrounding the heart.

Periosteum - Connective tissue surrounding bone.

Peripheral Nervous System - That part of the nervous system other than the central nervous system. Consists of nerves running to and from the C.N.S. and the various muscles, sense organs. gland, ets. (See Nerve).

Perissodactyia - An order of hoofed mammals containing horses. rhinoceroses. and tapirs. The main axis of the foot runs through the 3rd toe which is normally larger than the others and may be the only functional one.

Peristalsis - Rhythmic muscular contraction of the alimentary canal that causes food to pass along.

Peritoneum - Membrane lining the abdominal cavity and covering the organs within it.

Permian Period - Division of the Geological Time Scale.

pH - An index figure denoting the degree of acidity or alkalinity. ph7 is neutral. Lower figures are acidic, higher ones, basic.

Phagocyte - A cell, especially of the blood, that engulfs particles from its surroundings in the manner of Amoeba. Blood phagocytes are very important in defending the body against bacterial infection.

Phalanges - Bones of the fingers and toes.

Phalangida - An order of Arachnida containing the harvestmen or harvest spiders. Unlide true spiders, they have no slik glands and the body is not divided into two regions.

Pharynx - Region at the back of the mouth into which the windpipe opens. In fishes the gill slits open into the pharyns.

Phenotype - The visible appearance of an organism with respect to one or more characters, as opposed to the genetic constitution (Genotype). A certain genotype may give rise to several phenotypes according to the environment in which they live. Also a given phenotype may be produced by different genotypes; an heterozygous organism will exhibit the same features as a homozygous one possessing dominant genes.

Pholidota - Order mammals Pangotins.

Photophore - Light producing organ.

Phylogeny - The evolutionary relationships of a group of animals.

Phylum - The largest category used in classification. All members of a phylum have a basic similarity of structure but may differ greatly in detail.

Physiology - The study or living processes such as digestion.

Pinnipedia - Group of aquatic carnivores with flippers instead of legsseals and walruses.

Pisces - An old term covering all types of fish. It is not a true classificatory gouping.