When the oval window is set in motion its movements disturb the perilymph at the bottom of the vestibular canal. The pressure changes in the fluid within the vestibular canal are transmitted down the tympanic canal and each time the oval window bulges inwards another membrane the round window bulges outwards. The pressure changes in the cochlea affect the basilar membrane part of the spiral lamina. The membrane is a band of tissue running almost the length of the cochlea about one and a quarter inches. Arranged across it are a number of fibres, the lengths of which inctease as the bore of the cochlear duct gets smaller.

On top of these fibres is the spiral organ or organ of Corti. This contains hair cells, the hairs of which project into a gluey nap the tectorial membrane which lies above them. Different parts of the basilar membrane are sensitive to sounds of different frequencies. When it vibrates the hairs also vibrate. Tl: hair cells are linked with nerve fibres and movement of the hairs results in impulses being sent along the nerve to the brain. This translates the signals it recives as sound. The ear does not hear it merely receives sound waves which are transmitted as signals to the brain.
Balance.

Three responses are involved static respones, dynamic responses and the maintenance of tonus (this is the resting tension in muscles; at rest muscles are not limp, they have tonus).

Static responses are conerned with maintaining the centre of gravity of the body on movement (e.g. if a frog is held in the hand and tilted to one side, its head moves in order that the eyes keep the same horizon). Dynamic responses are concerned with the body's movement it accelerates rapidly in a straight line (as when going up in a lift) or in a curve (as when going up in a lift) or in a curve (as when spinning round and round).

The three semi circular canals are arranged at right angles to each other. They open into the utricle. All one end of each canal is a swelling or ampulla within which is a ridge of sensory cells together with supporting cells, the crista. The sensory cells have hairs that project into a jellylike mass, the cupula. Nerve fibres of the auditory nerve are intertwined round the bases of the hair cells. When the head rotates the fluid moving in the canals pushes against the cupula and the hair cells are thus stimiulated. Signals pass via the lower parts of the brain to the eye, back, and limb muscles which act to make amends for the bead movements.

The utricle and saccule communicate by a narrow tube branching off which is a blind tube the ductus endolymphaticus. Parts of the utricle and saccule lining contain a patch of hair cells and supporting cells (maculae) like those of the cristae. The hairs of the former are embedded in a jelly like mass though this is flattened. The jelly contains numerous small bodies consisting of calcium carbonate crystals mixed with protein. These are called otoliths and the weigh down the jelly so that it is in firm contact with the hair cells.

The maculae are upright but at right angles to each other so that each position of the head affects the hair cells in a different way. Signals from the sense cells of the maculae are continuous and supply the parts of the brain that control the motor nerves with information so that these can maintain the state of muscle tone. The sense cells of the cristae, however, are stimulated by movements of or pressure changes in the fluid round the cupulae. They, are concenrned with dynamic responses. Pressure receptors in the feet and the many proprioreceptors in the muscles of the body also provide the brain with information on the posture of the body and are thus concerned with maintaining balance.

Earwig - (See Dermaptera).

Earthworm - (See Oligochaeta).
Ecdysis. The periodic shedding of the hard cuticle in arthropods to allow for growth. By swallowing air or water the animal stretches the soft new cuticle which then hardens at a somewhat larger size than the old one.

Echinodermata - Large phylum of the entirely marine animals including the starfishes and sea urchins. They are radially symmetrical and the outer covering contains numerous chalky plates, sometimes produced into spines, which give the animals a rough appearance. There is no brain nor is there even any, structure that can be regarded as a head.

The nervous system consists merely of a network with thicker strands around the mouth and along the arms or radii. Sense organs are poorly developed and special excretory systems and respiratory organs are lacking in most of these animals. There is a true coelom, tiny projections from which reach the surface as sking gills. The skin is very thin and oxygen can diffuse in from the surrounding water. This is especially so in starfishes.

A feature unique to these animals is the water vascular system a system of water tilled canals which runs in the body. Tiny branches reach the surface and are known as tube feet. They are used for moving about and also aid respiration. In general, the sexes are separate but a few species are hermaphrodite. The sex cells are usually shed freely into the water where fertilisation occurs.

The young stages show similarities with certain chordates, rather than with any other invertebrates. The phylum has a long fossil history and there are many extinct groups. Five living classes are recognised, in the two subphyla. Sub phylum Eleutherozoa: Class Asteroidea. These are the starfishes such as Asterias. The arms are broad and each contains a branch of the digestive system. Class Ophiuroidea: the brittle stars. Although star shaped, these differsomewhat from the asteroids in that the central disc is very distinct and the arms do not contain branches of the gut.

Class Echinoidea contains the seaurchins rounded animals whose skeletal plates join up to form a complete shell or test. The tube feet project through tiny. The tube feet project through tiny pores in the test. Class Holothuroidea contains the sausageshaped sea cucumbers with tentacles around the mouth. Sub phylum Pelmatozoa contains only one living class Crinoidea. These are the sealilies which are fixed to the sea bed by a stalk for at least part of their lives. Some such as Antedon, break free when mature. Their delicately branched arms give them their name of feather stars.Echinoidea. Sea urchins (See Echinodermata).

Ecological Niche - A term used to describe any small habitat or food source that is Colonised or made use of by a group of animals. For example, cattle dung is a food and refuge for a number of insects. This is their ecological niche and here they live efficiently and protected from competition. Every animal has its niche and by filling it helps to maintain the balance of nature.

The same niches occur in all parts of the world but are filled by different animals(e.g) kangaroos in Australia and antelopes in Africa occupy similar niches as grazing animals.Ecology, The study of animal (and plant) communities and the ways in which they react with each other and to any changes in the environment.Ectoderm. Outer layer of embryo and all the tissues derived from that layer mainly skin and its derivatives, and nerves.

Ectoparasite - Parasite that lives and feeds on the outside of its host, e.g. flea, tick and louse.

Ectoplasm - The outer region of a cell's protoplasm. It is usually in a semisolid state and it is especially important in the movement of amoeboid cells.

Ectoprocta - A small phylum of coelomate animals, often joined with Endoprocta as the Bryozoa. They live mainly in the sea and are normally colonial-many individuals joining up so that their hard cases make branched fern like structures, or encrust rocks and sea weed. When feeding, the body emerges partly from the case and a ring of tentacles around the mouth collects small organisms from the water.Edentata. An order of mammals confined to South America.

Many strange mammals evolved in that continent while it was isolated during the Tertiary period. When it was reconnected to North America Most of these mammals died out in the face of competition from the invading carnivores but the edentates the armdilloes, ant eaters, and sloths have survived. The teeth are reduced to small pegs in the armadilloes, absent in the ant-eaters, and modified for grinding in the herbivorous sloths, but fossils show that the sloths passed through an insecteatin stage in their evolution. The claws are well developed for digging, or hanging in the case of sloths.

Eelworm - (See Nematoda).

Effector - An organ that acts in response to a signal. Muscles and glands are the main effectors. Receptors (q.v.), receive stimuli from outside the body and send messages to the brain. The brain then sends the appropriate message to the effectors which act to prepare the body for a coming event or to move the body away from some harmful thing.

Efferent - Leading away from (e.g) efferent branchial arteries leading from gills.

Elasmobranchii - (=Chondrichthyes). Sharks, rays, and other fishes whose skeletons are made entirely of cartilage with no trace of true bone. They are almost entrely marine, feeding on other fishes and bottom living invertebrates. The skin is rough and covered with placoid scales which are modified as teeth on the skin over the jaws. The tail is upturned or heterocercal and tends to drive the head down but this tendency is countered by the flattened head and the large pectoral fins which provide lift in much the same way as an aeroplane wing. There is no swim bladder. The gill slits are not covered by an operculum except in Chimaera which represent an early off shoot of the elasmobranchs. The class as a whole has a long fossil history but they become nearly extinct in Permian times. Modern forms have evolved from the few species that survived into the Jurassic.

Iuternally the sharks are characterised by a short intestine containing a spiral valve which effectively increases its length. The dogfish is normally taken as an example to show the typical elasmobranch structure.