Digestive enzymes are then poured over or into me prey and the spider then sucks up the resulting liquid, leaving only the hard skeleton of the insect. Most spiders possess tracheae resembling those of insects but they also breathe by means of lung boolks. The silk glands of spiders open at the hind end on to small projections called spinnerets. The eyes, although well developed, are simple: there is no trace of the compound eye found in insects.

Archaeopteryx - The earliest known fossil bird. Apart from feathers it looked very like a reptile, with claws on its wings and teeth. It also had a long, feathered tail. The fossils are about 150 million years old (Jurassic age).

Aristotle's Lantera - Jaw apparatus of sea urchins. It is made up of a series of chalky plates which rasp seaweed from the rocks.

Arterile - A small artery.

Artery - A vessel leading blood away from the heart and into the tissues. Arteries have thick muscular fibrous walls which withstand the high pressure set up by the pumping of the heart. As they get further from the heart, the arteries branch and get smaller, becoming arterioles, and finally capillaries. The latter join up after passing through the tissues and form veins which return the blood to the heart. Veins do not have such thick walls as arteries, and capillaries have no wall at all other than a single layer of cells. This layer is continous with the inner lining of the arteries and veins.

Arthropoda - A very large phylum of animals all of which have jointed limbs and are normally covered by a stout cuticle. The latter is usually impregnated with a horny protein called chitin and often with other materials such as lime. Like the annelid worms from which they are probably descended, the arthropods show definite segmentation. Peripatus a strange animal belonging to the class Onychophora shows several characteristics intermediate between annelids and arthropods.

Peripatus is the only arthropod that has cilia. The head is more marked in arthropods than in annelids and the nervous system more advanced. The limbs of the head are modified for feeding and for sensory functions. The large body cavity of most annelids is reduced and largely replaced by a haemocole in arthropods. (See Crustacea; Trilobita; Insecta; Onychophora; Myriapoda; Arachnida).

Artiodactyla - An order of hoofed mammals (ungulates) including pigs, hipopotamuses, deer, cattle, camels, arid sheep. The animals walk on the tips of their digits, toes three and four being equally developed and the rest reduced. Compare with Perissodactyla in which toe three is most developed. Apart from pigs and hippos, all artiodactyls chew the cud and their stomachs are specially modified.

Ascidian - A sea squirt.

Assimllation - The taking in and building up of simple materials into the complex proteins and other constituents of the body.

Association - The method by which animals 'remember' An encounter leading to an unpleasant experience is avoided at a future date. An encounter which ended in a reward will be repeated. In this manner animals can be trained (See Instinct).

Atlas - The first vertebra of the spine specially adapted to carry the skull. A nod of the head invoves movement between skull and atlas.

Atrium - The first vertebra of the spine specially adapted to carry the skull. A nod of the head involves movement between skull and atlas.

Atrium - The chamber surrounding the gill slits of Amphioxus and of seas quirts. Water passes in through the mouth of the animal, through the gillslits which extract food particles, and into the atrium. It then passes out through the atrial pore. (See Cephalochordata; Urochordata).

Auditory - (=Acoustic). Concerning hearing. The auditory capsule of the skull encloses the inner ear.

Auricle - A chamber of the vertebrate heart. There is only one in fishes but two in tetrapods, receiving blood returning from the body and from the lungs. (See Heart).

Autolysis - The destruction of cells by their own enzymes.

Autonomic Nervous System - The system of nerves spreading through the body is referred to as the peripheral nervous system. The outer or somatic part links up the sense organs that provide the animal with a picture of the outside world, with the effectors that move the animal towards food or away from danger. But in order that these sense organs and effectors function, the inner machinery of the body must be functioning smoothly and the rate at which it works must be adjustable so that the varying demands of the outer structures may be satisfied. When an animal is moving, its muscles require a greater supply of oxygen than when it is lying down. The heart must beat faster to supply more blood and this blood must be supplied with sufficient food and oxygen. The lungs must be filled with air and emptied more frequently, the blood vessels to the muscles must be expanded and more channels opened up there to cope with the increasing supple of blood.

The nerves that co ordinate the inner workings (e.g. smooth muscle of the gut and blood vessels, and the glands) form the visceral part of the peripheral nervous system. The visceral motor nerves are usually called the autonomic nervous system although they are controlled by the central nervous system just as the somatic motor system is. The autonomic nervous system is often referred to as the involuntary nervous system since we have very little conscious control of its activities, but  with training it is possible to change the size of the pupil of one's eye or to increase the speed of one's heartbeat.

The cell bodies of somatic motor nerve cells are inside the spinal cord but a characteristic of the visceral motor system is that the cell bodies of the final motor nerve cells (those that are in contact with the organ that they supply) are outside the spinal cord. These cell bodies are grouped together to form ganglia that lie either close to the spinal cord along nerves or actually in contact with the organs that they supply.

Thus, whereas the axon of a somatic motor heuron is long and carries impules cut from the spinal cord to the structure concerned, the typical visceral motor nerve is composed of two parts. One runs from the spinal cord to a ganglion and is called a preganlionic neuron; the other links with the preganglionic neuron in the ganglion and passes to the or an that it supplies. It is called a postganglionic neuron. In is called a postganglionic neuron. In mammals the preganglionic neurons usually have a fatty myelin sheath (they are myelinated), but the postganglionic neurons have no sheath and are said to be unmyetlinated.

The autonomic nervous system may be divided into two parts, sympathetic and parasympathetic. Generally their actions are opposite and their posifions in relation to the spinal cord are different. Sympathetic nerves arise from the spinal cord between the neck and waist region. Parasympathetic nerves arise from the head and from sacral region (between the waist and tail). Sympathetic preganglionic fibres leave the nerve cord by ventral roots as do somatic motor nerves, but para sympathetic fibres leave by both dorsal and ventral roots. Each preganglionic fibre is connected with many postganglionic fibres.

Though the two parts of the autonomic system arise in different parts of the spinal cord their branches spread nearly everywhere within the body. Most organs receive both sympathetic and parasympathetic neurons, though some (e.g. sweat glands) receive only one (in this case the sympathetic). The gut, for example (apart from the sphincter muscules), relaxes when it receives signals through sympathetic neurons.

Signals through parasymathetic fibres cause it to contract. The sphincter muscles are affected in the opposite way, contracting on the receipt of impules from the sympathetic and relaxing on receipt of signals from the parasympathetic. On The other hand the actions of the two systems may not strictly be opposite. the salivary glands produce a thick slime (mucus) when stimulated by the sympathetic system, but they produce the more watery saliva when stimulated through parasympathetic fibres.

Some of the Actions of Autonomic Nerves in Man

The action of one system on different parts of an organ system may vary considerably, Signals from sympathetic nerves increase the strength and rate of the heartbeat, but the main arteries leaving the heart relax, so allowing an increased now of blood, Sympathetic nerves usually cause the musculature of the rest of the blood system to constrict; During digestion the blood system to constrict. During digestion the blood now to the gut is increased by the gut blood vessels relaxing, At the same time the blood supply to the muscles is reduced to allow for this.

The actions of the autonomic nervous system are many and varied. Experiments indicate that the sympathetic part is concerned with preparing the animal for violent action and for withstanding abnormal conditions. The parasympathetic system is more concerned with reestablishing normal conditions in the animal once it has been involved in strenuous actions, so that it is ready once more to expend large amounts of energy if necessary. Parasympathetic nerves initiate peristalis (contraction of the gut) and stimulate the production of digestive juices by the associated glands. Parasympathetic fibres also supply the heart, lowering the frequency and strength of the heartbeat.

Autotomy - Deliberate shedding of part of a limb or tail when seized by an enemy. A lizard can part with its tail by snapping a vertebra by muscular action. Many arthropod limbs have a weak spot where they may break off. Limbs shed in this way usually grow again.

Aves - This class of chordates contains the birds. Their main features are the possession of feathers, a constant body temperature and the egg laying habit. The front limbs are modified as wings and the majority of birds can fly. All living birds are toothless.

Birds show traces of their reptilian ancestry in the scales on their legs and in their egg laying habit. The fossil bird Archaeopteryx, with teeth, clawed wings, and a long, feathered tail, shows a number of features intermediate between reptiles and birds and it is though that modern birds developed from creatures like this. All birds rely largely on sight to find food and to protect themselves and consequently we find that the optic lobe of the brain is well developed. The olfactory region (dealing with the sense of smell) is tiny in most birds. The jaws are extended beyond the head into beaks which vary considerably according to the diet of the bird. Feet are also varied according to the habits.

The classification of the class is described at the end of the book.

Axis - The second vertebra of the tetrapod spine.

Axon - The long, narrow part of a nerve cell.