Without this period of rest they seem unable to complete their growth and development.

Diaphragm - Sheet of muscle separating the thorax from the abdomen in mammals.

Diaphysis - The shaft of a long bone (See Bone).

Diapsida - Sub class of Reptilia, containing the extinct dinosaurs and all living reptiles except the turtles and tortoises.

Dictyoptera - Order of Insecta, containing cockroaches and mantids. They have biting mouths and show many other primitive insect features. The front wings are leathery and are folded flat over the back. Eggs are laid in horny cases (oothecae) the they hatch into nymphs not unlike the adults. Cockroaches are scavenging insects, feeding on a wide range of materials. This is what makes them such pests in kitchens and warehouses. Mantids are carnivorous and their front legs are specially adapted for catching other insects. Most of the order are tropical or sub tropical.

Differentiation - Change in the structure of cells and organs during their development.

Digestion - The breakdown of complicated food substances into simpler compounds which can be used by the animal to build up its own body and to provide energy. Digestion is performed by enzymes. mainly in the food canal, and the products are absorbed in to the body. In some animals, however, food particles are actually taken into the cells and digestion occurs there (See Coelenterata). The actual enzymes concerned vary according to the animal and the type of food it eats.

Plant eating animals rely largely in bacteria to break down the cellulose that forms a large part of heir diet. Man is an omnivorous creature, eating all kinds of food. His digestive processes therefore embody' all the main features of mamalian digestion.Digestion begins in the mouth where the food is mixed with saliva. This watery fluid contains ptyalin an enzyme that acts on starch and converts it to maltose sugar. Food is also broken down mechanically by the teeth.

The rhythmic movement (peristalsis) of the oesophagus carries the food down to the stomach whose lining cells produce hydrochloric acid. This sterilises the food and also enhances the effect of another enzyme pepsin which is produced by certain cells in the stomach wall and which begins to break down proteins. Rennin is produced in the stomach too and this enzyme clots milk. The food is thoroughly mixed in the stomach and forms a thick paste called chyme.DIGESTIVE ENZYMES AND THEIR WORK

 

A muscular ring regulates the passage of food from the stomach into the small intestine where bile from the liver and pancreatic juice from the pancreas are mixed with the chyme. Salts in the bile split fat droplets into smaller particles so that they can be digested more easily. Pancreatic juice contains amylase which breaks down starch to sugar, and lipase which breaks up fats.

It also contains trypsinogen which itself has no effect on the food, but when the pancreatic juice mixes with the intestinal juices, enterokinase from the latter acts on the trypsinogen to form trypsin, a powerful proteinsplitting enzyme. The acidic chyme is neurtralised in the small intestine and gradually the food becomes more and more liquid. Proteins become broken down into amino-acids and carbohydrates into sugars all relatively small molecules that can pass through the gut wall and into glycerol and fatty acids which pass into special duct (lacteals) in the ridges of the intestinal wall and join the blood stream later. Almost all the absorption occurs in the small intestine. The large intestine is concerned mainly withthe reabsorption of water from the undigestible remains which pass out of the gut as faces.

Digitigrade - Walking on the whole of the toes as opposed to just the tips (unguligrade) or the sale of the foot (Plantigrade). Cats and dogs, among others, are of this type.

Dimorphism - The existence of a species in two distinct forms. Sexual dimorphism in which males and females differ occurs in all bi sexual animals of course but it is particularly marked in birds and many insects. Many butterflies show seasonal dimorphism where spring and summer broods differ in colour and / or size. The occurrence of melanic and non melanic forms is also an example of dimorphism.

Dinosaur - Name given to a large assemblage of extinct reptiles now divided into two distinct orders according to the structure of the pelvic girdle. The order Saurischia had a typical reptilian pelvic girdle but the order Ornithischia had a bird like pelvic girdle with a pre pubic hone pointing forwards. Surprisingly, perhaps, this division does not correspond with the division into quadrupedal and bipedal forms: each order contains quadrupedal and bipedal species Saurischians included huge animals such as.

Brontosaurus and Diplodocus which, at 80 feet long, were the largest land animals ever to have lived, but not all dinosaurs were large some were no bigger than modern lizards. Brontostaurus and Diplodocus were herbivorous but the order also includes the fierce Typrannosaurus, a carnivore some 50 feet long and with dagger like teeth more than six inches long. The Ornithischia including such forms as Iguanodon and Triceratops, were all herbivorous and never reached the huge size of some Saurischians. Dinosaurs became extinct, for some as yet unexplained reasonmore. than 70 million years ago.

Diploblastic -  Having a body wall of only two layers.

Diploid - Having a double set of chromosomes in the nucleus. Normal body cells are dipolid. Sex cells are formed by a special process in which only one set of chromosomes goes to each cell. Cells with only one set of chromosomes are called haploid. When sex cellsjoin together at fertilisation the diploid number is regained in the resulting zygote.

Diplopoda - Millipedes sub class of Myriapoda.

Dipnoi - Order of crossopterygian fishes including the living lung fishes and related fossil forms. There are three living genera Neoceratodus of Queensland. Australia; Lepidosiren of South America; and Protopterus of tropical Africa. They live in sluggish rivers and all of them can breathe air.

The African and American forms can survive drought by burying themselves in the mud (See Aestivation). Compared with the early dipnoans of Devonian times the living forms show a great reduction of bone: there are no vertebrae and the notochord remains throughout life.Neoceratodus has lobed fins typical of the crossopterygians but the paired tins of the other two genera arc reduced to thin filaments.

 Diptera - Order of insects containing the true flies which have only one pair of wings. The hind pair are modified as balancing organs called halteres. These can be well seen in crane-flies (daddy long legs) but are less conspicuous in other groups. It is a very large order with such diverse types as the mosquitoes, bluebottles, and the large robber flies. They all have sucking mouths, frequently adapted for piercing and blood sucking. Metamorphosis is complete and the larvae are always legless. Many larvae live ill water.

Distal - The far end, away from the body. For example, the hand is at the distal end of the arm. Opposite of proximal.

Dogfish - Small cartilage fish of the calass Elasmobranchii, which is often studied in schools as an introduction to vertebrate anatomy.

Dominant - A gene whose character will be apparent in an animal whether it (the gene) is present on one or both of its particular pair of chromosomes.

Dormant - In a resting condition (e.g) hibernating.

Dorsal - On or near the side of an animal that is normally uppermost. Man's back is his dorsal side, for in most mammals that side is on top. Opposite of ventrul.

Dorsal Aorta - (See Spinal Nerve)Dorsal Root. (See Odonata),Deodenum. First part of the small intestine.

Dura Mater - Tough connective tissue covering the brain and spinal cord of vertebrates.