Images Dated 21st March 2003

Spiral shape of a fossilised ammonite shell

Ammonite. Fossilised spiral shell of an ammonite, an extinct squid-like cephalopod animal (sub- class: Ammonoidea) related to the living nautilus. The outer ridges of the shell are seen. Internally the shell was divided into chambers; by pumping water into and out of the chambers the animal could move down and up in the water. This sea creature lived in the outer chamber of the shell. Many species of ammonite flourished from the Devonian period (400 million years ago) until becoming extinct at about the same time as the dinosaurs at the end of the Cretaceous period (about 65 million years ago). This specimen was found in the Sahara desert, once a sea

© Martin Bond/Science Photo Library

Lungs, CT scans

Lungs. Sequence of six coloured axial CT (computed tomography) scans through the upper chest of a 52- year-old woman. The front of the chest is at top, and the right of the body is at left. The sequence moves down through the body as it runs from left- right along top then bottom. The two lungs (dark blue) and their branching airways (bronchioles, light blue) are separated by the mediastinal area (centre). This contains the trachea (blue circle) that splits into the two lung bronchi (images 4- 6). Major blood vessels (orange, upper centre) lead to the top of the heart (seen in images 4-6). The bone (dark orange) of the spine (lower centre) and ribs (surrounding the lungs) is also seen

© PASIEKA/SCIENCE PHOTO LIBRARY

DNA helices

DNA helices. Models showing the double helix and nucleotide base structure of DNA (deoxyribonucleic acid) molecules. The double helix is formed by two spiralling strands of sugar phosphates. Nucleotide bases (red, blue, yellow, green) are arrayed along these strands. The four bases can form two bonding pairs: adenine bonds to thymine and cytosine bonds to guanine. This complementary base pairing holds the two strands together to form the DNA ladder'. DNA is found in cell nuclei, and the sequence of its nucleotide bases forms the genetic code for protein synthesis and the functioning and growth of every living organism

© Lawrence Lawry/Science Photo Library