Perigynous. Having a flat or concave structure on which the sepals, petals and stamens are borne.

Perilymph. The fluid of the inner ear that fills the space between the bony labyrinth and the membranous labyrinth compare endolymph.

Periodontal membrane. The membrane of connective tissue that surrounds the root of a tooth and anchors it to its socket in the jawbone. Fibres of the periodontal membrane pass into the cement coverings the root, which provides a firm attachment.

Peripheral nervous system. All parts of the nervous system excluding the central nervous system. It consists of all the cranial and spinal nerves and their branches, which link the receptors and effectors with the central nervous system. (autonomic nervous system).

Perisperm. A nutritive tissue found in some seeds that is derived from the nucellus of the ovules.

Perissodactyla. An order of mammals having hoofed feet with an odd number of toes. They are all herbivores and include the tapirs, rhinoceros, and horse.

Peristome.
(1) A ring of toothlike structure near the opening of a mass capsule. The teeth tend to bend and twist in dry weather, thus opening the mouth of the capsule and allowing the spores to escape. In wet weather they close over the opening of the capsule.

(2) The area around the mouth in many invertebrates. It sometimes assists in food collecting. Examples are the spirally ciliated groove around the mouth of some ciliate protozoans and the first segment of th earthworm.

Perithecium. A type of fruiting body found in the ascomycete fungi.

Peritoneum. The thin layer of tissue (see serous membrane) that lines that abdominal cavity of vertebrates and covers the abdominal organs.

Permanent teeth. The second and final set of teeth that mammals produce after shedding the deciduous teeth. An adult human normally has 32 permanent teeth, consisting of incisors, canines, molars, and premolars (see dental formula). These usually appear between the ages of approximately 6 and 21 years.