In the obitect type the wings are stuck to the body and immovable, as in the chrysalis of a butterfly or moth; in the coarctate type an exarate  pupa develops within a hard barrelshaped puparium, as in the housefly and other Diptera.

Pure line. A population of plants or animals all having a particular feature that has been retained unchanged through generations. The organisms are homozygous and are said to ‘breedtrue’ for feature concerned.

Purkinje cells. Conspicuous pear – shaped nerve cells with extensive dendrities in cerebellar cortex, forming only axonal output.

Pustual. A blister – like area on a plant structure.

Putrefaction. Type of bacterial decomposition of protein containing substrate (largely anaerobic) with formation of bad-smelling amines rather than ammonia.

Pyranose ring. A monosaccharide with a six membered ring structure.

Pyrenoid. Small, spherical, highly refractive protein bodies found singly or in numbers embedded in chloroplasts of many algae but absent in majority of brown and red algae, in green algae associated with starch synthesis and surrounded by starch deposits.

Pyridoxine (Vitamin B₆). Vitamin of B group, forming a coenzyme (pyridoxal phosphate), important for many metabolic reactions; required by variety of organisms. e.g. some of the yeasts, insects, bacteria, birds, and mammal.

Pyridiumi. Type of capsule.

Pyrrole. A porphyrin building block that has a fine-membered heterocyclic structure and contains nitrogen.