Pineal Apparatus. Primitively consists of two outgrowths of roof of fore brain lying within skull, one behind the other (though may represent right and left of a pair). Anterior is parietal organ which forms an eye-like structure (pineal eye,) in some lizards and Sphenodon, a vestigial eye in lamprey (Cyclostomata), and is reduced or absent in other vertebrates.

Posterior forms a second, functional eye-like structure in lamprey, and the glandular pineal body or epiphysis of other vertebrates.

Pineal Body. (Epiphysis) Often assumed to secrete a hormone concerned with colour change; and melatonin, which strongly concentrates pigment within melanophores lightening colour, can be extracted from pineal body. Melatonin in mammals is concerned with the diurnal rhythm of activity, being secreted more in the dark than in the light.

Pithecoidea. Anthropoidea.

Pitressin. Extract of posterior lobe of pituitary,  causing when injected constriction of capillaries and arterioles (which raises arterial pressure) and diminished urine formation.

Placenta. (1) The organ in mammals and other viviparous animals by means of which the embryo is attached to the wall of the uterus, is composed of embryonic and maternal tissues: extensions of the chorion and allantois grow into the uterine wall so that so that materials (e.g. oxygen, nutrients) can pass between the blood of the embryo and its mother (there is, however, no direct connection between the maternal and embryonic blood). The placenta is eventually expelled as part of the afterbirth.

(2) A ridge of tissue on the ovary wall of flowering plants to which the ovules are attached. The arrangement of ovules on the placenta (placentation) is variable, depending on the number of carpels and whether they are from (sea apocarpy) or fused.