Peritoneal membrane see peritoneum

peritoneal sheath (ARTHRO: Insecta) A network of anastomosing muscle fibers that holds together the ovarioles of the ovary.

peritoneum n. [Gr. peri, around; tonos, strain; eous, composed of] A thin serous membrane lining the body cavity; covering and supporting the organs. peritoneal a.

peritreme n. [Gr. peri, around; trema, hold] 1. (ARTHRO: Che-licerata) In Acari, a concave plate surrounding a stigma (=spiracle). 2. (ARTHRO: Insecta) An annular sclerite surrounding a spiracle. 3. (MOLL: Gastropoda) see peristome.

peritrophic membrane (ARTHRO) An extracellular sheath in which chitin is present, separating the apical surface of the mid-gut that protects the gut cells from mechanical damage caused by abrasive food particles; usually loosening from the mid-gut and remaining around the food, passing with the feces.

perivisceral a. [Gr. peri, around; L. viscus, entrail] Surrounding the viscera.

permanent haplometrosis (ARTHRO: Insecta) A colony that is founded by a single female whose initial offspring are sterile females, then later towards the end of the annual cycle reproductives are produced. see temporary haplometro-sis, functional haplometrosis.

permanent hybrid Hybrid that maintains its heterozygosity by balanced lethal factors in its genotype.

permanent parasite A parasite living its entire adult life within or on a host.

permanent pleometrosis (ARTHRO: Insecta) In social Hymen-optera, the foundation of colonies through swarming. see primary pleometrosis.

permesothyridid foramen (BRACHIO) A pedicle opening found mostly within the ventral umbo.

peronium n.; pl. -ia [Gr. perona, fibula] (CNID: Hydrozoa) Thick epidermal tract from the base of the tentacle on to the bell.

peroral a. [L. per, through; os, mouth] By way of, or through the mouth.

perradius n.; pl. -ia [L. per, through; radius, ray] In Echino-dermata and Cnidaria, body parts and organs located along a limited number of radial planes; primary or 1st order radius.

pervious a. [L. per, through; via, way] Perforate or open.

petaloid a. [Gr. petalon, leaf] Resembling petals.

petasma n. [Gr. petasma, something spread out] (ARTHRO: Crustacea) Complex male copulatory organs with coupling hooks on the first pair of pleopods.

petiole n. [L. petiolus, little foot] 1. A stock or stem. 2. (ARTHRO: Insecta) a. In Hymenoptera Apocrita, the narrow constricted zone at the base of the gaster. b. In Formici-dae, a one or two segmented pedicel. petiolate a., petio-liform a.

petraliiform colony (BRYO: Gymnolaemata) In Cheilostomata, encrusting unilaminate colony loosely attached by the protruding basal walls of zooids or by basally budded kenozo-oids.

petricolous a. [L. petra, rock; colere, to inhabit] Dwelling within stones, crevices or in hard clay.

pH A symbol of a scale measuring the acidity or alkalinity of a medium, with a value of 7.0 indicating neutral, lower values indicating acidity, and higher values indicating increased values of alkalinity.

phacella; sing. -um [Gr. phakellos, bundle] (CNID: Scy-phozoa) Tentacle-like gastric filaments covered with gas-trodermis, nematocysts and gland cells.

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