Planula

(Lat. planus, flat). The oval ciliated embryo of certain of the Invertebrates.

Plastron

The lower or ventral portion of the bony case of the Chelonians.

Platyelmia

(Gr. platus, broad ; and helmins, an intestinal worm). The division of Scolecida comprising the Tape-worms, etc.

Platyrhina

(Gr. platus, broad; rhines, nostrils). The group of the Quadrumana.

Pleura

(Gr. the side). The serous membrane covering the lung in the air-breathing Vertebrates.

Pleuron

(Gr. pleuron, a rib). The lateral extensions of the shell of Crustacea.

Pluteus

(Lat. a pent-house). The larval form of the Echinoidea.

Pneumatic

(Gr. pneuma, air) Filled with air.

Pneumatocyst

(Gr. pneuma, air; and kustis, cyst). The air-sac or float of certain of the Oceanic Hydrozoa (Physophoridoe).

Pneumatophore

(Gr. pneuma, air ; and phero, I carry). The proximal dilatation of the coenosarc in the Physophoridoe which surrounds the pneumatocyst.

Pneumoskeleton

(Gr. pneuma; and skelelos, dry). The hard structures which are connected with the breathing organs (e.g., the shell of Molluscs).

Podophthalmata

(Gr. pous, foot; and ophthalmos, eye). The division of Crustacea in which the eyes are borne at the end of long foot-stalks.

Podosomata

(Gr. pous, foot; soma, body). An order of Arachnida.

Poephaga

(Gr. poe, grass, phago, I eat). A group of the Marsupials.

Pollex

(Lat. the thumb). The innermost of the five normal digits of the anterior limb of the higher Vertebrates. In man, the thumb.

Polychaeta

(Gr. polus, many; chaite, bristle). A name often applied to. the Tubicolar and Errant Annelides to distinguish them collectively from the Oligochaeta (Earthworms, &c).

Polycystina

(Gr. polus, many; and kustis, a cyst). An order of Protoza, with foraminated siliceous shells.

Polygastrica

(Gr. polus; and gaster, stomach). The name applied by Ehrenberg to the Infusoria, under the belief that they possessed many stomachs.

Polypary

The hard chitinous covering secreted by many of the Hydrozoa.

Polype

(Gr. polus, many; pous, foot). Restricted to the single individual of a simple Actinozoon, such as a Sea-anemone, or to the separate zooids of a compound Actinozoon. Often applied indiscriminately to any of the Coelen-terata, or even to the Polyzoa.

Polypide

The separate zooid of a Polyzoon.

Polypidom

The dermal system of a colony of a Hydrozoon or Polyzoon.

Poltpite

The separate zooid of a Hydrozoon.

Polystome

(Gr. polus, many ; and stoma, mouth). Having many mouths; applied to the Acinetae amongst the Protozoa.

Polythalamous

(Gr. polus; and thalamos, chamber). Having many chambers ; applied to the shells of Foraminifera and Cephalopoda.