Glossary, continued. 



OVOVIVIPAROUS— Refers to animals whose eggs 

 are fertilized, developed, and hatched inside the fe- 

 male, but receive no nourishment from her. See 

 OVIPAROUS and VIVIPAROUS. 



PALP— An organ attached to the head appendages of 

 various invertebrates; usually associated with feeding 

 functions. 



PARALARVA— A cephalopod mollusc in its first post- 

 hatching growth stage that is pelagic in near-surface 

 waters during the day, and that has a different life mode 

 than older conspecifics. 



PARASITISM — An obligatory association where one 

 species (parasite) feeds on, or uses the metabolic 

 mechanisms of the second (host). Unlike predators, 

 parasites usually do not kill their hosts, although hosts 

 may later die from secondary causes that are related to 

 a weakened condition produced by the parasite. Para- 

 sitism may also be fatal when high parasite densities 

 develop on or in the host. 



PARTS PER THOUSAND— A standard unit for mea- 

 suring salinity, abbreviated as %o or ppt. 



PARTURITION— The act of giving birth, e.g., the live 

 birth of bull shark pups. Compare to SPAWN. 



PATHOGEN— A microorganism or virus that produces 

 disease and can cause death. 



PEDIVELIGER— The larval stage of bivalves during 

 which a functional pedal (footlike) organ develops. 



PELAGIC— Pertaining to the water column, or to or- 

 ganisms that live in the water column and not near the 

 bottom. 



PELAGIVORE— A carnivore that feeds in the water 

 column. 



PELECYPODA— A synonym for the mollusc class 

 BIVALVIA. 



PHOTOPERIODISM— The responses of an organism 

 to changes in light intensity or in length of days; e.g., 

 seasonal and cyclic events such as migrations or 

 reproductive cycles of animals. 



PHOTOTROPH— An organism (e.g. phytoplankton and 

 other plants) using sunlight as a source of energy to 

 synthesize organic matter. Compare with AU- 

 TOTROPH and HETEROTROPH. 



PHYLLOSOMA— The larval stage of lobsters, being a 

 broad, thin, schizopod larva. 



PHYLOG EN Y— Refers to evolutionary relationships 

 and lines of descent. 



PHYTOPLANKTON— Microscopic plants and plant- 

 like protists (algae) of the epipelagic and neritic zones 

 that are the base of marine food webs. They drift with 

 currents, but may have some ability to control their 

 level in the water column. See ALGAE and DIATOMS. 



PISCIVOROUS— Refers to a carnivorous animal that 

 eats fish. 



PLANKTIVOROUS— Refers to an animal that eats 

 phytoplankton and/or zooplankton. 



PLANKTON— Microscopic aquatic plants, animals, and 

 protists have limited means of locomotion and drift with 

 currents. See PHYTOPLANKTON and ZOOPLANK- 

 TON. 



PLANTIGRADE— A young, newly settled post-larval 

 clam. 



PLEOPODS — Paired swimming appendages on the 

 abdomen of crustaceans. 



PNEUMATOPHORE— A root rising above the level of 

 water or soil and acting as a respiratory organ in some 

 trees (e.g., mangroves). 



POLYCHAETA— A class of segmented, mostly ma- 

 rine, annelid worms that bear bristles and fleshy ap- 

 pendages on most segments. 



POLYHALINE— A category in the Venice system of 

 estuarine salinity classification; water with salinity of 1 8 

 to 30 parts per thousand (%o). 



POPULATION— All individuals of the same species 

 occupying a defined area during a given time. Environ- 

 mental barriers may divide the population into local 

 breeding units (demes) with restricted immigration and 

 interbreeding between the localized units. See SPE- 

 CIES, SUBSPECIES, and SUBPOPULATION. 



POSTLARVA— larva following the time of absorption 

 of yolk; applied only when the structure and form 

 continue to be strikingly unlike that of the juvenile. 



349 



