Glossary 679 



N 



Nasopharynx— the upper part of the pharynx. 



Neck — a constricted part. 



Nekton— the larger animals of a body of water that are able to swim against 



the currents. 

 Nemathelminthes — the phylum of animals that contains the nematodes. 

 Neurilemma— the outer sheath of a nerve fiber. 

 Node — a swelling. 



Notochord — a dorsal cartilaginous supporting rod found in chordates. 

 Nucleolus — a small body found in the nucleus of a cell. 

 Nucleus — the controlling part of the cell. 



O 



Olfactory — pertaining to the sense of smell. 



Omniverous — feeding on foods of both plant and animal origin. 



Oogenesis— the process by which the egg is formed. 



Oral — pertaining to the mouth. 



Order- in classification, a category more inclusive than family and less than class. 



Organ — a structure adapted to perform a specific function or functions. 



Organelle — a specialized part of a protozoan cell performing a specific function. 



Organogeny — the embryological development of organs. 



Origin — the attachment of a muscle to the part that is not moved during 

 contraction. 



Orthogenesis — the development of organisms in a direct line. 



Osmosis — the passage of materials through a semipermeable membrane. 



Osteoblasts — bone-building cells. 



Osteoclasts — bone-destroying cells. 



Otolith — calcareous concretions found in sense organs concerned with maintain- 

 ing balance. 



Oviparous — laying eggs that subsequently hatch. 



Ovoviviparous — retaining the eggs within the body of the parent until hatched. 



Ovum — the egg ; the female germ cell. 



Paedogenesis — reproduction by larval animals. 



Palaeontology — the study of the life of past geologic periods. 



Palate — the roof of the mouth. 



Pancreas — a large compound gland present in most vertebrates. 



Papilla — a rounded projection. 



Parasitism — a condition in which two organisms are associated and only one 



benefits. 

 Parasympathetic — a physiological division of the autonomic nervous system. 

 Parenchyma — a soft connective tissue that fills spaces between organs in some 



animals. 

 Parietal — pertaining to the walls of a structure or cavity. 

 Parthenogenesis — reproduction by means of an unfertilized egg. 

 Pectoral girdle — a group of bones that supports the forelimbs. 



