172 GENERAL BIOLOGY 



N 



nematocyst. A stinging cell of Ccelenterates. 



nephridium. A much coiled tube serving as a kidney in 



annelids and other invertebrates. Typically they are 



paired segmental organs. 



nucleolus. A small dense spot within the nucleus, 

 nucleus. The central, usually spherical, portion of the cell 



which contains the chromatin. 



ocellus. A simple eye of an arthropod. 



operculum. A lid which closes the aperture of the shell of 



some snails; the covering of the gills of a fish; the 



skin which overgrows the gills of the tadpole, 

 oral. Pertaining to the mouth, opposed to aboral. 

 organ. A part of an organism capable of performing some 



special action which is essential to the life of the whole, 

 organism. An organized being or living body capable of 



independent existence; often the organism is com- 

 posed of organs. 

 osculum. The excurrent opening in the sponge through 



which water passes to the exterior, 

 ostiura. A mouth-like opening, as the pores on the outer 



surface of the sponge, and the pores in the heart of 



arthropods, 

 ovary. The organ which produces the ova or female sex 



cells, 

 oviduct. The duct or passageway from the ovary to the 



outside of the body, 

 ovipositor. The organ by which many insects deposit their 



eggs. 

 ovum. The egg or female sex cell. 



