410 BIOLOGY 



nephridia (Gr. nephros = kidney). The organs of the earthworm which 

 excrete nitrogenous waste, 161. 



nerve fibers. The separate fibers of which a nerve is composed, 170. 



nerve impulse, 314. 



nerves, 163, 194. 



nervous system of earthworm, 162; of frog, 192; of Hydra, 146. 



netted-veined leaves. Those with veins branching into a network, 114. 



nettle-hairs, 117. 



nettling cells. See nematocysts. 



neural arch (Gr. neuron = a nerve). The arch of bones on top of the ver- 

 tebrae, inclosing the neural foramen, 177. 



neuroglia (Gr. neuron = a nerve + gloios = glue). The connective frame- 

 work of the nervous system. 



neurons (Gr. neuron = a nerve). The nerve cells which are the units of 

 the nervous system, 169, 195. 



nictitating membrane (Lat. nictare = to wink). A semitransparent, inner 

 eyelid in the frog and some other animals, 175. 



nidamental glands (Lat. nidus = a nest). Glands connected with th<> 

 oviduct, that secrete the covering of eggs, 201. 



Nitetta, 29. 



nitrification. The production of nitrates in the soil from simpler nitrogen 

 compounds. 



nodes. The places on a stem where branches arise. 



nostrils. Openings into the nasal cavities; nares, 175, 186. 



notochord (Gr. notos = the back -f chorde = a string). A rod in the back 

 of vertebrate embryos that develops into the spinal column, 286. 



nucleolus. A small body in the nucleus of a cell, with unknown functions, 

 32. 



nucleoplasm. See karyoplasm. 



nucleus. The vital center of a cell, containing chromatin and controlling 

 constructive metabolism, 29, 32, 45. 



occipital bones. The skull bones which surround the foramen magnum, 

 180. 



occipital condyles. The rounded protuberances by which the skull articu- 

 lates with the first vertebra, 181. 



oesophagus. The tube from the throat to the stomach or crop, or from 

 the mouth into the body, 60, 158. 



olfactory lobes. Two small lobes of the brain in front of the cerebrum, 193. 



olfactory nerve. The first of the cerebral nerves, supplying the olfactory 

 sacs, 194, 196, 198. 



olfactory sacs. Minute sacs in the nasal cavities; the seat of the sense of 

 196, 198. 



