GLOSSARY. 437 



Mysis-stage. — A stage in the development of certain Crustaceans 

 (Pi awns), in which they closely resemble the adults of a genus (Jfysis) 

 belonging to a slightly lower group. 



Nascent. — Commencing development. 



Natatory. — Adapted for the purpose of swimming. 



Nauplius-form. — The earliest stage in the development of many Crustacea, 

 especially belonging to the lower groups. In this stage the animal has 

 a shori body, with indistinct indications of a division into segments, 

 and three pairs of fringed limbs. This form of the common fresh-water 

 Cyclops was described as a distinct genus under the name of Nauplius. 



Neuration. — The arrangement of the veins or nervures in the wings of 

 Insects. 



Neuters. — Imperfectly developed females of certain social insects (such as 

 Ants and Bees), which perform all the labours of the community. 

 Honce they are also called workers. 



Nictitating Membrane. — A semi-transparent membrane, which can be 

 drawn across the eye in Birds and Reptiles, either to moderate the 

 effects of a strong light or to sweep particles of dust, &c., from the sur- 

 face of the eye. 



Ocelli. — The simple eyes or stemmata of Insects, usually situated on the 



crown of the head between the great compound eyes. 

 (Esophagus. — The gullet. 

 Oolitic. — A great series of secondary rocks, so called from the texture of 



some of its members, which appear to be made up of a mass of small 



egg-like calcareous bodies. 

 Operculum. — A calcareous plate employed by many Mollusca to close the 



aperture of their shell. The opercular valves of Cirripedes are those 



which close the aperture of the shell. 

 Orbit. — The bony cavity for the reception of the eye. 

 Organism. — An organised being, whether plant or animal. 

 OrthOSPermOUS. — A term applied to those fruits of the Umbelliferse 



which have the seed straight. 

 Osculant. — Forms or groups apparently intermediate between and con- 

 necting other groups are said to be osculant. 

 Ova.— Eggs. 

 Ovarium or Ovary (in plants). — The lower part of the pistil or femaic 



organ of the flower, containing the ovules or incipient seeds ; by growth 



after the other organs of the flower have fallen, it usually becomes 



converted into the fruit. 

 Ovigerous. — Egg-bearing. 

 Ovules (of plants). — The seeds in the earliest condition. 



Pachyderms. — A group of Mammalia, so called from their thick skins ; 



and including the Elephant, Rhinoceros, Hippopotamus, &c. 

 Paleozoic. — The eldest system of fossiliferous rocks. 

 Palpi. — Jointed appendages to some ->f the organs of the mouth in Insects 



and Crustacea 



