316 GLOSSARY. 



Melanism. — The opposite of albinism ; an undue development of 

 colouring material in the skin and its appendages. 



Metamorphic Rocks. — Sedimentary rocks which have undergone 

 alteration, generally by the action of heat, subsequently to 

 their deposition and consolidation. 



MoLLuscA. — One of the great divisions of the Animal Kingdom, 

 including those animals which have a soft body, usually 

 furnished with a shell, and in which the nervous ganglia, or 

 centres, present no definite general arrangement. They are 

 generally known under the denomination of " shell-fish ; " the 

 cuttle-fish, and the common snails, whelks, oysters, mussels, 

 and cockles, may serve as examples of them. 



Monocotyledons, or Monocottledonous Plants. — Plants in 

 which the seed sends up only a single seed-leaf (or cotyledon) ; 

 characterised by the absence of consecutive layers of wood in 

 the stem (endogenous growth), by the veins of the leaves being 

 generally straight, and by the parts of the flowers being gener- 

 ally in multiples of three. {Examples, Grasses, Lilies, Orchids, 

 Palms, &c.) 



Moeaines. — The accumulations of fragments of rock brought down 

 by glaciers. 



Morphology. — The law of form or structure independent of 

 function. 



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

 (Prawns), in which they closely resemble the adults of a genu? 

 (Mysia) 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 short 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 NaupUus. 



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. Hence they are also called workers. 



Nictitating Membrane.— A semi-transparent membrane, which 



