308 GLOSSAEY. 



Alternation of Generations. — This term is applied to a peculiar 



mode of reproduction which prevails among many of the lower 



animals, in which the egg produces a living form quite different 



from its parent, but from which the parent-form is reproduced 



by a process of budding, or by the division of the substance 



of the first product of the egg. 



Ammonites. — A group of fossil, spiral, chambered shells, allied to 



the existing pearly Nautilus, but having the partitions between 



the chambers waved in complicated patterns at their junction 



with the outer wall of the shell. 



Analogy. — That resemblance of structures which depends upon 



similarity of function, as in the wings of insects and birds. 



Such structures are said to be analogous, and to be analogues of 



each other. 



Animalcule. — A minute animal : generally applied to those visible 



only by the microscope. 

 Annelids. — A class of worms in which the surface of the body 

 exhibits a more or less distinct division into rings or segments, 

 generally provided with appendages for locomotion and with 

 gills. It includes the ordinary marine worms, the earthworms, 

 and the leeches. 

 Antenna. — Jointed organs appended to the head in Insects, 



Crustacea and Centipedes, and not belonging to the mouth. 

 Anthers. — The summits of the stamens of flowers, in which the 



pollen or fertilising dust is produced. 

 Aplacentalia, Aplacentata or Aplacental Mammals. See Mam- 



malia. 

 Archetypal. — Of or belonging to the Archetype, or ideal primi- 

 tive form upon which all the beings of a group seem to be 

 organised, 

 Articulata. — A groat division of the Animal Kingdom character- 

 ised generally by having the surface of the body divided into 

 rings called segments, a greater or less number of which are 

 furnished with jointed legs (such as Insects, Crustaceans and 

 Centipedes). 

 Asymmetrical. — Having the two sides unlike. 

 Atrophied. — Arrested in development at a very early stage. 



Balanus. — The genus including the common Acorn-shells which 



live in abundance on the rocks of the sea-coast. 

 Batrachiaxs. — A class of animals allied to the Keptiles, but 



