66 MANUAL OF ZOOLOGY SECT. 



usually developed by modification of fibrous connective 

 tissue, the cells becoming distended with oily matter. 

 Cartilage is a firm but elastic material, readily cut with a 

 knife, which forms an important constituent of the skeleton 

 in higher animals. Bone differs from cartilage in being 

 much denser and harder, owing to its being strongly 

 impregnated with calcareous matter (carbonate and phos- 

 phate of lime). 



Muscular tissue is the material by means of which nearly 

 all the movements of the Metazoa are effected. It 

 consists of bundles of microscopic fibres, which in the living 

 condition have the special property of contractility, 

 contracting, i.e., becoming shorter and thicker, when 

 stimulated. Bundles or bands of these form the organs 

 known as muscles. Nerve tissue, which is the sensitive, 

 conducting and stimulating tissue of the body, consists of 

 nerve-cells and nerve-fibres ; groups of the former constitute 

 nerve-ganglia ; bundles of the latter form nerves. 



Associated with the multicellular character of the 

 Metazoa is the possession of a variety of different parts or 

 organs adapted for carrying on different functions in the life 

 of the animal. Such a formation of organs is faintly fore- 

 shadowed in the unicellular body of the Protozoa ; the 

 contractile vacuoles, the nucleus, the pseudopodia, flagella 

 and cilia, the gullet, etc , are all to be looked upon as organs 

 subserving certain functions. But in the Metazoa, with the 

 exception of some of the lower groups, the development of 

 organs for the carrrying on of the functions of animal life, 

 organs of locomotion, organs for protection and support, 

 organs of digestion, respiration, and reproduction is 

 carried much further. 



Some of the chief functions which are carried on in the 

 body of an animal have already been briefly referred to in 



