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 limey matter (carbonate and phosphate 

 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 condi- 

 tion 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 nej've-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 to carrying out 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 carrying 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 



