80 A PRIMER OF BIOLOGY 



the ciliary movements of many lower Algee and of 

 many reproductive cells in higher forms, such as 

 mosses and ferns, and the amoeboid movements of 

 the Slime Fungi, of the reproductive cells of many 

 lower Fungi and of Algae, &c. 



Naturally, it is in the animal rather than in the 

 plant world that we expect and find special organs 

 for locomotion. These are most varied in character 

 and comprise such types as the water-tube feet of 

 Organs of starfish and their allies, the jointed appendages of 

 motion. insects, crabs, lobsters, spiders, &c., the contractile 

 massive foot of the molluscs and the wings of all 

 grades of birds, bats, &c., the fins of fish and the 

 familiar limbs of mammals. Amongst these we 

 recognise three types, those adapted to terrestrial, 

 those adapted to aerial, and those adapted to aquatic 

 conditions, with, occasionally; as in the birds, ap- 

 pendages both for locomotion on land or in water 

 and for locomotion through the air. 



The subject of motion and locomotion in organisms 

 leads us naturally to the question of the skeleton or 

 Skeleton, hard parts, and to that subject we must devote the 

 rest of this chapter. 



The necessity for locomotion in search of food in 

 the animal is associated with the condensation of 

 the skeleton and the jointing of its various parts 

 whilst the uniform distribution of the skeleton in the 

 plant is associated with its fixed habit. We shall 

 see how this principle is exemplified and established 

 in the course of our discussion of the skeleton. 



Before discussing its composition let us, first of 



all, attempt to determine what functions the skeleton 



Functions fulfils by considering simple cases from the animal 



skeleton, world. Manifestly, it gives protection to soft parts. 



For example, the skull and vertebral column protect 



