THE ORGANS. 39 



c) Muscles. 

 § 32. 



The locomotion of the body exhibits itself in its simplest phase 

 as a change in the form of the body due to the contractility of 

 its protoplasm. When these changes in form follow one another 

 rapidly, and have all the same direction, the body either elongating 

 or sending out processes which attaching themselves to some fixed 

 point, are followed gradually by the rest of the semi-fluid body 

 (Rhizopoda), locomotion is effected. The difference between this 

 mode of locomotion and undefined change of form is seen to be 

 merely one of degree. The contractility of protoplasm may pro- 

 duce changes in position even when it is invested by a differentiated, 

 though soft, integument. This layer of integument will follow the 

 movements of the body it invests. In such cases, and they are very 

 common among the Protista, special organs of locomotion cannot be 

 said to exist, for the cilia have other functions to perform for the 

 organism in addition to locomotive ones ; such, for example, as that 

 of aiding in the ingestion of food. 



Specific organs of locomotion make their first appearance when 

 the contractile morphological elements known as muscle-fibres are 

 differentiated; these, in the simplest case, form a muscular layer 

 lying beneath the ectoderm. 



The genesis of this earliest musculature of the body is due to a 

 differentiation of the ectoderm (Hydroid polyps), the cells of which 

 give off flattened processes, which form a continuous layer of 

 contractile fibres. 



Each individual ectoderm-cell concerned in the formation of this 

 layer of fibres represents accordingly a sensory apparatus, which 

 stands in direct continuity with a contractile apparatus. The cell is 

 indeed replaced, when the musculature is differentiated, by groups of 

 muscles which work so as to balance one another, and completely 

 harmonise in their action (compare Sect. 31). We cannot yet say how 

 far this process, winch gives so deep an insight into the mode of 

 differentiation of the tissues as well as of the organs, is repeated in 

 the ontogeny of the higher forms of animals. In all divisions above 

 the Coelenterata we always find the separation between ectoderm 

 and muscle complete. It may therefore be doubted whether a 

 process of the kind described in the Hydroid polyps always accom- 

 panies the origin of the muscular system. But yet it is very 

 probable that something of the kind does occur. Even though the 

 process of differentiation in the higher organisms does not enable 

 us to recognise these processes in their case, yet it is not to be 

 assumed without further reason that the mode of origin of the 

 muscular tissue was in them primitively different, for Ontogeny 

 very seldom repeats phylogenetic processes in every detail. 



