CHAPTER XII. 



MOVEMENTS AND LOCOMOTION IN THE INVERTEBRATA. 



In this chapter we give an account of locomotion and other 

 movements in the Invertebrata. 



There is scarcely any species in the animal kingdom, which 

 is not more or less endowed with the power of movement or 

 motility ; but it is not essentially a property inherent in 

 organised matter ; for many histological elements are destitute 

 of it, and when an animal is only differentiated in a small 

 degree, motility is the attribute and the function of a special 

 tissue, at least in its most perfect mode. 



In the lowest animals, where there is no differentiation 

 of parts, the whole body is constituted of a substance which 

 is contractile and which changes its form perpetually — 

 emitting and retracting pseudopodia unceasingly. The 

 pseudopodia are the first organs of motion ; but they are 

 simply expansions of the substance of the body — viz., the 

 sarcode. The first effort of differentiation appears to be the 

 formation of cilia and flagella. In this case these expansions 

 are no longer transitory ; for they have a fixed and definite 

 form. They are persistent organs, constituting the principal 

 organs of locomotion in the Infusoria. As we ascend in the 

 zoological scale muscles become differentiated ; and by the 

 alternate shortening and lengthening of these muscles, move- 

 ments of the body are brought about. Muscles are present 

 in all but the simplest animals — i.e., in all animals higher 

 than the Protozoa and Porifera. 



