CHAPTEE III. 



OF MOVEMENTS IN THE ANIMAL KINGDOM. 



Without, as we have seen, specially appertaining to the animal 

 kingdom, motility is nevertheless a property infinitely more 

 spread and developed in the animal kingdom than in the vegetal 

 kingdom. There is scarcely any animal species which is not 

 more or less endowed therewith ; but it is not a property inherent 

 in organised matter : for many histological elements are destitute 

 thereof, and when the animal is perfectionated, differentiated in 

 some small degree, motility is the attribute and the function of 

 a special tissue, at least in its most perfect mode. 



At the lowest degree of animality, when all is still confused 

 in the living substance, it is the whole body of the animal which 

 is constituted by a substance, contractile, homogeneous, chang- 

 ing form perpetually, emitting and retracting expansions unceas- 

 ingly. This contractile, amorphous substance has been called 

 sarcode. It is sarcode which forms exclusively the body of the 

 amoebae of the lowest monerians {Bathyhius haeckeUi), also of the 

 rhizopods which move in emitting and retracting sarcodical 

 expansions. 



The first effort of differentiation seems- to be the formation of 

 vibratile cilia. Here the mobile expansions are no longer 

 transitory. They have a fixed, definite form. They are per- 

 sistent organs, constituting, as we have seen, the principal agent 

 of locomotion in the infusoria. 



In 'the hydroids, the hydra properly so-called excepted, there 



