Chap, x.j MOVEMENTS OF PROTOZOA. 371 



amorphous mass like an Amoeba is enabled, by the 

 withdrawal of one and the protrusion of another part 

 of its substance, to move about from place to place. In 

 the ciliated forms movement is due to the contractile 

 action or play of those delicate processes of proto- 

 plasm which form the cilia ; between the fine processes 

 that we ordinarily call by that name, and the coarser, 

 more lobate, processes that are distinguished as pseudo- 

 podia, the connection is very close, and under certain 

 conditions one form may be observed to pass into the 

 other. Among certain stalked Infusoria, such as 



O ' 



Vorticella, we observe a mode of movement which is 

 more rapidly executed than that of ordinary trans- 

 lation ; a Vorticella, or its branched ally, Carchesiuni, 

 may be seen to suddenly lower its bell, owing to the 

 rapid contraction in length of its stalk ; the agent by 

 which this is effected is a modified portion of the 

 protoplasm in the stalk (the so-called contractile band), 

 which presents a striatioii that calls to mind that of a 

 muscular fibre. Though agreeing with it functionally, 

 the stalk differs from it morphologically, in that it 

 is a modification of only part of a cell, and not of a 

 whole cell, or of a set of cells. These bands are not 

 confined to the stalked Infusoria, but are found in 

 other forms both of the Ciliata and of the Gregarinida; 

 without them, indeed, there can be but feeble move- 

 ments in the latter endoparasitic organisms, which are 

 without either cilia or pseudopodia. 



Among the lower Metazoa we find that the 

 movements of the young are at first effected not by 

 muscular tissue, but by cilia ; the free-swimming larva 

 being provided with cilia, which may be scattered 

 over the whole of the body, or confined to certain 

 definite and characteristic tracts, such as circlets, one 

 or more in number, or w^avy bands (Fig. 158). In all 

 groups, save that of the Porifera, the cilia are found 

 on the outer surface of the body or epiblast, and in 



