GENERAL ORGANOLOGY. 145 



cirripcds their featherlikc feet, and the mussel can actively 

 close its shell. 



Locomotion among the Lower Animals. In the lower 

 forms, the protozoa, progression is accomplished almost ex- 

 clusively by projections of the cell ; these may be pseudo- 

 podia, cilia, or flagclla. In the multicellular animals, this 

 is only very rarely the case. Amoeboid movements of the 

 epithelial cells, indeed, occur in the ccelentsrates and also in 

 many ivorms, but does not suffice for change of position. 

 More effective is the ciliated or flagellated epithelium, 

 which in ctcnopJiorcs, turbcllarians, and rotifers renders 

 swimming movements possible; this occurs, besides, in 

 many larvae of animals which, in the mature state, are unable 

 to change their location or do so only by the aid of muscles. 

 In the form of the planula, i.e., as a larva swimming by 

 means of cilia, nearly all ccelenterates, eckinoderms, mollusks, 

 and the majority of the worms leave the egg-membranes. 



Locomotion among the Higher Animals. The mus- 

 culature is alone adapted for energetic activity. The 

 arrangement of this varies with and depends upon the con- 

 stitution of the skeleton. Forms without a skeleton have 

 commonly the " dermo-muscular tunic," a cylinder of circu- 

 lar and longitudinal muscle fibres which is firmly united with 

 the skin. If a skeleton is formed by the skin, as in the 

 arthropods, then the cylinder resolves itself into groups of 

 muscles, which find points of attachment upon the dermal 

 skeleton ; if, on the other hand, as in the vcrtebratcd 

 animals, an axial skeleton is formed, a fixed point is fur- 

 nished for muscular action, so that the musculature obtains 

 a quite new character, in particular coining to lie deeper. 

 A locomotory apparatus quite unique is the ambulacral 

 system of the echinoderms, a system of delicate little tubes 

 with protrusible portions which function as feet. 



II. Nervous System. 



Its Regular Development. Scarcely an organic system 

 in the animal series shows such a regular development as 

 the nervous system. The different stages which can be 



