54 THE EVOLUTION OF THE META20A 



and which lives in the sea sand, has a polyp form; its surface is 

 covered with cilia w hich enable the animal to glide actively 

 and move in this way among small grains of sand (see Fig. 8C). 

 I have succeeded in showing (Hadzi 1959) that Halammobjdra 

 actually corresponds to a strongly modified Narcomedusa 

 whose muscular swimming apparatus has been much reduced 

 w^hile at the same time secondary cilia have developed. It is 

 certain that something similar could also be found in other 

 animal groups. 



The second type of locomotion is based on contractions of 

 muscular fibres w^hich had developed within the cellular cy- 

 toplasm. This specialized form of a general ability of the cyto- 

 plasm to contract has already been "invented" by Protozoa. 

 Metazoa have inherited it fully developed from their pro- 

 tozoan ancestors. These contractile fibres served first to make 

 possible movement on a fixed spot, bowing, the contraction 

 of stem-shaped forms, the constriction of cavities, e.g. of the 

 peristomial ciliary apparatus in the peritrichous and epitrichial 

 Infusoria (also in sphincters), etc. As a secondary result 

 and parallel to an increase of the size of the body, locomotion 

 by means of ciUa is given up, and the muscular movements of 

 the body begin to be used to effect locomotion. This can be 

 observed as early as in the larger flat Turbellaria, particularly 

 in Polycladida. The highest level in the development of the 

 means of locomotion in free water is reached in medusae which 

 possess special transverse stripes in their muscular fibres: in 

 these, movement is effected by means of regular rhythmic con- 

 tractions w^hich are controlled and led by nerves and which 

 remind us already of the pulsation of the heart (the latter is 

 first developed in MoUusca). 



A higher type of locomotion by means of contraction of 

 muscles has been developed with the formation of extremities. 

 Here we are less interested in this type because it has not been 

 developed in Ameria. 



The type of locomotion used by an animal is very important 

 and the symmetrical conditions in animals depend on it to 



