CNIDARIA AS THE ONLY COELENTERATA 55 



a large extent. We meet with this problem immediately when 

 we compare the first opposing pair: the mobility in contact 

 with the firm ground (vagility), and the immobility, the per- 

 manent attachment to the ground, and a stronger contractility 

 (sessility). Whenever we see that the free mobility has been 

 abandoned, a phenomenon w^hich can be observed quite often 

 in Protozoa as well as in Metazoa, we can regularly observe 

 how a simultaneous change of the carriage of the animal takes 

 place. The consequences of this change are great. A charac- 

 teristic property of animals which move or glide over a firm 

 surface is the bilateral symmetry of their bodies. The animal 

 moves forw ard with its anterior end placed in the direction of 

 the movement; this is the way the head region begins to 

 develop and with it, the great contrast between the anterior 

 and posterior ends of the body. The contacts of the two ends 

 with the environment are widely different. An almost equally 

 great difference can be observed in the contacts of the dorsal 

 and ventral sides and their environments: the flattened ventral 

 side touches the ground over which the animal moves while 

 the slightly vaulted dorsal side has contact with free water 

 only. The contacts of the two lateral sides (the left and 

 the right sides) are basically identical and this is why these two 

 sides are similar. In this way the bilateral organization and 

 form, and thus the bilateral symmetry, have been evolved. 



Bilateral symmetry exists in the whole animal world and it 

 must be considered as primitive All the other types of symmetry 

 can be deduced from the bilateral symmetry. The changes 

 in this symmetry are always due to the changes in the relation- 

 ship between the type of locomotion and the environment; 

 they can all be explained as modifications of the bilateral 

 symmetry. 



The most frequent change, which occurs as a more or less 

 distinct disturbance, is that of asymmetry. It is rather rare that 

 we find the assymetry to be conspicuous and character- 

 istic for larger groups of animals. It occurs, for example, in 

 Infusoria when they swim freely but not far from the sea 

 5* 



