60 THE EVOLUTION OF THE METAZOA 



general form of the body of animals. If we review all the 

 planktonic animals the impression we finally get is that there 

 has been no general regularities in respect. In these animals 

 all possible symmetrical conditions can be found exactly as in 

 the animals that live on the ground. And still there are diffe- 

 rences which are due, as we shall see later, to the fact 

 that in free water there is no contact between the firm ground 

 and the animal, i.e. its ventral side. In the Ctenophora we 

 can observe how life in free water promotes the develop- 

 ment of radial symmetry; this is due to the similarity of 

 contacts between the milieu and all parts of the animal 

 body. If there is, however, an eventual change of symmetrical 

 conditions, then this change is largely due to the type of 

 movement. In types of animals that move only slowly and 

 mainly by means of cilia or organs which have developed 

 out of these (e.g. the swimming plates in Ctenophora, the 

 circlets of cilia in trochophores), and in types that hang verti- 

 cally in water, the inclination to adopt the radial symmetry must 

 always be expected, and, as a matter of fact, it will be found 

 in them. Bilateral symmetry, however, remains preserved 

 a long time after the transition from a life in the benthos 

 to a life in the pelagic zone had taken place, if the movement 

 of the animal is more energetic and stronger and if it is effect- 

 ed by means of muscles and particularly by means of limbs. 

 This can be best observed in the case of Chaetognatha. 

 I derive them, in agreement with some other zoologists, from 

 the planktonic larvae of Brachiopoda. These brachiopod larvae 

 hang in water or they move slowly by means of a pulsation of 

 cilia. They are basically bilaterally symmetric animals but they 

 show a clear inclination to adopt radial symmetry. In Chaeto- 

 gnatha, however, which are predatory animals with strong 

 contractions of muscles and with an ability to move rapidly 

 head first, bilateral symmetry had been developed in spite of 

 their life in free water. Radiolaria, an extremely old protozoan 

 group which for a long time has been living exclusively in the 

 pelagic zone or in plankton, contain all possible forms among 



