CNIDARIA AS THE ONLY COELENTERATA 49 



to develop in the sea plankton. We can safely maintain that 

 this was so because, rebus sic stantibus, it could simply not begin 

 to develop here. It was only in a calm and shallow fresh water 

 where Volvox, and Anthophyta like "Lemna, could begin to 

 develop, either in free water or on its surface, and without 

 a contact with the bottom. 



Protozoa were in all probability the first animals that began 

 to live in the water of the free sea and continued to exist in it. 

 They evolved as Zooflagellata out of Phytoflagellata and fur- 

 ther continued their evolution into the planktonic Protozoa, 

 i.e. the rhizopodan Radiolaria. It is a priori improbable that 

 Metazoa developed out of planktonic Protozoa either such 

 as had formed colonies or those that consisted of several 

 nuclei (polyenergida). The reason is the same which prevented 

 metaphytes from developing in the sea plankton. 



The first Metazoa that inhabited the free water of the sea 

 were certainly the freely developing ontogenetic stages of 

 animals that lived on the sea bottom, first, because they had 

 evolved out of eggs that had been left freely swimming in 

 water, secondly, because they were small and light with a com- 

 paratively large surface, and thirdly, because they were covered 

 with flagella and cilia which they could use, even if to a smaller 

 degree, as a means of locomotion. Slight vertical streams were 

 sufficient to bring these future larvae into the upper strata of 

 water where they could find a rich food and other conditions 

 of Ufe. This is the way we have to explain the origin of plank- 

 tonic larvae. It was only later that these larvae were followed 

 by grown-up animal species into these strata of free water, 

 and these again had to make special adaptations to new con- 

 ditions. Or some larvae which belonged to animals that lived 

 on the sea bottom, remained constantly in the plankton so 

 that they changed by way of neoteny into planktonic animals. 

 The feeding of the first metazoic planktonic animals must have 

 been microphagous; it was only later when animal types de- 

 veloped, that macrophages began to feed on the former (Ce- 

 phalopoda, Vertebrate). 



