140 THE EVOLUTION OF THE METAZOA 



exception— any protonephridia. On the other hand we cannot 

 find, according to Lang, any protonephridia in the rather 

 large Polycladida that Uve exclusively in the sea, a fact which 

 must be explained as being due to their secondary disap- 

 pearance. Many other species of Turbellaria, especially those 

 that have adopted the parasitic way of life, have lost their 

 protonephridia, while at the same time these are preserved in 

 the small Temnocephala that live epizoically in the fresh water. 

 In my opinion we must attribute a greater importance to 

 the older data provided by v. Graff (1904-1908, in Bronn's 

 Classen und Ord. des Tierreiches), and by Lohners (1911), as 

 well as to those that have been published more recently 

 by Westblad (1948), according to which there can be found 

 an accumulation of excretory products in the form of 

 cellular and plasmatic inclusions in the external body layer, 

 in the so-called ectocyte, w^hich, however, is not equi- 

 valent to the ectoepithelium and thus to the epidermis 

 (e.g. in Varaphanostoma crassum, P. trianguliferum ^ the Convoluta 

 species, etc.). This is the same body layer which in Infusoria 

 regularly contains the contractile vacuoles. If the functioning 

 of these contractile vacuoles ceases because of the retrogression 

 of the emunctory function, the excrement is deposited as 

 firm bodies (eventually as crystalloids) in the cytoplasm either 

 of the mesenchymal or of the digestive cells. 



(2) It is therefore not surprising, in view of the fact that 

 the protonephridia had been so labile both during their 

 development as w^ell as in their occurrence, that they have 

 been lost during the evolution from the turbellarian ancestors 

 to the Cnidaria, at first semi-sessile and slowly creeping, and 

 afterwards to the completely sessile and immobile animals. 

 In addition to this, first Cnidaria were in all probability small 

 animals. In these circumstances, the protonephridia became 

 superfluous and they did not stand any longer under the 

 protection of Natural Selection. It is quite possible that in 

 the period of the separation of Cnidaria from their turbellarian 

 ancestors the formation of protonephridia had been very labile. 



