196 THE EVOLUTION OF THE META20A 



to the formation of a firm skeleton that can be polished and 

 rubbed. Even in this Cnidaria do not show an originality. 

 The first steps in this direction had again been made bv 

 Turbellaria, even if in these this development has preserved 

 a local and moderate character due to the fact that Turbellaria 

 are freely moving animals. As a matter of fact, not even Tur- 

 bellaria have been the first to have discovered this possibiUty— 

 similar things can be found as early as in Protozoa. Calcium 

 salts have been used as a skeleton material in two different 

 ways, either as a substance which impregnates the periderm, 

 or as biocrystals (spicula). Both these elements can also be 

 found elsewhere in the animal world; Mollusca and Brachio- 

 poda can serve as examples of the former development; Ces- 

 toda, Nematoda (male animals), Brachiopoda with their spi- 

 cula, Tunicata, and others, as examples of the latter. These 

 spicula or calcoblasts have been occasionally cemented together 

 into a firm skeleton (e.g. in Octocorallia, etc.) 



In two species of Turbellaria (J^urhella klostermanni and 

 Sidonia elegans) L. v. Graff (1882) was able to observe skin 

 inclusions that produced an effervescent effect when treated 

 with the acetic acid; they had therefore been impregnated with 

 a lime -containing carbonic acid. Similar morphological ele- 

 ments have been described by O. Steinbock (1931) as "small 

 calcareous bodies" in connection with the primitive Acoela 

 Nematoderma bathycola; he did not determine, however, the 

 true chemical nature of this substance. 



This abiUty to deposit mineral salts in their own cells or 

 in the products of their cells (traces of which can be found 

 in the freely moving Turbellaria) has undergone further 

 progressive development during the transition to a sessile way 

 of life and to the formation of cormi. The madreporic corals 

 and so-called Hydrocoralha (Milleporidae and Stylasteridae) 

 are some of the most important absorbers of the sea chalk. 

 Frequently it has been maintained (e.g. by Kiikenthal, Kastner, 

 etc.) that the calcareous skeleton is formed by the ectoderm 

 only. This is not true for Octocorallia, and probably it is not 



