PREVIOUS INTERPRETATIONS OF CNIDARIA 195 



periderm. The same can also be found in Scyphozoa, even if 

 here to a lesser degree because scyphopolyps are less inclined 

 to form cormi. The formation of a periderm is widely develo- 

 ped among the Hydrozoa (Hydroidea), especially in the most 

 primitive Thecata. These have even developed devices in the 

 form of a lid by means of which the animal is enabled to close 

 the opening of its theca so that finally the entire body of the 

 hydranth is protected by its periderm. The Athecata have 

 ceased to form the hydrothecae and they have replaced the 

 personal protection with the so-called renovation of dama- 

 ged or destroyed hydranthes. 



The formation of a periderm is not an invention of Cnidaria; 

 similarly as in the case of cnidae, Cnidaria had inherited this 

 inclination and ability from their turbellarian ancestors. It is 

 natural that in Turbellaria as freely moving animals a general 

 and strongly formed periderm has never been developed. In 

 Turbellaria a thick and firm cuticle can appear locally only and 

 under special circumstances. Thus cuticular thorns (they are 

 probably built of scleroprotein) can be found on the dorsal 

 side of the genus Bnantia, among Polycladida. Much more 

 frequently we find firm cuticular formations developed in 

 connection with their genital organs (the male organs of co- 

 pulation). It should be mentioned in passing that this ability 

 to form a firm cuticle had also been inherited by numerous 

 types of Ameria. The best parallel to Cnidaria can be found in 

 Endoprocta as sessile descendants from Turbellaria which usu- 

 ally form cormi. In these, similarly as in Cnidaria, w^e can find 

 in one and the same individual all possible transitions from 

 a ciliate and bare epidermis, to such that one has cilia and that 

 is covered by a thin and extensible cuticle, and finally to one 

 that is covered by a thick periderm which can be occasionally 

 equipped with spines. This periderm can serve, as it does in 

 Cnidaria, as a true exoskeleton. 



In Anthozoa and in Hydrozoa, especially in the former, the 

 formation of cormi is attended with a secondary impregnation 

 of the organic skeleton matter with mineral salts; this has led 



