194 THE EVOLUTION OF THE METAZOA 



such a reduction has actually taken place. It is in agreement 

 with our interpretation of the evolution of Cnidaria that this 

 ciUation is best preserved in those Antho?:oa which have re- 

 mained solitary animals, i.e. in Actiniaria and in Ceriantharia. 

 In those species that have become purely sessile and colonial 

 animals, as well as in other sedentary animal types, the external 

 ciliation has been limited more or less to the surface of the 

 tentacle apparatus and to its surroundings, especially to the 

 entrance into the intestinal cavity. Those freely moving 

 (swimming) species have ceased to form cilia whose size has 

 been increased and whose active movement is effected by 

 means of contractions of muscles (medusae, Siphonophora); 

 these cilia can, however, return if the size of the species has 

 been subsequently diminished (Halammohydra, sporosacs of 

 the hydroids, Dicoryne conferta, see Fig. 8). 



There is a great need of protection in the sedentary animal 

 types. The protective measures regularly take place on the 

 free surfaces of the animals; these measures are such that they 

 do not prevent the animal from making partial movements 

 (usually contractions). This protection is usually achieved in 

 such a way that part of the free surface of the animal body 

 becomes covered with a firm cuticle, and a smaller intermedi- 

 ate part with an extensile cuticle (Endoprocta, Ectoprocta, 

 Phoronidea, etc.). This is also the situation in Cnidaria. To 

 this protection have additionally come secretions from the 

 cells of the skin glands as a new means of protection. In 

 Cnidaria this has largely been done by cnidae. 



As early as in Octocorallia we can see the formation of the 

 so-called periderm to be connected with the formation of 

 colonies (Stolonifera). It is a secretion of an organic substance 

 in the form of a cuticle which must not remain in its entire 

 extent connected with the skin epithelium and which is in 

 spite of this able to increase constantly its thickness. The con- 

 tractile and extensible part of anthopolyps is covered with a 

 thin extensible cuticle so that in case of need the polyp is able 

 to retract entirely into the broadened part (the theca) of its 



