PREVIOUS INTERPRETATIONS OF CNIDARIA 191 



elastic capsule), and partly remains a viscous fluid (within the 

 capsule) ; it can gush powerfully out of the cnida and it produ- 

 ces physiological effects. The cell itself can mostly be found 

 in the ectoderm and does not actually belong to it but rather 

 to the middle layer (mesohyl or mesenchyme); it is amoeboid 

 and it can function once only. The cell itself (the cnidoblast or 

 nematoblast) as the producer of the cnida is destroyed when 

 this cnida is used in the explosion. 



If we disregard the few and therefore very interesting cases 

 of the so-called kleptocnidae, where a predatory species takes 

 over and uses those weapons of its prey which had not yet 

 been used, we find that there are several animal types w^hich 

 are able to produce either genuine cnidae or some closely 

 similar forms (gland cells with a partly formed secretion). Is 

 it a pure coincidence that it is only in Turbellaria that such 

 forms are produced quite generally and in large quantities, 

 in spite of the fact that they have remained on a lower level 

 in their organization and in their phylesis? This again can be 

 easily understood in view of the fact that Turbellaria have 

 remained freely moving animals while Cnidaria have adopted 

 the sessile w^ay of life. It has not yet been clarified whether 

 Nemertinea possess their own cnidae. The fact, however, seems 

 to be much more important that even among Protozoa species 

 or even larger taxons can be found which produce either 

 genuine cnidae or forms which resemble closely a cnida (cni- 

 doid). Cnidaria can therefore not be considered as animals 

 v/hich have "invented" cnidae or which alone can be found 

 in the possession of cnidae. 



Before we begin to study in detail the problem of cnidae, 

 let us first make a critical and comparative study of the skin. 

 Similarly, as the Cnidaria owe their name to cnidae, the Turbel- 

 laria have been so named because of the tiny water current pro- 

 duced by the ciliate cover of their skin. The surface of their skin 

 remains bare due to this ciliate cover, by means of which the 

 animal is able to move actively, in the same way as in Ciliata 



