200 THE EVOLUTION OF THE METAZOA 



gland cells (rhabdites, rhamnites, sagittocysts) shows clearly 

 that the development has gone here into considerable 

 width. 



Here we can bypass the problem whether genuine and auto- 

 nomous cnidae can be found as early as in Infusoria; the so- 

 called cnidotrichocysts oixhtProrodon are, according to Kriiger 

 (1934, 1936), certainly very close to a genuine cnida. Since we 

 consider that the Turbellaria as the lowermost Eumetazoa, had 

 been developed from the ciliate Protozoa, we accept 

 it as certain that Turbellaria had inherited their tendency to 

 gland cells with a strongly gushing and partly firm secre- 

 tion from their protozoan ancestors; in Cnidaria this same 

 ability has developed into a real virtuosity (Fig. 36). 



It seems, especially on the basis of studies made by Meixner 

 and by Kepner and his school on Microstoma^ that no genuine 

 cnidae have been developed anywhere among the Turbellaria. 

 In Turbellaria the highest level reached in the development 

 in this direction seems to be in sagittocysts, a fact which, as 

 has already been mentioned, can easily be understood it we 

 take into consideration the way of life on one hand of Cnidaria 

 and on the other of Turbellaria. 



The Nemertinea, too, had inherited from their turbellarian 

 ancestors, even if to a lesser degree, the ability to produce 

 forms similar to cnidae. It may not be a mere chance that, 

 especially in Nemertinea, such secretion products can be found 

 on their unarmed proboscides. Frequently one can find in 

 the epidermis cells of the anterior part of their proboscis rod- 

 like structures w^hich are very similar to rhabdites of Turbel- 

 laria. It is improbable that these rod-Hke structures could 

 be of a foreign origin. Genuine cnidae have been found in 

 the wall of the proboscis of several species belonging to 

 the genera Micrura, Cerebrattdus, and "Linens; these cnidae 

 show a regular distribution (in two longitudinal lines); their 

 origin has unfortunately not yet been determined, so that 

 we do not know^ whether they are autonomous. The cnidoid 

 forms, however, cannot be expected in Nemertinea with an 



