254 Dr Wagner on the Structure and 



nads. Here, therefore, a moving globule and a monad are con- 

 sid'/red as identically the same. In order, however, to establish 

 this, they should have been seen to take in colouring matter, 

 and to contain internal cavities, which Ehrenberg has done with 

 regard to all the true monads. Ingenhous, Treviranus, Horns- 

 chuch and Nees Von^Esenbeck, if I mistake not, have said that 

 the Priestley's matter consists of dead infusoria, and that from 

 this congealed vegetable mass, as it is called. Confervas, Ulvae, 

 Tremellae, nay even the Hypnum riparia, have all been formed. 

 Here I do not doubt the fact; times without number have I seen 

 the Priestley's matter take its origin from green infusoria, such 

 as the Euglena viridis, and the green pellicle in pools and in- 

 fusions very generally consists of the dead bodies of infusoria, 

 but never have I seen plants formed from this green matter ; 

 it continued long fresh, and became afterwards yellow, brown, 

 run together, and fell to the bottom. When Confervas grew 

 in the infusion, or in the midst of these masses, there was never 

 a transition between the dead infusoria and the living plants. 

 Whether these plants arose from the spontaneous development 

 of globules, or from the presence of infinitely small seeds in the 

 water when drawn, I will not pretend to determine. Still less 

 can I believe in the observations of Wiegmann, that the En- 

 tomostraci, Cypris and Cyclops are generated in putrid water 

 from Priestley's matter, and after their death are changed into 

 confervse. He even asserts that he has seen confervas springing 

 out of the feet and antennae. It is very possible that these ani- 

 mals have arisen on confervae, but notjrom them. 



VI. Structure of the Cercarice. 



The exact observations and inimitable drawings which have 

 been given of the Cercariae by Nitsch, must be known to every 

 one who has occupied himself with researches into the infusoria. 

 Subsequently Baer has made some highly important observa- 

 tions on their formation, after Bojanus had called our attention 

 to the subject. During last summer I had an opportunity of 

 observing the living Cercariae in some water which had been 

 drawn from a pool for the purpose of examining the infusoria, 

 and which had been at rest in a glass. It contained also many 



