354 The Microscope. 



For a description of A. Eichhornii and of its habits, see 

 " Fresh-water Rhizopods of North America," by J. Leidy, p. 259, 

 plate XLI. 



We will pass at once to the special subject in hand, beginning, 

 for convenience, with the simplest or youngest heliazoan. 



Development. — Let it not be understood that the order in which 

 I am now to describe the different stages of development is the order 

 in which I observed them. On the contrary, what I shall first 

 describe really came about last in my observations, since I did not 

 at first take the youngest stages of this heliazoan to have any connec- 

 tion with the larger heliazoa. My observations began with an 

 undoubted heliazoan of this species (Fig. 13, of my plate), and from 

 that I worked both ways, but principally to the younger. It would 

 have been impracticable to have watched the development of a single 

 heliazoan from the very youngest individual to the full grown animal, 

 since it would have required not only a constant observation for a 

 much longer time than I could spare, but would also have needed 

 some little care. As it was, I could watch a young heliazoan until 

 it had developed a few stages, and had considerably lessened the 

 near supply of food, and then I could find another heliazoan of the 

 same stage as the one just discarded but which was in more favorable 

 circumstances for further growth. As indicated, the number of 

 heliazoa was enormous, and the different stages represented by the 

 scores. Had I suspected these various stages to have been what 

 they were, there would have been no trouble in finding a complete 

 set, for every gradation, from the youngest to the adult, was present 

 in great quantities. Fortunately, there were quite a number of 

 worms — Dorylaimus stagnalis — in the water, and their constant wig- 

 gling about kept the heliazoa and other animals in perpetual motion, 

 so that they came in contact with one another where, otherwise, they 

 would not have done. 



A far greater number of observations were made than I shall 

 here describe. Enough were chosen, however, to form a complete 

 series, and accurate drawings made of them. I shall, therefore, 

 describe only those observations which I have illustrated, hoping 

 that the series will be full enough for our purpose. 



I think it is safe to say that were this minute mass of protoplasm 

 which constitutes the youngest heliazoan observed by itself for a lit- 

 tle while, no one would mistrust its true nature or relations. Indeed, 

 it was only after a long and continued observation, and that under 

 the most favorable circumstances, that I became convinced of its time 



