NEMATOCYSTS OF MICROSTOMA. 1 



WILLIAM A. KEPNER. 



For the past three autumns many brown Hydras and Micro- 

 stomas were found in a fish pond northeast of the University of 

 Virginia. In both these forms of animals were present oval, 

 refractive bodies, which were the nematocysts. These nema- 

 tocysts when everted have in both cases slender filaments with 

 three barbules radiating from their base. The filament at its 

 base is attached to the neck of a pear-shaped "poison-sack" 

 or capsule. Since Oersted (1844) nematocysts were known to 

 occur in flat worms as well as in Ccelenterata. Associated with 

 the nematocyst of Hydra there is a cell which has elaborated the 

 nematocyst and cares for it until it is discharged. This cell is 

 known as a cnidoblast or nematocyte. Each cnidoblast of Hydra 

 has a small spine-like structure the cnidocil. This is supposed 

 to receive the stimulus that results in the discharge of the nema- 

 tocyst. When one examines the living Hydra and the living 

 Microstoma found in this vicinity no difference can be detected 

 between the nematocysts of the two forms, except that there is 

 no cnidocil associated with the nematocysts of the flatworm. 

 Moreover the nematocysts in both Hydra and Microstoma when 

 undischarged and when being discharged appear to be normal 

 parts of the respective animals. This indigenous appearance and 

 ready discharge of the nematocysts of Microstoma led men to 

 look upon them as structures elaborated by the cells of Micro- 

 stoma. Such was my view after I had repeatedly, during three 

 years, studied the nematocysts of living Microstoma. According 

 to Martin (1908) until 1903 all zoologists held that the nemato- 

 cysts of Microstoma were its own products. 



Recently the inference has been made that the nematocysts 

 of Microstoma have been derived from ingested ccelenterata, 



1 The author wishes to thank Professor A. H. Tuttle, of this laboratory, Professor 

 E. G. Conklin and Professor Ulric Dahlgren, of Princeton University, for valuable 

 suggestions that were helpful in the preparation of this paper. This paper was read 

 before the Philosophical Society of the University of Virginia. 



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