38 ILLINOIS BIOLOGICAL MONOGRAPHS [232 



paired excretory tubules, simple pharynx, and ventral mouth. The main 

 differentiating characters are: first, the presence or absence of a preoral 

 intestinal diverticulum; second, the habit of asexual budding possessed by 

 the Microstominae to a surprising degree, and not presen t at all in the second 

 group. 



Microstoma caiidahmi Leidy 1852 



Figures 52, 53, and 56 



The Microstominae contain two genera, one of which is found in this 

 country, four species having been reported from one or two places in New 

 York and Michigan. One of these. Microstoma caudatum, is present in large 

 numbers in an artificial pond at Grinnell, Iowa. It has been taken as late as 

 the last of November from heavy masses of Cladophora floating near the shore, 

 and to the naked eye appears very much like Stenostoma leiicops, which was 

 also to be found in some parts of the pond. Almost all of the specimens showed 

 three distinct fission planes, two nearly complete pharyngeal cavities, and tv/o 

 others at a much earlier stage of development. 



This v/as clearly Microstoma caudatum but in several minor points it dif- 

 fered from the original descriptions of that species. In size it was about two 

 millimeters long, the averages of the whole number of specimens taken varying 

 within very narrow limits. The anterior end was not as large and round as 

 the eastern type. It seemed slightly pointed at the apex, being very little 

 broader than the posterior tail end. The tail part also was somewhat different 

 from that of the original type in that it was round and blunt and not at all 

 pointed. The whole surface was smooth and regular, showing almost no in- 

 dentations at the lines of fission. The color was a very transparent, almost 

 iridescent, pale, yellowish-green. The intestine when empty was also sur- 

 prisingly clear, showing hardly any yellow at all, blendmg w'th the surrounding 

 parenchyma. There was the greatest difference, however, when it was more 

 or less filled with food material, as then the color was a distinct gray giving an 

 entirely different tinge to the body as a whole. 



The epithelium is very thin and transparent but bears extremely long cilia 

 which are few in number, of the same size all over the body, and rather heavy. 

 The most noteworthy details of structure are the nematocysts, which are very 

 conspicuous and correspond to those of the hydra. They are nearly one-third 

 as large as the pharynx and are spherical in shape. Before being discharged, 

 they appear to have a small, sharply-tipped cone-shaped structure within 

 and lie parallel to the surface of the body. When the cell is set off, the thread 

 of the sting is shot out from the tip of the cone. These nematocysts are 

 scattered very evenly over the body, and are about fifty or sixty in number. 



The muscular system is very slightly developed and together with the par- 

 enchyma forms only a small part of the body make-up. The pharynx is deep 

 and broad, cup-shaped, with the mouth opening very large. When closed, 

 however, the mouth makes only a narrow sHt on the ventral side, standing 



