No. 2.] FRESH-WATER PROTOZOA. 55 



secondary nucleus. Anus and contractile vacuole termi- 

 nal. Body very contractile, assuming a variety of forms, 

 as shown in Figs. 223, 224, and 225, which may, however, 

 be regarded as separate species. 



M. sigmoides CI. & L., Figs. 223-225. 

 Spirostomum Ehrbg. 



Very contractile, elongated, cylindrical or thread- 

 formed. Peristome a long furrow reaching to the middle 

 of the body and leading into a short pharynx. Strong 

 membranellae on the left side of the peristome, but no un- 

 dulating membrane. Striation spiral. Nucleus ellipsoidal 

 or bead-like. Anus terminal ; a contractile vacuole ex- 

 tending almost to the anterior end. 

 Sp. teres CI. & L., Fig. 222. 



Sp. ambigmim Ehrbg., has also been found, but no figure 

 of it is given. 

 Bursaria O. F. Miill. 



Inflexible, somewhat purse-shaped. In front obliquely 

 truncate. Behind broadly rounded. Ventral surface flat. 

 Dorsal surface convex. Peristome extending from the 

 anterior backwards into a deep funnel, which opens on the 

 ventral side by a slit reaching as far as the middle of the 

 body, and passing imperceptibly into a pharynx-like tube. 

 Adoral zone on the left side of the peristome composed of 

 small membranellje. Striations regular; nucleus long and 

 ribbon-like and many secondary nuclei. Many small con- 

 tractile vacuoles and a terminal anus. 

 B. truncatella Miill, Fig. 231. 

 Stentor Ehrbg. 



Fixed, or free swimming. When attached the body is 

 elongated and trumpet-formed, with a more slender pos- 

 terior end, sometimes inclosed in a jelly-Hke lorica. When 

 free swimming, pear-shaped to ovate. A spiral row of 

 strong cilia extending around the truncated anterior end, 

 beginning on the ventral side with an inwardly projecting 

 curve, and extending from there over the right dorsal and 

 left side again to the ventral side, where it ends in a mouth 

 leading into a short pharynx. Striations evident; cilia- 

 tion fine and regular ; nucleus ellipsoidal, thread-formed or 



