EU CI LI AT A, HOLOTRICHIDA 363 



species in the intestine of sea urchins belonging to the genus 

 Strongylocentrotus. 



Entorhipidium echini Lynch (Fig. 155, i). Large, 253 

 microns long. With a single micronucleus. 



Family 3 Microthoracidae Schouteden 



Genus Microthorax Engelmann. Body small, up to 60 

 microns in length. Roughly ovoid; ventral side flat, dorsal side 

 convex. Cytostome at the posterior end, with an undulating 

 membrane. Cilia few. Rounded macronucleus central. A con- 

 tractile vacuole between the macronucleus and cytostome. 

 Fresh water. 



Microthorax sulcatus Engelmann (Fig. 155, j). 45 to 60 

 microns long. In pond water and infusion. 



Genus Cinetochilum Perty. Body irregularly ovoid or len- 

 ticular. With d-eep spiral furrows. Beginning at the posterior 

 end on the ventral surface, there is a short oral groove, at the 

 anterior end of which a cytostome surrounded by an undulating 

 membrane, is located. Macronucleus rounded, central; a con- 

 tractile vacuole. Fresh water. 



Cinetochilum margaritaceum Ehrenberg (Fig. 155, k). Body 

 about 40 microns long. Fresh water. 



Family 4 Parameciidae Grobben 



Genus Paramecium Hill ( = Paramaecium O. F. Miiller). 

 Cigar-shaped; circular or ellipsoidal in cross-section. With a 

 single macronucleus and one to many micronuclei which are 

 either vesicular or compact. The peristome long, broad, and 

 conspicuous. Cosmopolitan and common in the stagnant water 

 and infusion. Several species of which nine are here described 

 briefly. 



Paramecium caudatum Ehrenberg (Fig. 156, a). The most 

 widely distributed and, therefore, most frequently observed 

 species. Length 200 to 260 microns. With a compact micro- 

 nucleus and a massive macronucleus. Two contractile vacuoles 

 on the aboral surface. Posterior end bluntly pointed. 



Paramecium aurelia Miiller (Fig. 156, h). Length 120 to 250 

 microns. Two small vesicular micronuclei and a massive macro- 



