HETEROTRICHIDA 



373 



in length. For our purpose, we ignore the varieties. Length 500 

 microns to 2 mm. when extended. The macronucleus monili- 

 form. 



Spirostomum teres Claparede and Lachmann (Fig. 160, h). 

 In habitats similar to the last species. Smaller with an oblong 

 macronucleus. Length 150 to 400 microns. 



Fig. 160 a. Plagiotoma lumbrici. X 140 (After Stein). 



b, c. Nyctotherus ovalis. X130 (After Stein), c, cyst, 

 d, e. N. cordiformis. X130 (After Stein), e, cyst. 



f. Blepharisma lateritia. X 100 (After Stein). 



g. Spirostomum ambiguum. X70 (After Stein). 

 h. S. teres. XI 25 (After Stein). 



i-k. Metopus sigmoides. i, from life ( X 100 after Stein) ; 

 j, a stained specimen (X250 after Noland); k, 

 encysted exconjugant after life (X210 after Noland). 



Genus Metopus Claparede and Lachmann. Body elastic. 

 When extended it is oblong or fusiform, with rounded extremi- 

 ties. Circular or oval in cross-section. Anterior part of the body 

 spirally twisted. Peristome with an adoral zone on its left side; 



