82 University of California Puhlications in Zoology [Vol. 13 



nature and function and assist in the process of defecation, or whether 

 they serve merely as supporting structures to the rectum it is not 

 yet possible for me to say. 



Anal opening. — The anal opening {an., pi. 4, fig. 3; pi. 6, figs. 

 18, 19; pi. 7, figs. 32, 33), which is a mere slit, is located at the pos- 

 terior extremity of the body close to the ventral side. 



The caecum, rectum, and anus have to do with the discharge of the 

 more solid particles, i.e., the undigested remnants of the bacteria upon 

 which the animal feeds. The fluid excreta are gotten rid of by means 

 of the contractile vacuoles. 



Organs of Excretion 

 These are the contractile vacuoles {ant. c. v. and post. c. v., pi. 4, 

 fig. 3; pi. 6, figs. 15, 17; pi. 7, figs. 28, 30), which are also ectoplasmic 

 structures. They are two in number and anterior and posterior in 

 location. The anterior contractile vacuole {ant. c. v., pi. 4, fig. 3), is 

 located close to the dorsal wall in the median sagittal plane about mid- 

 way between the dorsal membranelle zone and the micronucleus and 

 just to the left of the macronucleus. The posterior contractile vacuole 

 {post. c. v., pi. 4, fig. 3), is similarly located in the posterior half of the 

 body close to the mid-dorsal wall, half way between the micronucleus 

 and the posterior end of the body and just to the left of the macronu- 

 cleus. Bach vacuole when distended is ellipsoidal in shape, slightly 

 larger than the micronucleus, and is surrounded by a slightly differ- 

 entiated ectoplasm previously described. Each vacuole opens on the 

 dorsal surface through a small canaliculus and a njinute pore {ant. c. v., 

 pi. 4, fig. 3). For the species D. ecaudatum these contractile vacuoles 

 have proved almost absolutely constant both in number and in position. 

 In fact, out of the large number of individuals of this species which 

 have been observed by me during the past three years, not more than a 

 dozen have shown the single vacuole as figured by Fiorentini (1889, 

 pp. 15, 16, pi. 3, figs. 1-2) and by Eberlein (1895, pp. 262, 263, pi. 18, 

 figs. 18-19). 



Neuromotor Apparatus 



We now come to the description of what is believed to be the most 

 interesting structure in the anatomy of this organism, a structure so 

 intimately and so peculiarly connected with the motile parts of the 

 body that its function as a neuromotor apparatus is strongly indicated. 



