80 University of California Publications in Zoology [Vol. 13 



tures we have given the names dorsal disk, operculum, and oral disk. 



Dorsal disk. — The dorsal disk {d. disk, fig. B; pi. 3, figs. 1, 2; pi. 6, 

 fig. 15; pi. 7, fig. 33), located between the dorsal membranelle zone 

 and the operculum {op., fig. B), is an exceedingly elastic structure. 

 Normally it has the shape of a spherical wedge, resembling one of the 

 carpels of an orange. The equator corresponds in position to a line 

 drawn from the middle of the operculum (op.) to the middle of the 

 dorsal membranelle zone, and the axis, or diameter, corresponds to 

 a line connecting the right and left extremities of the inner dorsal 

 lip {i. d. lip) . The size and shape of the dorsal disk depend entirely 

 upon the degree of contraction or relaxation of the operculum and 

 dorsal membranelle zone. 



Operculum. — The operculum {op., fig. B; pi. 3, figs. 1-2; pi. 7, fig. 

 33), also elastic and contractile in its nature, functions as a protective 

 structure to the organs of locomotion and nutrition, when these are 

 retracted. It is located between the dorsal disk and the dorsal curve 

 of the outer adoral lip and is attached to the bases of, the right and left 

 skeletal areas respectively by its right and left extremities, which thus 

 serve as skeletal attachments. The size and shape of the operculum 

 depend largely upon its state of contraction. This structure will be 

 further considered under observations on the living animals. 



Oral disk. — The oral disk {or. disk, figs. A, B; pi. 4, fig. 3; pi. 7, 

 figs. 20, 21), resembles the dorsal disk only slightly. It is circular in 

 form when viewed from above and dome-shaped when viewed from the 

 side. The oral disk almost completely surrounds the mouth when that 

 organ is open and entirely surrounds it when it is closed. It is sur- 

 rounded externally by the adoral membranelles and bounded internally 

 by the oral cilia which separate it from the mouth opening. The oral 

 disk is thicker on the right side than on the left, thus throwing the 

 mouth opening rather to the left of the center of the disk. The func- 

 tion of this disk is to support the oral cilia, give shape to the mouth 

 opening, and act as a valve to close the oral aperture. 



Organs op Defecation 



The organs of defecation are the caecum {caec), rectum {rect.), 

 and anus {an.). 



Caecum. — The caecum {caec, pi. 4, fig. 3; pi. 7, fig. 33), is situated 

 in the posterior one-fourth of the body close to the ventral wall and 

 slightly to the left of the median plane. The size and shape of the 

 caecum depend entirely upon the amount of excreta which it contains. 



