1914] Sharp: Diplodinium ecaudatum 65 



the underlying skeletal structures. It is thrown into fine longitudinal 

 ridges which mark off the little diamond-shaped depressions, which, 

 as already pointed out, give the mottled appearance so characteristic 

 of these areas. 



At the anus {an., pi. 4, fig. 3) the cuticle is continuous with the 

 rectal sheath and at the mouth with the lining of the oral cavity and 

 oesophagus {or., pi. 4, fig. 3). At the dorsal and adoral zones of cilia 

 it dips down into the furrows (pi. 4, fig. 3) and covers the lips, disks, 

 and operculum. Over the lips and operculum it is much thickened, 

 but in the furrows and over the disks it is thin. 



Ectoplasm. — The ectoplasm {ect., pi. 4, figs. 3-5), which is entirely 

 covered by the cuticle and separated from the entoplasm by the very 

 distinct boundary layer {hd. I.), is not a hom^ogeneous substance, but 

 shows a rather- definite alveolar stroma highly modified in certain 

 regions, as noted above, to form skeletal structures. This layer of 

 ectoplasm varies much in thickness in different parts of the body, 

 being very much the thickest in the anterior region {ect., pi. 4, figs. 

 3, 4; pi. 6, figs. 14-16; pi. 7, figs. 23-27), and thinnest over the left 

 side {ect., pi. 4, fig. 5.) In the anterior and posterior ends of the 

 body the thick ectoplasm fills in the inequalities of the outer form of 

 the body, so that the enclosed entoplasm is smoothly rounded off, and 

 in these regions does not conform to the general contour of the body. 

 Here also the reticular structure is coarser, the meshes appear larger, 

 and may therefore be more easily studied. That part which lies close 

 to the cuticle loses its irregular arrangement and forms a rather well- 

 defined alveolar layer in which, in cross-sections, the alveoli appear 

 irregularly quadrilateral. A similar layer in which the individual 

 alveoli may be even more easily distinguished lies next to the boundary 

 layer {hd. I., pi. 4, fig. 4). The ectoplasm also encloses the first part 

 of the oesophagus {oes., pi. 4, figs. 3-4), the rectum {rect., pi. 4, 

 fig. 3), macronucleus and micronucleus {mac. and mic, pi. 4, figs. 

 3, 5), and the two contractile vacuoles {ant. c. v. and post. c. v., pi. 4, 

 fig. 3). It is noteworthy that in the immediate vicinity of the con- 

 tractile vacuoles the alveolar structure is again modified, in that the 

 meshes are larger and the interalveolar walls are finer in structure and 

 stain less heavily than the remaining ectoplasm (c. v. r., pi. 4, fig. 3; 

 see also microphotograph, pi. 7, fig. 27). No streaming whatever of 

 the ectoplasm has been observed. 



Skeletal Structures. — The skeletal structures noted in the descrip- 

 tion of the cuticle are undoubtedly of ectoplasmic origin, but if, as 



