THE INFUSORIA 443 



myonemes, or just external to this level, below the alveolar layer (Fig. 186, 

 B, D) ; the cilium passes outwards in the edges of the alveoli that is to say, 

 along the lines in which the walls of contiguous alveoli touch at their corners 

 and pierces the pellicle to pass to the exterior. In Anoplophrya, Collin (50) 

 describes root-like fibrils which pass inwards through the cndoplasm, and 

 are inserted on the membrane of the macronucleus. Khainsky (170 '5) also 

 describes fibrils passing inwards from the basal granules of the cilia of Para- 

 mecium. In the remarkable form Pycnothrix monocysloides, which possesses 

 an ectoplasm of great thickness and distinctness, there are, according to 

 Schubotz (868), two layers of basal granules, one more superficial, the other 

 deeper. Each granule of the outer layer gives off a cilium on its outer side, 

 and on its inner side a fibril connecting it to a granule of the deeper layer, 

 from which, again, a fibril passes inwards and becomes directly continuous 

 with a myoneme. 



The free cilium shows, according to Schuberg (44), a distal " end-piece,'* 

 which stains more lightly and is of finer calibre, and a basal, thicker, and 

 darkly-staining portion (Fig. 186, A) ; the basal portion is of eveh thickness, 

 and is about double the length of the distal end-piece, which tapers to a fine 

 point. Motile cilia are not stiff, but change their form by bending in a heli- 

 coid spiral, or in a portion of such a spiral, like the flagella of the Flagellata 

 and of spermatozoa. Cast-off cilia often coil up at their proximal end into a 

 fine loop. 



A cilium is composed of two different parts an elastic axial filament of 

 firm consistence covered by a sheath of more fluid contractile substance. 

 According to Khainsky (170 '5), the sheath of the cilium is in continuity with 

 the substance of the pellicle. The end-piece represents the axis exposed and 

 continued beyond the sheath. The axis is compared by Schuberg to that of 

 the axopodia (p. 48), and is the form-determining element for the fluid 

 sheath. Cilia perform active movements even when separated from their 

 basal granules, which are not to be regarded, in Schuberg's opinion, as kinetic 

 centres ; the movements caused by the fluid envelope are probably due to 

 alterations in surface-tension (p. 200, supra). 



The basal granules of the cilia are not regarded by Maier or Schuberg as 

 centrosomic in nature. Maier considers that they probably arise as cytoplasmic 

 bodies at the surface of the cell, and are to be interpreted as special thicken- 

 ings at the roots of the cilia ; Khainsky (170'5) takes a similar view. In this 

 connection, however, attention should be drawn to the observations of 

 Entz (53), who finds that in the division of Tiniinnidce the new peristomo 

 arises in the interior of the cytoplasmic body as a split or cavity, and that 

 the basal granules appear first, the pectinellae later ; the basal granules are 

 stated to be formed in connection with the nuclear apparatus, and their 

 substance to be formed either from the macronucleus or micronucleus. The 

 connection between the basal granules and the macronncleus described by 

 Collin (50) would seem also to indicate a nuclear origin for them. The 

 question of their centrosomic nature must remain, therefore, open for the 

 present. According to Schuberg, the basal granules of each row of cilia arc 

 connected with one another by a fine longitudinal fibril. 



The typical motile cilia described above become modified in various 

 ways, chiefly by fusion of separate cilia to form more complex structures. 

 The stiff, tactile bristles have precisely the same structure and mode of 

 insertion as the ordinary cilia (Fig. 186, H, i.e.), and in this case the change 

 is purely one of function or substance rather than of perceptible cytological 

 structure. The undulating membranes found in the pharynx of the Hymeno- 

 stomata are formed by fusion or adhesion of a single row of cilia, of which 

 the basal granules are ranged in a series to form a " basal rim " (Basalsaum) 

 from which the membrane takes origin (Fig. 186, E, u.m.). According to 

 Schuberg, the fibrils of which the membranes, membranellae, etc., are made 

 up correspond, not to a whole cilium, but to its axial portion alone. Some- 

 times, however, more than one row of cilia contribute to the formation of 

 an undulating membrane ; the two membranes which compose the adoral 



