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THE PROTOZOA 



The myonemes are ectoplasmic differentiations which are contrac- 

 tile in nature and are formed, according to Butschli and Schewiakoff, 

 from the walls of the alveoli which make up the sub-cuticular layer of 

 the membrane. Although probably arising in the peripheral alveolar 

 region, these threads occasionally become separated from this position 

 and are then found in the cortical plasm or even in the endoplasm. 

 Myonemes are most highly differentiated and are best known in the 

 Vorticellidas, where the sudden contraction of the bell, or the instan- 

 taneous rolling-up of the stalk, are due to their action. While the 

 most conspicuous myonemes in Vorticella run from the centre of the 

 disk to the very base of the stalk, Entz ('91) has described additional 

 fibrils which have a similar but less important function. According 



A 



B 



C 



D 



Fig. 95. — Supposed change of position of the mouth in Ciliata. [BUTSCHLI.J 

 A. Original position (as in Holophrya). B. The mouth has become elongated (as in Enchelys 

 or Spatkidmm), C. Similar stage from the ventral side. D. The mouth has become closed be- 

 hind, leaving the opening away from the body extremity. The markings on the membrane now 

 meet in the line represented by the original mouth-slit {e.g. in Glaucoma). 



to this observer there are two sets of myonemes, one internal, the 

 other external. Each set includes two groups of myonemes, one cir- 

 cular in its course, the other longitudinal. The external layer, 

 observed by Lachmann ('56) and Stein ('59, '67) but denied by many, 

 is formed of a large, single fibre composed of fibrillar, which winds 

 spirally about the bell from the junction of the peduncle to the centre 

 of the disk. It is this myoneme, Entz maintains, which gives the 

 annulate appearance to the bell, and, like a muscle-fibre, it is charac- 

 terized by fine transverse striations. A second circular set is formed 

 by another single fibre, which, however, is confined to the peristome disk, 

 and is located deep in the ectoplasm (Fig. 91, D). This fibre takes 

 only a few spiral turns at the base of the elevated disk and around the 

 edge of the collar, and functions as a sphincter-muscle to close over 

 the disk. Two sets of longitudinal myonemes complete the muscular 

 system. Of these, the external set, lying between the two circular 

 myonemes, consists of fine fibres running from the peduncle to the 



