MYONEMES— EXTRUSION FILAMENTS 



45 



sitic in the rumen of cattle, such as some of the complicated forms like 

 Diplodinium (Fig. 520), the anterior region of the body is highly developed, 

 while in association with this there is a complicated system of contractile 

 fibres which enables the organisms to withdraw the whole anterior ciliated 

 region of the body into a def)ression, which becomes closed over it. In 

 a similar manner the ciliated peristomal region of Vorticella and its allies 

 can be suddenly retracted or withdrawn. The curious elongate ciliate 

 Spirostomum is well supplied with 

 longitudinal myonemes, which enable 

 it to retract suddenly to the globular 

 form when stimulated (Fig. 509). 



The presence of these myonemes 

 often renders it extremely difficult to 

 obtain satisfactorily fixed specimens 

 in the fully expanded condition, as 

 stimulation of the fixing fluid causes 

 immediate contraction of the myonemes, 

 and consequent rounding up of the body. 



EXTRUSION FILAMENTS.— In some 

 Protozoa special structures occur which, 

 on stimulation, have the property of 

 discharging filaments of varying length. 

 These may be protective or aggressive in 

 function or serve the purpose of fixation. 



As organs of protection they are 

 known as trichocysts, and are found 

 amongst the Ciliata such as Parame- 

 cium, Prorodon, Dileptus, and many 

 other forms. They appear as minute 

 ovoid bodies embedded in the ectoplasm 

 (Fig. 29). From the blunt end there arises a fine process which extends 

 as far as the pellicle or outer layer of the ectoplasm. When stimulated, 

 the fine process is ejected as a tapering filament. Several explanations 

 of the sudden formation of the filament have been suggested. One is 

 that a very rapidly coagulating fluid is discharged. Whether this is the 

 correct explanation or not, it does not appear that the filament as such 

 exists in the trichocyst before it is visible externally. A larger organ 

 with a similar function is the Nessel's capsule or nematocyst. It is 

 present in Epistylis, and is arranged in pairs (Fig. 529). 



Another type of filament which can be suddenly discharged occurs 

 in the Cnidosporidia (Fig. 30). In this group the resistant cysts or spores 

 are provided with one or more polar capsules from which long filaments. 



Fig. 29. — Trichocysts as seen in 

 Sections of Paramecium cauda- 

 tum ( X ca. 1,500). (From Minchin, 



1912, AFTER MaIER.) 



A. Body Surface. 



B. Mouth and oesophagus. 



T. Trichocysts; /.t'.,food vacuoles; M.m., 

 undulating membrane formed of fused 

 cilia in the oesophagus . 



