94 PROTOPLASM OF PROTOZOA 



The stalk consists of an external wall, an inner liquid, and a spirally 

 wound contractile cord. Within the contractile cord is an excentrically 

 placed myoneme often called a spasmoneme, the contraction of which 

 causes the stalk to become coiled like a spring. The distal end of the 

 stalk is usually attached to the substratum, but in Vorticella natans and 

 V. mayen the organisms are never attached but swim, stalk first, through 

 the water. In the former the stalk rolls up into narrow spirals, but in the 

 latter the stalk swings in a wide loop on contraction somewhat like a 

 flagellum. Belehradek and Paspa (1928) have reconstructed a myogram 

 from moving pictures of the stalk of a vorticellid and find that the 

 spasmoneme does not function like a true muscle but like a modified 

 flagellum. Various attempts have been made to explain the contraction 

 and extension of the stalk of vorticellids as caused by the complex action 

 of two opposing bundles of fibers, or in terms of internal pressure against 

 coiling brought about by elastic fibers. 



Myonemes are also well developed in the gregarines, where longi- 

 tudinal and circular myonemes are apparently responsible for bending 

 and peristalsis-like movements. Myonemes called myophrisks are also 

 present in certain Radiolaria. Here they are associated with the spreading 

 out of the gelatinous cortical layer, previous to their decrease in specific 

 gravity and subsequent rise to the surface (Schewiakofi^, 1927). 



ROPINESS, OR THREAD FORMATION 



Living material is said to be ropy if it can be drawn out into threads; 

 ropiness is thought to be due to the micellar structure of the material. 

 The ability of a drop of a pure liquid to resist distortion is due to its 

 surface tension only, and in heterogenous mixtures the surface film may 

 approximate a solid consistency. A column of fluid breaks up into a 

 number of smaller spherical bodies. The formation of pseudopodia, 

 especially of filopodia, axopodia, and myxopodia, demonstrates the pres- 

 ence of solid structures in protoplasm. Then, too, protoplasm may be 

 drawn out into fine strands of considerable elasticity and tensile strength: 

 the ectoplasm of Amoeba has been drawn out into fine strands by Hyman 

 (1917) and many others; Schultz (1915) was able to draw out the 

 protoplasm of the foraminiferan Astrorhiza into long threads; this was 

 more marked in the outer layers than in the inner mass of protoplasm. 



