i8o 



THE PROTOZOA 



rows like a string of beads. Without going deeply into the subject, 

 which is far from settled, it will suffice here to state that two views 

 are now held as to the seat of contractility in the stalks of Vorticella. 

 One set of observers hold that the outer membrane of the stalk is the 

 contractile portion, and that the contained thread merely counteracts 

 the force of the membrane, which tends to contract and roll up the 

 stalk. In other words, the myonemes of the spasmoneme are regarded 

 as elastic and not as contractile fibrils, at rest when the stalk is coiled, 

 active when the bell is extended (Cohn, '62 ; Metschnikoff, '63 ; Rouget, 



d 



a 



Fig. 97. Myonemes and cilia. [METSCHNIKOFF, BUTSCHLI, and JOHNSON.] 

 a, b, d, e. Cuticle and myonemes of Stentor c&ruleus. d. More highly magnified piece of 

 myoneme. g. Optical section through the body wall of Holophrya discolor. 



'61; Schaaffhausen, '68; Entz, '91). Entz described the membrane of 

 the stalk and the spasmoneme as antagonistic elements. The former, 

 which stretches out while at rest and contracts when irritated, opposes 

 the latter, which acts in the reversed manner. The axoneme he re- 

 garded as a sort of nerve-centre. The opponents of this view hold 

 that the rolling of the stalk is accomplished by the contraction of 

 the muscle-like spasmoneme (Stein '67, Clap. & Lachmann '58, 

 Engelmann '76, Biitschli '88, etc.). 



The myonemes lie in minute canals according to Biitschli ('88) and 

 Schewiakoff ('89) ; in direct contact with the plasm according to John- 

 son ('93) and Entz ('91), and they probably vary in position in differ- 

 ent forms. The structure and position of a myoneme in the ectoplasm 

 can be more easily seen from the accompanying figure than from a 

 description (Fig. 97). 



