VORTICELLA 49 



of the animal. The ectoplasm also continues throughout the 

 length of the stalk. It should be noted that it is in the ectoplasm 

 that the specialized contractile fibers, or myonemes, are formed. 

 The endoplasm of Vorticella, as in other protozoan forms, makes 

 up the greater portion of the animal's body, but it does not con- 

 tinue into the stalk. In the endoplasm are numerous gastric vacu- 

 oles, a large U-shaped macronucleus, and a small micronucleus. 

 There is usually a single, large contractile vacuole. 



Stalk and Axial Filament. In the center of the stalk and 

 running its entire length is a noteworthy structure, the axial 

 filament. This is the definite contractile element and consists 

 of a great number of fine, individual fibers, the myonemes, that 

 arise in the ectoplasm of the bell and converge to form the axial 

 filament in the stalk. The myonemes are of interest because they 

 represent the lowest type of differentiated contractile fibers which 

 may be compared in their function to the contractile muscular 

 elements of higher animals. (B. f. 3.) 



When the axial filament of the stalk is contracted, it looks like 

 a tightly wound spiral spring, and all degrees of contraction can 

 be observed between this condition and a nearly straight rod. 

 When a contraction of the axial filament occurs, the myonemes 

 in the wall of the bell usually also contract, and this brings about 

 a contraction of the flaring peristome and the disc of the cell body. 

 As a result, the bell-shape is temporarily modified, and the body 

 of the animal becomes almost spherical. A number of such indi- 

 viduals are shown on the under surface of the leaf in figure 3. 



Cilia. The cilia do not form a uniform coating over the outer 

 surface as in Paramecium, but are restricted in the attached, or 

 sessile, animals to the edges of the peristome and epistome, and to 

 the vestibular space where they are differentiated to form a vibrat- 

 ing, or undulating membrane. The chief function of the cilia in 

 Vorticella is to create a water current which will sweep food 

 particles into the vestibule and thence through the mouth into the 

 gullet. The waste products of the animal are also removed from 

 the vestibule by ciliary currents. 



The discussion of Vorticella, so far, has been confined to the 

 sessile animals with stalks. But as a result either of unfavorable 

 environmental conditions or of cell division, free-swimming ani- 

 mals, without stalks, may be found occasionally. In such cases 

 an additional band of cilia specialized for locomotion is developed. 



