PROTOPLASM OF PROTOZOA ,93 



pulex) springing movements are brought about by the strong contrac- 

 tion of the outer layer of the body, the resulting locomotion being inde- 

 pendent of flagellar movement. Certain flagellates have an extraordinary 

 superficial resemblance to medusae both in appearance and in method of 

 locomotion: (Cystoflagellata: Leptodiscus, Craspedotella; Phytomona- 

 dida: Medusochloris; Dinoflagellata: Clipeodin'ium). In these types ac- 

 cording to Pascher (1917), the movements represent a special form 

 of metaboly and two mechanical systems should be present, one radial 

 and dilating, the other peripheral and contracting. The former could 

 of course be elastic only and bring about passive return after the con- 

 traction of the latter. 



In the Infusoria there are many examples of specialized retractile 

 organelles: the tentacles of Suctoria may be retracted and extended much 

 as may the axopods of the Heliozoa. The structure of these tentacles is 

 quite similar to that of the axopods ( Roskin, 1925). The remarkable 

 tentacles of the ciliate Actinobolina may be extended to a length twice 

 the diameter of the body or may be completely retracted. They are as- 

 sociated internally with two groups of fibrils which seem to wind up to 

 retract and unwind to extend them (Wenrich, 1929). In many ciliates, 

 such as Stetjtor and Spirostotiium, there are actively contractile myonemes 

 in the ectoplasmic layer. In other related forms, such as Climacostomum, 

 the corresponding structures are elastic only and have been referred to 

 as morphonemes. The myonemes may even appear to be striated (Dierks, 

 1926, and others) but this has been denied (Roskin, 1923). 



A most unusual case of extension and retraction occurs in the ciliate 

 Lacry^nana olor. Here the "neck" may be extended to fifteen times the 

 length of the body, the form of which remains unchanged. This "elas- 

 ticity" is associated with the presence of what appears to be a series of 

 spiral striations. However, upon complete extension of the neck, Penard 

 (1922) has observed that there is only a single continuous spiral. No 

 experimental work has been done upon this form, so that the nature of 

 the extension and retraction is not understood. However, Verworn 

 (1899) has cut the neck of L. olor free from the head and body. The 

 neck retains its extensile and contractile properties exactly as when in 

 connection with the body. 



The finer structure of the contractile stalks of the Vorticellidae has 

 been studied by Koltzoff (1912), Fortner (1926), and many others. 



