DERIVED ORGANIZATION 125 



Spirostomum ambiguum may be 2 mm. in length but, on irritation, 

 it suddenly contracts to one-quarter that size, or a Trachelocerca 

 phoenicopterus contracts to one-twelfth its original length (Lebedew) ; 

 a Folliculina ampulla with its great peristomial lobes widely out- 

 spread quickly folds itself completely into its comparatively narrow 

 tube (Figs. 94, 206), or an entire colony of widely distended indi- 

 viduals of Zooihamnium arbuscula contracts instantly into a minute 

 ball. These varied movements which are quite independent of 

 movements of translation or rotation, are due to the contraction 

 of specialized muscle-like fibrils, the myonemes. These are long, 

 delicate, contractile threads, circular or band-like in cross-section 

 situated in the cortical zone and running throughout the entire 

 length of the body, either straight (Stentor) or spirally (Spiro- 

 stomum). In some cases a second set of myonemes run transversely 

 about the body as in the peristomial regions of Campanella umbellaria 

 or various species of Stentor. The myonemes of Stentor coeruleus 

 or Prorodon feres- lie in characteristic canals, which appear hyaline 

 in contrast with the granular adjacent "ribs" of the ectoplasm. 

 Their finer structure has been made out in only a few types, in 

 Stentor coeruleus perhaps better than in any other. Here Schroder 

 describes a typical cross-striping due to alternate rows of light and 

 dark substance (Fig. 69, d). 



In the majority of cases the contractile effect of the activity of 

 myonemes is possible only by their intimate connection with the 

 firm membranous cortex which encloses the entire animal, a con- 

 nection which makes it possible for a coordinated contraction of the 

 whole animal at once. A retraction of special regions of the organ- 

 ism involves the attachment of one end of the contractile element 

 to some relatively fixed structure, as muscles in vertebrates are 

 attached to the endoskeleton (Fig. 70). In many cases the general 

 cortex serves this purpose as in the sphincter-like myonemes of the 

 Vorticellidse (Schroder), or the retractile elements of the "seizing 

 organ" or "tongue" of Didinium nasutum (Fig. 98, p. 187), or the 

 closing apparatus of the operculum-bearing types of ciliates. In 

 some cases, however, especially in parasitic ciliates like Ophryoscolex 

 or Diplodinium ecaudatum, there is a specialized differentiation of 

 the "cuticle" discovered by Gunther and well described by Sharp. 

 These peculiar differentiations function according to the latter 

 observer, as endoskeletal structures for the attachment of conspic- 

 uous band-form myonemes, which serve as retractor strands for 

 drawing into the body a characteristic gullet and adjacent organ- 

 oids. These skeletal elements are formed from the ectoplasm and 

 are hardened, according to Eberlein, by a deposit of silicic acid which, 

 as Sharp implies, may be the explanation of their rigid but brittle 

 nature. 



Myonemes or analogous organoids are not confined to the ciliates 



