MORPHOLOGY AND TAXONOMY OF THE INFUSORIA 479 



zone," around the margin of the peristome, but a dorsal ring of 

 membranelles is present in some parasitic forms (e. g., Diplodinium, 

 Fig. 2, p. 20). Here also limited "arches" of apparent membran- 

 elles are variously distributed about the body (Fig. 1-4). Cirri 

 are combinations of cilia of usually the ventral surface, but they 

 may encroach on the dorsal surface (e. g., Uronychia); they form 

 groups, as a rule, named according to their position, frontal, ventral, 

 anal and caudal cirri, the number and arrangement forming a 

 basis for diagnosis of genera and species. 



At the present time there is need of a more precise characterization 

 of cirri. The Order Hypotrichida, for example, is described by 

 Kahl (1931) as including forms which possess no cilia but only 

 membranelles, membranes and cirri. This distinction upsets the 

 classification, in use for half a century, as given by Stein. Having 

 no cirri, the former hypotrich family Peritromidae is removed to 

 the Heterotrichida while the accepted cilia of the Urostylidae (here 

 included in the Oxytrichidae) are now regarded as simple combina- 

 tions of cilia, i. e., cirri. Marginal cirri are more complex, frontals 

 and anals still more so, while the great steering and jumping organs 

 of Uronychia, Diophrys, etc., certainly call for more descriptive 

 terms than cirri. Temporarily the need may be met by use of the 

 expressions: simple cirri, caudal cirri, tactile cirri, frontal, anal 

 and marginal cirri and giant cirri. 



The activities of the motile organs are coordinated through a 

 system of longitudinal and transverse fibrils connecting the basal 

 fibrillae coining from the cilia or groups of cilia (p. 152). A coor- 

 dinating center, termed the motorium, regarded by numerous 

 observers as an artefact (Rees, 1931; Turner, 1933, etc.) has been 

 demonstrated in some forms (Diplodinium Sharp, 1914; Euplotes 

 Yocom, 1918; Balantidium MacDonald, 1922; Kidder, 1932, ^ al.). 

 The "silver line" system, discovered by Klein, is a complex 

 meshwork of granules and fibrils in the cortex arranged in patterns 

 which appear to be characteristic of different species. This, appar- 

 ently, is a universal coordinating system of the Infusoria (see p. 80). 

 Myonemes also are widely distributed in the group. In Stentor 

 they lie in superficial canals within the cortex and in some cases 

 appear to be conducting as well as contractile elements. In Epis- 

 tylis Schroder has described myonemes running longitudinally from 

 the stalk to the peristome where they terminate in the basal plates 

 of the membranelles (Fig. 70, p. 120); distally they combine to form 

 the contractile strand of the stalk. 



A well-defined mouth is present in almost all ciliates (absent in 

 an entire group, only in Astomida). In gymnostomida it is closed 

 save at times of food ingestion; in all other groups it is perma- 

 nently open. In these latter cases the form of the mouth varies 

 from circular to elliptical, crescentic or triangular openings and in 



