294 GENEEAL HISTORY OF THE INEUSORIA. 



being curved and are more elastic, and, besides all this, they exhibit trans- 

 verse folds or constrictions, of different depths, which are rendered still more 

 evident when the animals contract and shorten themselves upon their stems. 



The rigid stems of Opercularia (XXX. 1) and EpistyUs (XXX. 11) are 

 solid, without internal canal and contractile matter ; fi^equentlj they appear 

 finely striated longitudinally, and in several species {e. g. OiDercidaria articu- 

 lata) present transverse lines (XXX. 1), along which they more readily fracture. 

 These last are commonly described as articulations or joints ; but they occur at 

 irregular distances, and are, even in the same species, neither constant in num- 

 ber, in distinctness, nor in distribution, and are consequently worthless in 

 specific descriptions. 



The substance of these rigid stems is, however, not imiform, but divisible 

 into a cortical layer and an inner or medullary substance. This is manifest by 

 the fact of the transverse lines, which become more evident during the Hmited 

 undulating movements of the stem, penetrating only through its cuticle or 

 covering. " On the addition of concentrated sulphuric acid," says Stein (p. 

 112), " the pedicle swells up, and both longitudinal stria) and transverse lines 

 or folds vanish, the whole mass appearing homogeneous and hyaline. Tincture 

 of iodine coloiu's it yellow ; but sulphuric acid being added, it is again rendered 

 colourless." 



CoMPOUN^D Special Oeoans of Locomotion axd Peehensioit. The Peeistom 



AND ROTAEY OR CiLIlTED DiSK. ThE SpIEALLT-COILED HeAD OF SpIEOCHONA. 



— Before entering on the description of the internal organization of the Ciliated 

 Protozoa, there is one set of organs, belonging to the important genera Vortl- 

 cellina and Ophrydina (Ehr.), which demands our attention. The organs in 

 question are appiu-tenances of the head, and consist of a cihaiy wi^eath and 

 a retractile cihated disk. 



Ehrenberg appeai-s not to have recognized the existence of the ciliated disk 

 as a special structure ; for in his several generic descriptions of VorticelUna 

 and OiDlirydina, he speaks of the head as simply crowned by a wreath of 

 cilia, more prominent at one part, which he called the forehead, and inter- 

 rupted at one spot by a sort of gap where the oral apertiu'e is placed. Stein's 

 researches, however, show clearly that the armatui'e of the head, in most of the 

 genera of those families, is much more complex. The excepted genera are 

 Stentor, Tricliodina, Urocentnmi, and Tintinnus, which are, in fact, not true 

 members of the family. Stentor furnishes an example of the structui'e of 

 cihary wreath, presumed by Ehi^enberg to belong to all VorticelUna, being 

 in fact a single line of cilia fringing the periphery of the head, and bending 

 do^vn spii-ally to the mouth (XXYIII. 16 ; XXIX. 7, 8). Tricliodina is 

 very curiously fringed with an anterior and posterior wreath of cilia, and 

 has besides a fii'm collar-like ring, within which is a circlet of stiff uncini 

 (XXIX. 15, 16, 17). 



In the genus Vorticella the apparatus is most simple ; it is shghtly more 

 developed in Ojjihrydium and in Vaginicola, still more so in Eijistylis, and 

 most of all in Ojyercidaria and Lagenophrys ; lastly, in Sjnrochona, Chcetospira, 

 and Lagotia, totally exceptional forms occur. "When examined closely, 

 Lachmann says (A. N. H. 1857, xix. p. 118), we find the wreath is a spiral, 

 and not a complete circle (XXIX. 1, 2, 3, 4, 7). It begins in the vicinity 

 of the orifice of the vestibule, nms above it towards the left, and roimd the 

 margin of the ciliaiy disk ; but before it again reaches its starting-point, 

 it descends, upon the stem of the rotary organ, into the commencement of 

 the digestive apparatus (i. e. the vestibiilum) .... The portion of the ciliaiy 

 spiral, which is outside the vestibulum, is not of equal length in all Vorticel- 

 Una ; in many — VorticeUa, Oarchesimn, Zoothamnium, ScyphicUa (XXIX. 3), 



