284 GENERAL HISTORY OF THE INFUSORIA. 



In spealdng of the attachmeut of the sheath, we have mentioned only that 

 by the base, with or without a stalk. But there are a few fonns which affix 

 themselves to foreign bodies by one side of their sheath, e. g. Vaginicola 

 decumhens (Ehr.) and the genus Lagenophrys. In such cases the attached 

 side is flattened, so as to increase the sui^face in contact. 



But, apart from the mode of attachment, the sheaths of different genera 

 vary in figm^e ; and as to size, there is no constant relation between that of the 

 ease and that of the enclosed being. The figure of the sheath, even in one 

 and the same species, is subject to modification fi'om age and from suri'ounding 

 circumstances. Thus, in Vaginicola crystallina it is usually cylindrical and 

 truncate (XXYII. 11), but at times it may be bellied posteriorly (XXYII. 10), 

 or, otherwise, have its anterior border expanded and curved outwards, or be 

 narrowed in front, or comj)ressed in one direction. Nevertheless there is 

 usually a general resemblance in figui-e among individuals of the same species or 

 genus, sufficient to furnish descriptive characters. For example, Cothurnia 

 imberbis has commonly a cylindiical sheath, bellied posteriorly and shghtly 

 contracted anteiioiiy (XXX. 15), whilst C. Sieboldii is campanulate, and has 

 its anterior half compressed in one direction, and its angles in front prolonged 

 and tapering (XXX. 13, 14). In the genus Lagenophrys, when adherent by 

 its flattened side, the sheath appears ovoid or shaped like a bellied oil -jar, ^ith 

 a contracted truncate mouth (XXX. 29, 30). A peculiar form of sheath is 

 presented to us in the genus Lagotia (XXYIII. 21, 23), which may be de- 

 scribed as retort- shaped, the relative diameter and length of the body and 

 neck differing in different specimens or species. In one species, at least, the 

 neck has the further peculiarity of being throT^^l into spiral or, otherwise, 

 annular folds or rings (XXXI. 7, 8), the presiuned form and origin of which 

 have just been described. 



We are fiu'ther indebted to the discoverer of Lagotia for the recognition of 

 a remarkable valvular structure within the tubular sheath of a species of Va- 

 ginicola, which he in consequence names Vag. valvata (XXYIII. 18, 19). 

 Dr. Wright states (Edin. New Phil. Joiirn. April, 1858) — " On examining 

 the valve in situ, I found it to consist of a rigid plate imbedded in a thick 

 layer of transparent sarcode (XXYIII. 18 b), which latter was continuous at 

 the lower end of the valve with a thin layer of the same substance, lining 

 the whole of the interior, and coating the upper part of the exterior of the 

 tube. The valve was closed by a contractile process passing from its under- 

 surface to the wall of the tube .... I am disposed to consider the whole ap- 

 paratus to consist of an oval plate of soft sarcode, supported by an included 

 bar or narrow plate of horn or chitine .... In some specimens the tube was 

 marked w^ith close transverse or cii'cular striae." 



In Stentor Mi'dleri (XXYIII. 16, 17), we have the curious instance of an 

 animal living indifferentl}^ with or without a sheath, and enjoying fi^eedom of 

 movement. Amidst numerous specimens of this species, not a few (says Cohn) 

 may be seen swimming freely about, or, otherwise, attached, enclosed within a 

 roomy ovate sheath, composed of a soft gelatinous substance, and open at one end 

 (XXYIII. 17). The animalcule is fixed by its posterior extremity (apparently 

 converted for the time mto a suctorial disk) to the closed end of the sheath ; but 

 it is still able to evert its spiral ciliary wreath, and to extend itself beyond the 

 open mouth, or to retract itself in a contracted condition witliin its interior. 

 Ehrenberg remarked the exudation of a mucous sheath around this animal- 

 cule when kept confined for some time for observation within small glass 

 tubes, but mistook it for a sort of morbid act preparatory to death. Cohn, on 

 the contrary, has shown {Zeitschr. 1853, iv. p. 263) that it is in no way con- 

 nected with disease or "v^dth approaching death, but happens with individuals 



