of the Polygastric Infusoria. 439 



are more scattered, and are principally developed at the anterior 

 and posterior extremities of the body. Each bristle (as these 

 cilia are properly called) is articulated at the base, and is conse- 

 quently susceptible of a distinct motion, whilst in the ordinary 

 cilia their motions appear dependent upon the striae on which 

 they are situated (fig. 2). 



3) Locomotive organs belonging to the posterior part of the body. 

 Several forms exhibit in this spot cilia which are not remarkable, 

 but merely resemble those described under 2 ; others have small 

 fibres, with which they fix themselves (Stentors) ; others again 

 exhibit parts in which the muscular system in its primitive form 

 may be more perfectly studied than elsewhere : I allude here 

 especially to the Vorticellce. These animals are situated upon the 

 extremities of simple or divided trunks, the structure of which, 

 in those having the power of springing back, is as follows : — A 

 sheath (muscular sheath), fig. 3 s, incloses a simple muscle, which 

 disappears a little above the part at which the sheath is attached 

 to foreign bodies. The evident connexion between the motions 

 of the body with those of the muscular peduncle shows us that the 

 muscle ramifies within the animal itself. I have only succeeded 

 in observing this ramification in Vorticella nebulifera. I saw 

 two very distinct, although very small (not perceptible without a 

 power of 400 diameters) fibres, fig. 3 v v, stretching inside the 

 body. Ehrenberg saw a similar extension of the muscle in the 

 body of V. Convallaria. When this peduncle is not contracted, 

 the whole body of the animal is in a state of full extension ; but 

 as soon as it contracts this, especially when it draws in the oral 

 cilia, the sheath and the muscle both become shortened (the 

 whole peduncle becoming spirally coiled) and the animalcule 

 springs back on its peduncle ; if the body becomes again ex- 

 tended, and especially if the oral cilia are very distinctly unfolded, 

 the peduncle also passes from its contracted into the elongated 

 state. The oral cilia and the whole of the anterior part of the 

 body appear to be of importance in this retraction, since expan- 

 sion and contraction of the trunk and body appear mutually 

 conditional. What influence upon the motions we have just 

 described must be attributed to the muscular sheath, and what 

 to the muscle, has not yet been satisfactorily determined. This 

 much however is certain, that for perfect retraction three con- 

 ditions are requisite ; — an uninjured state of the muscular sheath, 

 an uninjured condition of the muscle, and attachment of the entire 

 peduncle ; for in Vorticellce, in which the muscle was torn in the 

 uninjured sheath, I observed, it is true, a contraction of the body, 

 but it had no influence on the extension and contraction of the 

 peduncle ; in others, the sheath of which had disappeared, the 



