OF THE PROTOZOA. CILIATA. 313 



mouth or alimentary tube : thus in Ophryoglena, Bursa ria, Opercularia, 

 Epistylis, and Zoothamnium it lies close upon the vestibulum within, or 

 almost within, the region of the ciliary wreath (XXVII. 16; XXX. 9-11) ; 

 in Vorticella and Vaginicola it is placed against the upper part of the ali- 

 mentary tube, and in Tncliodina, Nassula, and many others, near it at its 

 termination (XXX. 5, 6, 17; XXIX. 4). Exceptions to this position are 

 met ^\iih. in Colejps and Colpoda (XXIX. 35-37), where it occupies the 

 posterior extremity, placed very close to the external sui^face. When two 

 vesicles exist, they are often placed on opposite sides of the body, the one 

 more or less anterior to the other, as seen in Paramecium (XXIX. 29, 30). 

 In Chilodon Ciicullidus a third is sometimes seen near the posterior extremity 

 (XXIX. 48). 



On watching these clear spaces, they are observed to disappear for a few 

 moments and again to reappear — in other words, to exhibit rhythmical con- 

 tractions, a feature which distinguishes them from any other vesicular spaces. 

 The contraction is known as the ' systole,^ the re-expansion as the ' diastole ; ' 

 these movements may be either regular or irregular, and they differ in dura- 

 tion in different species. Perty states that the pulsations in Stylo7iychia 

 pustidata occupy from six to seven seconds : in Spirocliona and Colpoda they 

 are more prolonged ; indeed, as Stein affirms, they are slower in the former 

 genus than in any other animalcule he has examined. When more than one 

 vesicle is present, no uniformity in the order of their movements has hitherto 

 been proved, although Siebold believes they must follow some nile. As evi- 

 dence of the independence of the vesicle of the general contents of the body, 

 Lachmann records {A. N. H. 1857, xix. p. 126) the fact that, even after the 

 contents of an animalcule have been sucked out by an Acineta, the vesicle 

 lodged in the still present and contractile layer may continue to pulsate for 

 several hours. 



With regard to the number of these vesicles in particular species, much 

 discrepancy has existed among observers. Siebold affirms that Ehrenberg has 

 proceeded in a pui-ely arbitrary manner in calling one a contractile or sper- 

 matic sac, and others, indistinguishable from it, gastric cells, and quotes in 

 illustration the Berlin Professor's description of the vesicles of Amphileptus 

 meUagris and of A. loyigicollis. To this objection Eckhard rejoined by assert- 

 ing that Ehrenberg was guided in detennining the natiu^e of vesicles by cer- 

 tain appreciable differences in the character and contractions of different sacs, 

 and that Siebold had erroneously represented lateral abdominal vesicles in 

 Stentor, and an elongated one in Spirostomum amhiguum. In this, however, 

 he was WTong, for the description of Siebold has been confii-med by Lachmann 

 and others (XXIX. 7) ; and on the other hand, Ehrenberg is not so much in 

 error respecting the numbers of these vesicular spaces as Siebold was led to 

 suppose. 



It is, indeed, only by careful and repeated observations that such variations 

 can be reconciled. In pronouncing a space contractile, a sufficient criterion 

 seems to be found in the cii^cumstance of a like organ being found, in aU 

 specimens of the same animalcule, constant in position, and rhythmical in 

 its movements. Gastric cavities or alimentary vacuoles may collapse and 

 disappear ; but this movement is not followed by renewed acts of disappear- 

 ance and reappearance in regular succession, and in the same spot ; for if one 

 such vacuole do replace another, a general movement onwards in the course 

 of the internal cyclosis may be discovered. Another test to distinguish a 

 stomach -vesicle fi^om a true contractile sac may be found in the use of coloured 

 food. Xow that the special contractile sac is admitted generally to be merely 

 the central organ of a system of contractile vessels disposed at various parts 



