OF THE PKOTOZOA. CILIATA. 293 



detached from their stems, alternately contract and extend their bodies ; and 

 yet no one pretends to see any distinct lines or bands in their interior to be 

 termed muscles." 



Stein's conclusion therefore is, that the contained substance of the stalk of 

 the contractile VortkeUina is not muscular, although it is the organ through 

 which the wiU of the animal is exercised over the pedicle. Fiu'ther, as the 

 action of chemical reagents upon the enclosing tube or sheath of the pedicle 

 coiTcsponds vdih. theii' action upon the cuticle of the body, so also is there a 

 similar correspondence, in chemical relations, between the axis-cylinder and 

 the internal tissue of the body. 



Czeimak, in his essay on the stem of Vorticellce (ZeitscJir. iv. p. 442), describes 

 in that of Carchesium three distinguishable structures: — 1, the hyaline 

 colourless sheath ; 2, a yellomsh contained fibre or band ; and 3, a finelv- 

 granular fibre lying parallel to the last (XXX. 10). These thi^e portions he 

 terms three isotropous helicoids, ^ith reference to their spiral mode of con- 

 traction. Eckhard supposed the eftective cause of the contractility to consist 

 in the constant intimate connexion between the motions of the stem and 

 those of the body : but there is no such constant connexion ; for the ciHary 

 wreath may be retracted frequently without any contraction of the pedicle. 

 According to Czermak, the explanation of the movements is to be found in 

 the external hyaline fibre or tube being elastic, and tending naturally to keep 

 the stem outstretched, whilst the yellow contained filament is contractile 

 serving to thi^ow the stem into folds, — the one consequently antagonistic to 

 the other. To the third or granular element, he is disposed to attribute only 

 a vegetative function. The elastic force of the stem is constant, whilst the 

 contractile is momentary in operation ; the result of this, coupled with its 

 tubular structiu^e, aff'ords an explanation of the particular spii^al mode of 

 contraction. This, Czeimak has taken much pains to elucidate by reference 

 to physical laws, and an appeal to arguments which we deem unnecessary to 

 reproduce here. 



More recently, the idea of the muscular nature of the axis-cylinder of 

 Vorticellce has been re\ived by Lachmann (oj). cit. p 229), who does not 

 hesitate to call it a stem-muscle, and ^' cannot allow any value to Stem's 

 objection, that it stUl contracts even when the stem is not attached to another 

 object ; for the muscle does not thus loose its insertion, as it is attached to the 

 sheath of the stem itself by its hinder extremity, and not to the foreign object." 

 This reply to the objection seems perfectly admissible, although for our part 

 we do not at all perceive the necessity of regarding the axis-matter as muscle 

 in the exact sense of the term, even if it is in function homologous Tvdth that 

 compound tissue of higher animals. A further statement made by Lachmann 

 is, that the muscular tissue of the stem extends upwards into the body, where 

 it joins with the supposed muscular lamina lining the cortical laj'er. 



The manner in which the axis-cylinder is produced and disposed, is shown 

 by Stein to afi'ord a distinction between the allied genera Carchesium and 

 Zoothamnium. In the former, each branch developes its own canal and its 

 own central substance, so that neither of them is directly continuous with the 

 canal or the contractile matter of those portions previously formed (XXX. 9) ; 

 in Zoothamnium, on the contrary, both the sheath and the axis-cylinder of 

 the stalk are continuous throughout the ramified pol^qDidom (XII. 69). It 

 is in this genus, particularly, that the oldest portion of the stem is often solid : 

 indeed imperfectly-developed stems occur, in which after one or more divisions 

 this same solid and rigid condition is seen. Such varieties, as Stein points 

 out (o/j. cit. p. 218), are to a certain extent difficult to distinguish from species 

 of Epistylis ; nevertheless they are never so rigid as the latter, but admit of 



