292 GENERAL HISTORY OF THE INFrSORIA. 



Structure of Pedicles. — The intimate structm-e of the stem of the Vor- 

 ticeUina is different in the contractile and in the rigid forms. The highly- 

 sensitive, contractile, simple pedicle of the genns Vorticella has challenged 

 especial study. It is evidently a compoimd structui'e, consisting of a hollow 

 tube containing a cylindiical band. The tube is a portion of the general 

 integ-ument, and continuous with it ; in diameter it is uniform throughout, 

 except at its point of junction with the body, where it undergoes a very slight 

 expansion. Owing to the excessive rapidity of its spii-al contraction, this act 

 can ^^dth difficulty be observed, except after the adcUtion of a weak solution 

 of corrosive sublimate, which renders it so much slower that its progress may 

 be watched. Ultimately, indeed, the solution kills the animal. 



The contained band, or, to borrow a term from general anatomy, the '' axis- 

 cylinder," does not fill the cavity of the tube, but is disposed within it in a 

 loose spiral manner. Opinion has been much divided as to the nature of 

 this structui-e. Ehrenberg, judging from its active contractility, pronounced 

 it a muscle, and went so far as to represent it as striated, i.e. as belonging to 

 the liighest- developed condition of muscular tissue, which, however, com- 

 parative anatomy teaches us is absent in the lowest classes of animals. Many 

 other writers have united with Ehrenberg in considering the band muscular, 

 and some few also striated, whilst others, again, have regarded it as a simple 

 primitive contractile substance, less elevated than muscle proper in the range 

 of tissues. Indeed, when we contemplate the contractility exhibited by 

 certain plants, and can find nothing more than spiral vessels which can 

 be conceived the seat of this property, we are forced to admit that muscular 

 tissue is not the only actively contractile element in organized bodies. 



Stein, after remarking that the histology of the stem in Vorticella, 

 Carchesium, and Zoothamnium is essentially similar, proceeds to describe the 

 axis-cylinder as an opaque, solid, finely-granular mass, presenting delicate 

 longitudinal lines or stripes. In Vorticella nehulifera, V. convallaria, V. 

 CamjKinula, and in Carcliesiimi polypinum (XXX. 9), it extends into the body 

 as a single tapering band or sti'eak, and in other Vorticellina in two' such 

 diverging from one another, as remarked by Ehi'enberg, who concluded them 

 to be two muscular cords. When the stem contracts spirally, transverse lines 

 or stripes appear in the axis-matter, which are no other than cross folds, not 

 parallel, and most strongly marked on the concave side (XXX. 10) ; they 

 have therefore no homology with the transverse strise of muscle. That the 

 contractile power is dependent on the contained axis-cylinder is shown by 

 the facts, that where this is deficient at any part, as not unfrequently hap- 

 pens in Zoothamnium, that portion is ligid, as in Ejnstylis or OpercuJaria ; 

 that when destroyed by maceration, or by chemical agents, the stem is out- 

 stretched and remains immoveable ; and that, as is not seldom seen both in 

 Carcliesium and Zoothamnium, this axis-matter may be torn across, at one or 

 more parts, without the external sheath being injured : the contractility is 

 destro3'ed, except in that segment which is still in continuous union with the 

 body of the animal ; and generally the pedicle is only so far and so long con- 

 tractile as its axis-cylinder continues its unbroken connexion with the body. 



"Although," observes Stein (p. 80), "these phenomena are in favour of 

 the axis-matter being a muscle, yet there are others sufficiently conclu- 

 sive against the notion. Eor instance, were the axis a muscle, its move- 

 ments should cease when it loosens its hold from the object it is affixed 

 to : but this, although asserted by Eckhard, does not happen ; for when 

 Vorticellce and Carchesia relax their hold and swim freely about with their 

 stems, these last are seen to actively contract in theii' usual spiral manner, 

 and presently again to extend themselves. In like maimer Vorticellce, when 



