96 ANDREWS. 



contracting substance, is now seen to have manifest physical 

 advantages for such a set of physical charges, controlled by 

 contractility. 



These phenomena were found gradually to be a key to a 

 whole host of contractile phenomena of the living substance, 

 as seen in optical changes of a visible, or inferred, vesicular 

 structure. All apparent contradictions were found to fall 

 under the same explanation, when based on the finer struc- 

 ture of the interalveolar substance itself ; wherever the con- 

 tinuous substance in contracting areas was seen to thin as it 

 elongated in sympathy with mass extension, and to thicken 

 in sympathy with contraction, it was found to be itself the 

 organized foam, or area of contraction. That is, the sur- 

 rounding substance bore but a passive part in the contraction 

 phenomena, and the striae, if there were more than one, acted 

 in harmony but without that structural dependence seen in 

 the Epistylis cuticle. In such cases, then, the structure of 

 Biitschli, or the visible organized structure, was not the active 

 basis of physiological function. 



In the highly elastic cuticle of certain rotifers, Philodinaea, 

 where there is a beautiful organization of the elements as 

 in Epistylis, it was noticed that striae, after becoming, as in 

 Epistylis, thicker and more obvious until the alternate set 

 of lines was almost obliterated, then began to grow thinner 

 and still more highly refractive, while extension of the mass 

 and of their length continued. In other words, the limit of 

 displacement of the interalveolar substance upon a basis of 

 Butschli's structure being reached, extension was prolonged 

 upon a basis of the finer foam ; the strial substance then 

 functioning as true fibril. 



All such cases, whether stable or intermittent, or only 

 occasional, fall under the head of vmsciilar fibrils of the 

 substance. There are areas whose structure superficially re- 

 sembles that found in the cuticle of Epistylis, which yet are 

 formed of these independent, but allied fibrillations. I have 

 found such in forms nearly allied to Epistylis. Single fibrils 

 of this sort when they lie in areas of rather fluid, vesicular 

 structure, slip easily in their extension or contraction through 



