INTRODUCTORY NGTE TO THE INFUSORIA. 
Constant labors in the whole department of microscopy, and that, 
too, with greatly improved instruments, during the past few years, have 
materially changed the face of the class Infusoria since the issue of this 
work. There have been numerous and signal researches among all the 
lower forms of animal life; and the imperfect and undeveloped forms of 
others, which are higher, have been wrought out with an accuracy and 
detail before unknown. 
These movements have all tended to diminish the numbers of the so- 
called Infusoria, and it remains to be seen how large the proper class will 
be when these researches shall have been further extended. By some even 
it is believed that it will be entirely resolved into other classes; this view, 
however, would appear far from being warranted by our present knowl- 
edge; for, while, on the one hand, whole genera have been shown to be 
only larval worms (Bursaria, Paramecium, &c., from Planaria ),* yet, on 
the other, some forms have manifested phenomena and changes leading 
us to place them almost unhesitatingly among individual animals, In 
its best aspects, however, the subject has many perplexing points; and, in 
its present unsettled state, it is almost hazardous for a scientific man to 
entertain anything like positive views thereon. 
I need scarcely allude to the vegetable, algous character which whole sec- 
tions of the Polygastrica have recently assumed ; and the limits of this work 
will not allow me to discuss in detail this and other interesting points. But 
there are two or three topics of the highest physiological import, which 
are prominently introduced by these studies. These are, What is a plant? 
What is an animal? and, Are the animal and vegetable kingdoms on their 
lowest confines separate and distinct from each other 2 
As is well known, all the older criteria by which animals were separated 
from plants have long since been regarded invalid; and some of those 
which in late years have been regarded among the most constant, have, 
quite recently, been declared as equally unsound. Cellulose has been 
shown to be a component of animal as well as of vegetable structures, and 
Kolliker t has insisted that some forms which have neither mouth nor stom- 
*Agassiz, Ann. Nat. Hist. VI. 1850, p. } Kélliker. Siebold and Kélliker’s Zeitsch. 
156. I. 1849, p. 198. 
