INTRODUCTORY NOTE 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, ParaviCBcium, &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 ? 



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 

 KiiUiker^ has insisted that some forms which have neither mouth nor stom- 



*Agassiz, Ann. Nat. Hist. VI. 1850, p. t Kclliher. Siebold and Kdltikcr's Zcitsch. 



156. I. 1849, p. 198. 



