46 THE ANATOMY OF INVERTEBRATED ANIMALS. 



rather than the exception among the lowest plants, that at 

 one stage or other of their existence they should be actively 

 locomotive, their motor organs being usually cilia, altogether 

 similar in character and function to the motor organs of the 

 lowest animals. Moreover, the protoplasmic substance of the 

 body in many of these plants exhibits rhythmically pulsating 

 spaces or contractile vacuoles of the same nature as those 

 characteristic of so many animals. 



No better illustration of the impossibility of drawing any 

 sharply-defined distinction between animals and plants can be 

 found than that which is supplied by the history of what are 

 commonly termed " Monads." 



The name of " Monad " ' has been commonly applied to 

 minute free or fixed, rounded or oval bodies, provided with 

 one or more long cilia (flagella), and usually provided with 

 a nucleus and a contractile vacuole. Of such bodies, all of 

 which would properly come under the old group of Monadi- 

 da3, the history of a few has been completely worked out ; 

 and the result is that, while some (e. g., Chlamydomonas, 

 zoospores of Peronosjiora and Coleochaste) are locomotive 

 conditions of indubitable plants, others {Radiolaria, Nocti- 

 luca) are embryonic conditions of as indubitable animals. 

 Yet others (zoospores of Myxornycetes) are embryonic forms 

 of organisms which appear to be as much animals as plants ; 

 inasmuch as in one condition they take in solid nutriment, 

 and in another have the special morphological, if not physio- 

 logical peculiarities of plants ; while, lastly, in the case of 

 such monads as those recently so carefully studied by Messrs. 

 Dallinger and Drysdale, the morphological characters of which 

 are on the whole animal, while their mode of nutrition is un- 

 known, it is impossible to say whether they should be regarded 

 as animals or as plants. 



Thus, traced down to their lowest terms, the series of 

 plant forms gradually lose more and more of their distinctive 

 vegetable features, while the series of animal forms part with 

 more and more of their distinctive animal characters, and the 

 two series converge to a common term. The most character- 

 istic morphological peculiarity of the plant is the investment 

 of each of its component cells by a sac, the walls of which 

 contain cellulose, or some closely analogous compound ; and 



- 0. F. Miiller, " Historia Vermiura," 1773. " Vermis inconspicuus, sim- 

 plicissimus, pellucidus, punctiformis." 



