THE GROUND BETWEEN ANIMALS AND PLANTS. 655 



blance to Heteromita, and, like it, grouped under the general name of 

 "Monads," which, nevertheless, can be observed to take in solid nu- 

 triment, and which therefore have a virtual, if not an actual, mouth 

 and digestive cavity, and thus come under Cuvier's definition of an 

 animal. Numerous forms of such animals have been described by 

 Ehrenberg, Dujardin, H. James Clark, and other writers on the In- 

 fusoria. 



Indeed, in another infusion of hay in which my Heteromita lens 

 occurred, there were innumerable infusorial animalcules belonging to 

 the well-known species Colpoda cucullus.^ 



Full-sized specimens of this animalcule attain a length of between 

 To ^^ Too" of ^'"^ iiich, so that it may have ten times the length and a 

 thousand times the mass of a Heteromita. In shape it is not alto- 

 gether unlike Heteromita. The small end, however, is not produced 

 into one long cilium, but the general surface of the body is covered 

 with small, actively-vibrating ciliary organs, which are only longest 

 at the small end. At the point which answers to that from which the 

 two cilia arise in Heteromita^ there is a conical depression, the mouth ; 

 and in young specimens a tapering filament, which reminds one of the 

 posterior cilium of Heteromita., projects from this region. 



The body consists of a soft granular protoplasmic substance, the 

 middle of which is occupied by a large oval mass called the " nu- 

 cleus ; " while at its hinder end is a " contractile vacuole," conspicu- 

 ous by its regular rhythmic appearances and disappearances. Obvi- 

 ously, although the Colpoda is not a monad, it differs from one only 

 in subordinate details. Moreover, under certain conditions, it becomes 

 quiescent, incloses itself in a delicate case or cyst^ and then divides 

 into two, four, or more portions, which are eventually set free and 

 swim about as active Colpodm. 



But this creature is an unmistakable animal, and full-sized CoJpodce 

 may be fed as easily as one feeds chickens. It is only needful to 

 diff'use very finely-ground carmine through the water in which they 

 live, and, in a very short time, the bodies of the Colpodce are stuffed 

 with the deeply-colored granules of the pigment. 



And if this were not sufficient evidence of the animality of Col- 

 poda, there comes the fact that it is even more similar to another 

 well-known animalcule, Paramcecium, than it is to a monad. But 

 Paramecium is so huge a creature compared with those hitherto dis- 

 cussed it reaches y|-g- of an inch or more in length that there is no 

 difficulty in making out its organization in detail ; and in proving 

 that it is not only an animal, but that it is an animal which possesses 

 a somewhat complicated organization. For example, the surface-layer 

 of its body is different in structure from the deeper parts. There are 

 two contractile vacuoles, from each of which radiates a system of 

 vessel-like canals ; and not only is there a conical depression continu- 



' Excellently described by Stein, almost all of whose statements I have verified. 



