INFTJSOBIAI ANIMALCULES. 19- 



and decisive, until at length no single chai-acter is sufficient to dis- 

 tinguish them. Thus, motion, digestive structiu-e, composition, the 

 products evolved, &c., taken singly, are of little avail in separating 

 an animal from a vegetable organism. Eecent reseai'ches have rather . 

 increased these difficulties. The fashion of the present day is to 

 magnify the argoments in favour of vegetable life and physical- 

 motions, while those on the side of an animal existence, are slm'ed 

 over. It is, therefore, desirable to pause before offering an opinion, 

 especially when every distinction hitherto proposed, is seen to vanish 

 if rigorously tested. The organisms of a doubtful animal nature, are 

 principally found in the families Ilonaditia, Vibrionia, and Jiacillaria, 

 which are fully described in Part III. 



1 . Motion. Tliis is an excellent animal character, where its volimtary 

 and spontaneous nature can be clearly perceived, but in microscopic . 

 bodies, vision being obtained by one eye only, and that imder unusual, 

 conditions, difficulties present themselves which do not occur in 

 common ^dsion. Again, the germs, or spores of minute Algae, and 

 other vegetable organisms, swim about in water until they find a 

 proper place for attachment, when they grow as a plant ; hence some 

 natiu'alists have supposed that animal Life is ti'ansformed iato vegeta- 

 ble, as the name zoospores implies. (See Vibrionia). The mollecular 

 motions of Dr. R. Browne — namely, those seen imder a deep magnifier 

 in a drop of water, in which fiaely divided gamboge or other organic 

 substances have been triturated ; these motions have been compared 

 with the spermazoa of animals and plants, which are now considered 

 as physical motions only. The circulation or cyclosis in plants, so 

 well exhibited in the Chara, have been compared with the motions in 

 the Closterina and Bacillaria (see Part III), and hence they are only 

 allowed a vegetable Ufe. (See M. Thuret on the Zoospores of Algae, 

 Ann. des Sciences Nat. Sieme series Tom XIV., 1850.) 



2. Cilia. The presence of these organs for locomotion, is a strong 

 argument in favour of the animal nature of an organism, but alone 

 are insufficient, as the minute spores of some Algae possess them. 



3. Digestive Organs. The presence of a stomach would strongly 

 tend to the establishment of an animal, but plants have been dis- 

 covered which possess a cavity for admitting water, and thus re- 

 sembling a digestive sac in its simplest form. While if imbibition 



c 2 



