OF THE MONADINA. 489 



Genus MONAS (XVIII. 1, 2, 15, 17, 19, 21).— The animalcules of this 

 genus — the true Monads — are described (see table) by Ehrenberg as destitute 

 of an eye, with projecting lip and tail, and as always swimming in the direction 

 of the longitudinal axis of the body, their mouth being situated at the anterior 

 end. It is another distinguishing character of the true Monad, that it is never 

 seen to cluster, like others of its family, so as to form a berry-like mass ; and 

 hence it is designated single, in contradistinction. Amongst the several 

 species distinguished, some few are green, yellowish, or of a reddish tint ; 

 but the majority are colourless ; coloui', moreover, is not a characteristic to 

 be rehed upon. Monads may often be present in water, under inspection, with- 

 out being seen, o^ving to the magnifying power employed being insufficient. 

 They will be sought for in vain with a power of less than 300 diameters ; and 

 even this, in some cases, will be found insufficient. They are, besides, as a 

 genus, difficult to be accurately determined, not only on account of their ex- 

 ceeding minuteness, but because the young of other genera are so likely to be 

 mistaken for them, — for instance, the young of the Bacterium, Vibrio, Uvella, 

 Polytoma, Pandorina, Goniuin, &c., when separated from their clusters. And 

 this difficulty in discriminating them will be more likely to happen when they 

 are not observed whilst undergoing the process of self- division, or when seen 

 in water containing but a small number of them ; under which circumstances, 

 however anxious we may be to ascertain their name, we must often rest con- 

 tented with probable sm^mise. When the water swarms with the creatures, 

 the decision vtiU be far easier, and more trustworthy, since the characters are 

 then more easily discoverable, and theii' possible variations appreciable. The 

 observer may, however, be gTiided to a certain extent by the following rule : — 

 Suppose that in a droj) of water containing species of the genus Vibrio, Bac- 

 terium, Uvella, or Polytoma (easily distinguished by their clustering forms), 

 separate Monad-like bodies were to be observed ; the probability is that they 

 would be either single forms, or the young of the clusteiing animalcules ; and 

 if there were no great difference in the size of the separate individuals and 

 those forming the clusters, this conclusion would be generally correct : and 

 this rule apphes equally to those green Monad-hke creatures found amongst 

 Pandorina and Gonium. Chlamidomonas Pidvisculus, when young, is very 

 deceptive, and may often be mistaken for an illoricated and eyeless green 

 Monad. 



The only locomotive organ which has been discovered in the genus is the 

 single fihform proboscis (filament) issuing from near the mouth. The niuner- 

 ous cilia sometimes aj)parent thereabouts are nothing more than this filament 

 in a state of vibratory or rotatory motion. This organ, Ehrenberg observes, 

 has a twofold office, one being locomotive, and the other to provide the creature 

 with food, and hence may be called a purveying organ. 



Vacuoles are readily seen in some of the species {e, g. M. Guttula and M. 

 vivipara) without the aid of coloured food ; in others {M. Termo, M. Guttula, 

 and M. social is), its aid is required. 



The propagative apparatus Ehi^enberg represented in M. Guttula and M. 

 vivipara to consist of a vast niunber of granules formed into a net-like mass, 

 dispersed generally throughout the creature, having a comparatively large 

 spherical body (the nucleus) which divides in the process of self-fission. 



Monads multiply rapidly by self-division, either transversely, as in Monas 

 Guttula, M. hyalina, M. gliscens, M. Ohenii, and M. socialis ; or longitudi- 

 nally, as in M. Punctum (XVIII. 2) : both methods have been observed in 

 M. vivipara. 



As the members of this genus are chiefly cmious on account of their extreme 

 minuteness, only the leading characters and size of the several species are 



