OF THE PHTTOZOA. 131 



Very little observation and reflection will soon convince the student that 

 the members of this group of beings can be distinguished by no such constant 

 definite characters as suffice to establish genera and species with any pre- 

 cision ; their history is too imperfectly known, and their individuality is un- 

 proved. If they make their appearance in a fluid, it is only transitory ; for 

 they are soon replaced by a different series of existences, and direct observa- 

 tion has shown many of them to be no other than transitional phases of life 

 of other organisms. Thus, Dujardin advances as an apology for his attempted 

 classification of Monadina, that the generic distinctions he has essayed to 

 make " are entirely artificial, and simply intended to facilitate the naming of 

 Infusoria which may have been met Avith in any particular infusion, but 

 which, when better known, may prove in some instances mere varieties of one 

 and the same species " (Hist. lufus. p. 273). Siebold entirely rejects this 

 family of Monadina from the Infusoria, believing them only embryonic forms, 

 and chiefly zoospores of Confervae, &c. (Anat. d. luirheUos. Tliiere, 1848, 

 p. 8). In so doing he has had many approvers, — among them the eminent 

 natiu-alist M, Agassiz, who thus writes: — ''Recent investigations upon the 

 so-called Anentera have satisfactorily sho^ii, in my opinion and in that of 

 most competent observers, that this type of Ehrenberg's Polygastrica, without 

 gastric cavities and without alimentary tube, are reaUy plants belonging to 

 the order of xilgse in the widest extension of this group, while most of the 

 Monas tribe are merely moveable germs of various kinds of other Algae " 

 (A. N. H. 1850, vi. p. 156). Nevertheless the character of this treatise renders 

 it necessary for us to present Ehrenberg's views of organization. According to 

 these, "■ the Monadina are illoricated, Tvith a homogeneous body, and no external 

 appendages except cilia, having many separate gastric sacs or vesicles, but 

 no alimentary canal connecting them, and a bisexual or hermaphrodite pro- 

 pagative system. They multiply by simple and complete self-division of the 

 body into two, foui', or more indi\'i duals. The uniformity or unvarying 

 external form may be considered one of the principal characteristics of the 

 family ; for no one of the Monadina can voluntarily alter the shape of its 

 body, nor can it extend any portion of it and then contract it again. Pro- 

 pagation by ova is assiuned of all the Monadina, and by living young, or 

 vivij)arous reproduction, in Monas vivijxira. Some of them have an eye- 

 speck, but no vascular or respiratory system is discernible." 



Although the general characters of the Monads are rightly delineated in 

 this account, yet the peculiar hypothesis implied will not at the present day 

 find supporters. Dujardui denied the presence of an enveloping skin or 

 integument ; and if a separable distinct tunic is intended, that natiu^alist is in 

 the right ; yet it would be an error to ignore the existence of a layer of 

 different consistence to the contained matter, i. e. of a pellicle. Besides such 

 a peUicle, some Monads, at least, have the power of secreting around them- 

 selves a second external envelope or cyst, or of ' encysting ' themselves. 

 When thus transfomied, Ehrenberg would not recognize them as Monadina, 

 but as Cryptomonadina, or loricated Monadina. Hence one source of error 

 in his distribution of these minute microscopical forms. 



The invariability of form and incapability of extending and retracting the 

 body, so prominently advanced as special features of Monadina, Dujardin 

 does not admit as facts, but, on the contrary, states them to be without 

 integ-ument, and susceptible of adhesion to one another or to foreign par- 

 ticles, and to be capable of stretching themselves out so as to alter their form, 

 even so far as to produce an expansion which may at times be mistaken for 

 another filament. Some Monadina, he adds, can, while freely swimming 

 about, change theii' fonn, and by so doing approach the character of Amoehce. 



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