An Introduction to the study of the protozoa with special reference to the parasitic forms / by E.A. Minchin.

  • Minchin, Edward Alfred.
Date:
1912
Catalogue details

Licence: In copyright

Credit: An Introduction to the study of the protozoa with special reference to the parasitic forms / by E.A. Minchin. Source: Wellcome Collection.

Provider: This material has been provided by Royal College of Physicians, London. The original may be consulted at Royal College of Physicians, London.

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    Siebold's generalization was a great step in advance, introducing clear and orderly ideas into the place of the chaotic notions pre- viously held, and setting definite limits to the group Protozoa by excluding from it various types of organisms, such as Sponges, Rotifers, etc., which had hitherto been classed as Protozoa, but ^\•hich were now referred definitely to the Metazoa. Nevertheless Siebold's definition presents many difficulties, especially when con- sidered from a wider standpoint than that of the zoologist. This will be apparent if the two words of the definition given above, unicellular animals, be considered critically. 1. Unicellular.'''—Accepting the standpoint of the cell-theory, it has already been noted that many other organisms besides Protozoa must be regarded as single cells. Moreover, it is found that many organisms which must be classed as Protozoa appear constantly in a multicellular condition ; such are the well-known genus Volvox and its allies, besides examples of other orders. In aU cases of this kind, however, the constituent cells are morphologically equivalent, and are to be regarded as complete individuals more or less inde- pendent, sho-\ving as a rule no differentiation, or, if any, only into reproductive and vegetative individuals ; and the multicellular organism as a whole is to be regarded as a colony of unicellular individuals primitively similar but secondarily differentiated, it may be, in relation to special functions. Such multicellular Protozoa present, in fact, a perfect analogy with the colonial forms seen in many groups of animals higher in the scale, especially the Coelentera, where also the members of a colony, primitively equivalent and similar amongst themselves, may become differentiated secondarily for the performance of distinct functions by a process of division of labour among different individuals. It is not possible to con- found the multicellular Protozoa with the Metazoa, in which the organism is not only composed of many cells, but exhibits also cell-differentiation based on mutual physiological dependence of the cells on one another, leading to the formation of distinct tissues ; that is to say, aggregations or combinations of numerous cells, all specialized for the performance of a particular function, such as contraction, secretion, and so forth. The essential feature of the Protozoa, as contrasted with the higher animals or plants, is to be sought in the independence and physiological completeness of the cell-individual. The Metazoa are tissue-animals, in which the primitive individuality of the cell is subordinated to, or has a restraint imposed upon it by, the corporate individuality of the cell-aggregate. In the Protozoa the cells are complete individuals, morphologically and physiologically of equal value. If, however, as few will doubt, the Metazoa have been evolved from simple unicellular ancestors, similar to the