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PREFACE.IT is now some ten years since the author, then but a recruit in the ranksof practical microscopists, elected to concentrate his attention upon thegroup of organisms that form the subject of this treatise. At a very earlyperiod of his investigations, formidable obstructions to substantial progressin the course mapped out, presented themselves in connection not onlywith the very backward condition of the literature of this country relatingto this topic, but by reason also of the exceedingly wide and scattered areaof Continental bibliography that had to be explored and sifted before itwas possible to arrive at any adequate idea of the state of contemporaryknowledge concerning almost any given type that might be the subject ofexamination. It was the recognition of, and continual contact with thesedifficulties that suggested to the author the advantagesthat would accrueboth to himself and all English-speaking microscopists, from the compilationof a treatise, brought up to date, that should contain a concise descriptionof the innumerable speciesknown to science whose descriptions were distributedthroughout many scattered sources, and that led to the efforts,now carried into execution, to supply this desideratum.It was in the first instance suggested that this Manual should be basedupon the same lines as the, at the time, only other English treatise devoted'to the subject, A History of the Infusoria/ by Andrew Pritchard, the fourthand last edition of which was published so long since as the year 1861 ;that it should include in a similar manner an account of the several distinctgroups of microscopical organisms known as the Rotifera, Desmidiaceae,Diatomaceae, and other Protophytes which form,as being a reproductionof Ehrenberg's 'Infusionsthiere/ so conspicuous a feature of Mr. Pritchard'sbook. It soon became apparent, however, that to compass so comprehensivea task with any degree of efficiencywould extend the size of thistreatise far beyond convenient limits, and that indeed more than sufficientmaterial for a work on the same scale as the one above-named hadaccumulated in connection with the Infusoria in the most limited andrestricted sense as represented by the Flagellate, Ciliate, and TentaculiferousProtozoa.Those readers and subscribers, therefore, who at first sight mayI LIH 3