26 LOWER INVERTEBRATES. 



Class IV. — INFUSORIA. 



The Infusoria are the highest of the Protozoa. They are so regarded because they 

 usually have a definite shape, owing to the fact that the outer portion of their bodies is 

 much more dense than the inner. It may, in fact, be said, with slight freedom in the 

 use of words, tliat they are surrounded by a skin, or, to use the term of science, an ecto- 

 sarc ; the jirolongations of tlieir protoplasm for the purpose of locomotion and prehen- 

 sion are permanent, not transient, as in the pseudopodia of the groups just passed, 

 extended or withdrawn at pleasure. In all but the lowest the food is received into the 

 body by one or more mouths ;. and, with rare exceptions, tliey are active m their move- 

 ments. Additional points of su])eriority will be seen further on. 



Few Infusoria are individually visible without the aid of a microscope ; but some- 

 times they form large colonies, which are readily seen. All are aquatic, and wherever 

 standing water appears there are Infusoria. " They abound in the full jjlenitude of life 

 alike in the running stream, the still and weed-grown pond, or the trackless ocean. 

 Nay, more, . . . every dew-laden blade of grass supports its multitudes, while in the 

 semi-torpid, or sporular state, they permeate as dust the atmosphere we breathe, and 

 beyond question form a more or less considerable increment of the very food we 

 swallow." 



Anthony van Leeuwenhoek has the honor of first publishing an account of an infu- 

 sorian. His "Obs. . . . concerning little animals observed in Rain, Well, Sea, and 

 Snow Water, as also Water wherein Pejjper had lain infused," apjjeared in the Philos- 

 ophical Transactions, Vol. XII., 1677 ; the recorded discoveries were made during the 

 two previous years. In Observation I. (1675), four forms are described; of the first he 

 says, " The first sort by me discovered [in rain-water which had been standing four 

 days] I divers times observed to consist of 5, 6, 7, or 8 clear globules. . . . When 

 these animalcula, or living atoms, did move, they put forth two little horns, continually 

 moving themselves ; the place between the horns was flat, though the rest of the body 

 was roundish, sharpening a little towards the end, where they had a tayl near four 

 times the length of the whole body." There is no difliculty in recognizing by this 

 description a species of Vorticella. It adds fi-esh interest to these charming animalcules 

 to know that they were the first of their numerous kindred to be discovered. 



The name " Infusoria " was first used by M. F. Ledermuller in 1763. Until quite 

 recently it was applied to a heterogeneous assemblage of minute animals and plants 

 having little in common but minuteness. The limits of the group are now pretty well 

 defined ; there are still differences of opinion concerning certain forms, but all students 

 of the Protozoa now agree to relegate the diatoms, desmids, and rotifers to other and 

 very diverse relations. The structure of the individual infusorian as at present limited 

 will now be discussed. 



The zooids of this group of the Protozoa are essentially unicellular ; in the lowest 

 forms they may consist of a naked cell {gymnocyta), or in the higher they may possess 

 a cell membrane {lepocyta). Ehrenberg held that they were complex or multicellular; 

 but this view resulted from his " polygastric " theory, put forth in 1830, which has been 

 shown to be not well founded. Among the distinguished investigators who have also 

 advocated the multicellular theory, may be mentioned Diesing, O. Schmidt, L. Agassiz, 



