754 MICROSCOPIC FOKMS OF ANIMAL LIFE 



is difficult to decide what is their relationship to other groups of 

 animals. Notwithstanding the wide zoological separation between 

 these two kinds of animalcules, it seems most suitable to the plan 

 of the present work to treat of them in connection with one another ; 

 since the microscopist continually finds them associated together, 

 and studies them under similar conditions. 



SECTION I. INFUSORIA. 



This term, as now limited by the separation of the Rhizopoda 

 on the one hand, and of the Rotifer a on the other, is applied to a 

 far smaller range of forms than was included by Professor Ehren- 

 berg under the name of ' polygastric ' animalcules. For a large 

 section of these, including the JJesmidiacew, Diatomace<i j , Volvocinece, 

 and many other protophytes, have been transferred by general 

 (though not universal) consent to the vegetable kingdom. And 

 it is not impossible that many of the reputed Infusoria may be but 

 larval forms of higher organisms, instead of being themselves com- 

 plete animals. Still an extensive group remains, of which no other 

 account can at present be given than that the beings of which it is 

 composed go through the whole of their lives, so far as we are ac- 

 quainted w r ith them, in a grade of existence which is essentially 

 protozoic, each individual apparently consisting of but a single cell, 

 though its parts are often so highly differentiated as to represent 

 (only, however, by way of analogy} the 'organs' of the higher 

 animals after which they are usually named. 



Among the ciliate Infusoria, which form not only by far the 

 largest, but also the most characteristic division of the group, there 

 is probably none save such as are degraded by parasitic habits 

 which has not a mouth, or permanent orifice for the introduction 

 of food, which is driven towards it by ciliary currents ; while a 

 distinct anal orifice, for the ejection, of the indigestible residue, is 

 not infrequently present. The mouth is often furnished with a 

 dental armature, and leads to an wsopkageal canal, down which 

 the food passes into the digestive cavity. This cavity is still 

 occupied, however, as in rhizopods, by the endosarc of the cell ; but 

 instead of lying in mere vacuoles formed in the midst of this, the 

 food-particles are usually aggregated, during their passage down 

 the oesophagus, into minute pellets, each of which receives a special 

 investment of firm protoplasm, constituting it a digestive vesicle 

 (fig. 589) ; and these go through a sort of circulation within the 

 cell- cavity. 



The ' contractile vesicles,' again, attain a much higher develop- 

 ment in this group, and are sometimes in connection with a network 

 of canals channelled out in the ' ectosarc ; ' while their rhythmical 

 action resembles that of the circulatory and respiratory apparatus 

 of higher animals. There is ample evidence, also, of the presence 

 of a specially contractile modification of the protoplasmic substance, 

 having the action (though not the structure) of muscular fibre ; 

 and the manner in which the movements of the active free-swimming 

 Infusoria are directed so as to avoid obstacles and find out passages 



