MICROSCOPIC FORMS OF ANIMAL LIFE. 25 



SECTION I. INFUSORIA, 



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

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

 smaller range of forms than was included by Prof. Ehrenberg under the 

 name of ' polygastric ' animalcules. For a large section of these, includ- 

 ing the DesmidiacecB, Diatomacece, Volvocinecv, and many other Proto- 

 phytes, 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 complete 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 acquainted with them, in a grade of existence which is essentially 

 Protozoic ( 391); 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 * f organs' of the higher animals 

 after which they are usually named. 



413. 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 which has not a mouth, or permanent orifice for the intro- 

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

 tinct anal orifice, for the ejection of the indigestible residue, is also gen- 

 erally present. The mouth is often furnished with a dental armature; 

 and leads to an wsophageal canal, down which the food passes into the 

 digestive cavity. This cavity is still occupied, however, as in Khizopods 

 (403), by the enaosarc 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. 299); and these go through a sort of circulation 

 within the cell-cavity. 



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

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

 canals chanelled-out in the ' ectosarc ' while their rhythmical action 

 resembles that of the circulatory and respiratory apparatuses 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, seems to indicate that another 

 portion of their protoplasmic substance must have to a certain degreee 

 the special endowments which characterize the nervous systems of higher 

 animals. Altogether, it may be said that in the Ciliate Infusoria the 

 Life of the Single Cell finds its highest expression.* 



1 The doctrine of the unicellular nature of the Infusoria has been a subject 

 of keen controversy among Zoologists, from the time when it was first definitely 

 put forward by Von Siebold(" Lehrbuch der vergleich. Anat.," Berlin, 1845) in 

 opposition to the then paramount doctrine of Ehrenberg as to the complexity of 

 their organization, which had as yet been called in question only by Dujardin 

 "Hist. Nat. des Infusoires," Paris, 1841). Of late, however, there has been a 

 decided convergence of opinion in the direction above indicated; which has been 

 brought about in great degree by the contrast between the Protozoic simplicity 

 of the reproductive and developmental processes in Infusoria, and the com- 



