INFUSORIA 755 



seems to indicate that another portion of their protoplasmic sub- 

 stance must have to a certain degree the special endowments which 

 characterise the .'le/'rottx 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 



Before proceeding to the description of the ciliate Infusoria, 

 however, it will be of advantage to notice two smaller groups the 

 flayellate and the suctorial which, on account of the peculiarities 

 of their structure and actions, aYe now ranked as distinct, and of 

 whose ' unicellular ' character there can be no reasonable doubt, 

 since they are, for the most part, * closed ' cells, scarcely distinguish- 

 able morphologically from those of protophytes. 



Flagellata. Our knowledge of this tribe has been greatly aug- 

 mented in recent years, not only by the discovery of a great variety 

 of new forms, but still more by the careful study of the life-history 

 of several among them. The monads, properly so called, 2 which are 

 amongst the smallest living things at present known, are its simplest 

 representatives ; but it also includes organisms of much greater 

 complexity ; and some of its composite forms seem to have a very 

 remarkable relation to sponges. The Monas lens, long familial' 

 to microscopists as occurring in stagnant waters and infusions of 

 decomposing organic matter, is a spheroidal particle of protoplasm, 

 from or/out-h to 5 oV>oth of an inch in diameter, enclosed in a delicate 

 hyaline investment or ' ectosarc,' and moving freely through the 

 water by the lashing action of its slender flayellum, whose length 

 is from three to five times the diameter of the body. Within the 

 body may be seen a variable number of vacuoles ; and these are 

 occasionally occupied by particles distinguishable by their colour, 

 which have been introduced as food. These seem to enter the body, 

 not by any definite mouth (or permanent opening in the ectosarc), 

 but through an aperture that forms itself in some part of the oral 

 region near the base of the flagellum. In some true Monadinw 

 neither nucleus nor contractile vesicle is distinguishable, but in 

 the majority a nucleus can be clearly seen. The life-history of 

 several simple Monadince presenting themselves in infusions of 

 decaying animal matter (a cod's head being found the most pro- 

 ductive material) has been studied with admirable perseverance 



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

 keen controversy amongst zoologists from the time when it was first definitely put 

 forward by Von Siebold (Lehrbuch der vergleicJi. Anat. Berlin, 1845) in opposition 

 to the then paramount doctrine of Ehreiiberg as to the complexity of their organisa- 

 tion, 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 de- 

 velopmental processes in Infusoria, as, for example, shown by Dallinger and Drysdale, 

 and by the former alone in the life-histories of the Saprophytes, and the complexity 

 of the like processes as seen in even the lowest of the Metazoa, which has been 

 specially and forcibly insisted on by Haeckel (' Zur Morphologic der Infusorien,' 

 Jenaische Zeitschr. Bd. vii. 1878). An excellent summary of the whole discussion 

 was given by Professor Allman in his Presidential Address to the Linnean Society in 

 1875. 



- The family Mottadina of Ehreiiberg and Dujardin consists of an aggregate of 

 forms now known to be of very dissimilar nature, many of them belonging to the 

 vegetable kingdom 



3c2 



