2b THE MICROSCOPE AND ITS REVELATIONS. 



415. Before proceeding to the description of the ciliate Infusoria, 

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

 and the suctorial which, on account of the peculiarities of their struc- 

 ture and actions, are 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 distinguishable morphologically from those 

 of Protophytes. 



416. FLAGELLATA. Our knowledge of this tribe has been greatly 

 augmented 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, 1 which are the 

 smallest animals at present known, are its simplest representatives; but 

 it also includes organisms of much greater complexity; and some of its 

 composite forms have a very remarkable relation to Sponges ( 508). The 

 monas lens, long familiar to Microscopists as occurring in stagnant waters 

 and infusions of decomposing organic matter, is a spheroidal particle of 

 protoplasm, from l-2000th to l-5200th of an inch in diameter, inclosed 

 in a delicate hyaline investment or 'ectosarc' and moving freely through 

 the water by the lashing action of its slender flagellum, whose length is 

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

 be seen a variable number of vacuoles; aud these are occasionally occu- 

 pied by particles distinguishable by their color, which have been intro- 

 duced 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 the smallest Monadince, neither nucleus nor contractile vesicle is dis- 

 tinguishable; but in larger forms a nucleus can be clearly seen. The 

 life-history of several simple Monadince, presenting themselves in infu- 

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

 ductive material), has been studied with admirable perseverance and 

 thoroughness by Messrs. Dallinger and Drysdale, of whose important 

 observations a general summary will now be given. 2 



417. The Monad-form most recently and completely studied by Mr. 

 Dallinger with all the advantages derived from trained experience, and 

 under objectives of the highest quality and greatest magnifying power 

 is the Dallingeria Drysdali (Kent) represented in Plate xiii. Its normal 

 shape, as seen in fig. 1, is a long oval, slightly constricted in the middle, 

 and having a kind of pointed neck (a), from which proceds a flagellum 

 about half as long again as the body. From the shoulder-like projections 

 behind this (b, c) arise two other long and fine flagella, which are directed 

 backwards. The sarcode body is clear, and apparently structureless, with 

 minute vacuoles distributed through it; and in its hinder part a nucleus 

 (d] is distinguishable. The extreme length of the body is seldom more than 

 the 1-4, 000th of an inch, and is often less. This Monad swims with 



plexity of the like processes as seen even in the lowest of the Metazoa ( 391) 

 which has been specially and forcibly insisted on by Haeckel (" Zur Morphologic 

 der Infusorien," Jenaische Zeitschr., Bd. vh., 1873). An excellent summary of 

 the whole discussion was given by Prof. Allman, in his Presidential Address to 

 the Linnaean Society in 1875. 



1 The Family monadina of Ehrenberg and Dujardin consists of an aggregate 

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

 the Vegetable Kingdom. 



2 See their successsive Papers in the " Monthly Microsc. Journ.," Vol. x. (1873). 

 pp. 53, 245; Vol. xi. (1874), pp. 7, 69, 97; Vol. xii. (1874), p. 261; and Vol. xiii. 

 (1875), p. 185; and " Proceed. Roy. Soc.," Vol. xxvii. (1878), p. 332. 



