OF THE PROTOZOA. CILIATA. 327 



unless the body is much occupied by food and opaque particles of any kind. 

 If not at once apparent, it is demonstrable by the disniption of the body by 

 pressui'e ; by the process of diffluence, which disperses the surrounding tissues ; 

 or by the addition of acetic acid, which dissolves the rest of the animal, 

 leaving the nucleus more or less completely isolated. 



It occurs as a well-defined, finely-granular, more or less opaque body, 

 having a more solid look than the surrounding parts, and frequently also 

 a tawy or sHght yellow blush (XXIX. 28 c, 30 c, 48 c; XXX. 1, 11 d, 

 12 h, 27/). It varies both in position and shape in different species, and 

 either presents one or more internal spots or small bodies of a circular outline 

 which represent the nucleolus or nucleoli, or this organ may appear as a 

 distinct appendage to it (XXVIII. 9-15 ; XXIX. 28). The nucleus is im- 

 bedded in or closely united with the cortical lamina ; and although it may be 

 thrust aside by the impetus of passing particles of food, it retains its hold. 

 Under the usual point of view of an animalcule, its position will look more 

 central than it really is ; for it is either in advance or in rear of the real 

 centre, or to one side or other of it, and often lies across the ahmentary tube 

 when elongated or band-like. But what is curious about this organ is, that 

 it is not at all firmly fixed in its position, but is pushed forward or backward 

 to one side and to the other by the movements of the animal, particularly by 

 those of the retractile ciliary wreath, and also by the ingestion of food. This 

 may be witnessed in Opercularia and Eplstylis. Lastly, even in examples of 

 the same species its position is not constant. 



The usual figure of the nucleus is circular or oblong, but it may be clavate or 

 reniform, or sinuous and band-like. The first type of outline prevails in Para- 

 mecium (XXIX. 28), Colpoda (XXIX. 37), Nassida (XXVIII. 1), Chilodon 

 (XXIX. 48), Spirochona (XXX. 17), and Stylont/chia (XXVIII. 10 d). A 

 reniform or kidney-shaped one is seen in Epistylis plicatdis, in Opercrdaria 

 articidata (XXX. 1), and in 0. berberina ; a horse-shoe figure in Vorticella and 

 Zoothamnimn ; whilst in Epistylis brancMophila, in Ophrydium (XXX. 5, 6), 

 Carchesium, Trichodina (XXIX. 16, 17), Lagenophrys (XXX. 29, 30), &c. 

 it is still more elongated and band-like and much curved, or actually sinuous. 

 Cohn represents it as having a thick clavate figure in Nassida elegans. The 

 figui^e, moreover, is very much modified during the reproductive processes, 

 and in the metamorphoses which befall some at least of the Ciliated Protozoa ; 

 these modifications, however, we shall not here consider, but reserve them to 

 the details on development. Again, even among examples of the same species, 

 sHght variations occiu' in length and width, and in curvature or sinuosity, 

 where no reproductive act is discernibly in progress. Lastly, not a few of 

 the nuclei, which are at first sight simply oblong, are, on closer examination, 

 seen to have a depression or sulcus on one side, and consequently to be, strictly 

 speaking, bean- or kidney-shaped. This is exemplified in the nuclei of Para- 

 mecium, certain Nassulce, and in Prorodon. 



Where the nucleus is elongated, it is a common event to see it bent par- 

 tially round the pharynx or the oesophagus, at some little distance from it. 



The nucleus being the last of the soft contents to break up after death, 

 is presumably of a more solid texture. Its tissue may be described as 

 normally homogeneous ; but various changes are ever occurring in it, render- 

 ing it at one time more transparent, at another more granular and opaque. 

 It must owe a certain degree of resistance to external injuries to the fact 

 that it is enclosed by a tough elastic membrane or sac, which sometimes is 

 separated from it by a clear interspace or areola, but at other times is closely 

 adherent, and only demonstrable by artificial means, such as the aj^plication 

 of chemical reagents, or of a solution of potash, or of acetic acid: this 



