OF THE PROTOZOA. CILIATA. 



285 



in fiill ^'ital activity and surroimded by favourable external conditions, and 

 adds that gemmation frequently proceeds in these encased beings, and that, 

 ■when from evaporation of the surrounding fluid or other prejudicial cause the 

 animals are threatened with injury, they quit their sheaths and swim away, 

 the pre^'iously suctorial extremity resohdng itself into a pencil of bristles. 

 The result of these observations of Cohn is to disassociate this phenomenon of 

 sheath-formation in Stentor from that of the encysting process, to which 

 Ehrenberg's account of it would have led it to be referred. 



Dr. Strethill Wright coincides "\^'ith Cohn in denying the relation between 

 the presence of the sheath of Stentor Mulleri and the diseased or dying state 

 of the animalcule. Indeed he speaks of the presence of a gelatinous case 

 as the rule, and adds that '' as the zooids (animalcules) di\dde they form a 

 gelatinous mass, which is attached to weeds and often to the siuface of the 

 water, from which I have seen some 10 or 15 combined Stentors hanging 

 with their heads downwards." 



Cilia ais^d Ciliaey Action. — The most common, and at the same time the 

 characteristic external appendages of the Ciliated Protozoa are the cilia, Avhich 

 constitute their most active and powerful locomotive organs. Cilia are, 

 moreover, not wanting internally, but are there comparatively few, since 

 they are appendages only of free surfaces. They are met with lining the 

 oesophagus, where they, no doubt, seiwe to facilitate the ingestion of food and 

 of the water taken in for the purposes of aeration. 



The nature and cause of ciliary movement have been much debated. To 

 account for the energetic and peculiar movements of cilia, Ehrenberg imagined 

 the existence of a muscular aj)paratus at their globular roots, consisting of four 

 muscles, each pulling in an opposite dii'ection, but, by acting in succession, 

 causing the apparent rotation of the axis around the fixed base. This bold 

 idea has met with no favour among physiologists, who condemn it as purely 

 imaginary and as opposed to the simplicity of natiu-e, to all analogy, and to 

 aU the admitted facts and principles of liistology. Most inquirers despair of 

 attaining a satisfactory explanation, of ciliary action, and treat it as an ulti- 

 mate fact. However, Cohn, looking to the peculiar structure of the integu- 

 ment of Paramecium {Loxodes) Bursaria (XXIX. 26), fancied that ciliary 

 motion admitted of explanation, since, on the supposition of an inherent 

 contractility in that membrane, each Kttle pyramid might be imagined to 

 contract its sides in turn, and make the cilium surrounding it revolve in the 

 figure of an inverted cone. But granting the possibility of this explanation 

 in the case of the animalcule cited, it could in no wise be applied generally 

 to cihary motion ; for a similar structiu-e is found in comparatively few other 

 examples, and the innate contractility of the supporting membrane, assumed 

 in the instance in question, has certainly no existence in many ciliated sur- 

 faces, and involves nearly an equal stretch of imagination to conceive as 

 Ehrenberg's muscles. 



Returning from this digression on the nature and cause of ciliary action, 

 let us briefly review the mode of distribution of cilia in the Protozoa. In. 

 many genera they are chstributed universally over the surface (XXIX. 20, 

 28, 48 ; XXYIII. 1, 8, 31), not at random, however, but in definite parallel 

 lines, more or less approximated, usually traversing the length of the body. 

 A distribution in parallel lines is also not unfrequently obsei-ved across 

 or around the body. Even where generally difi'used over the body, they are 

 commonly more developed at certain parts, as about the mouth, the head, and 

 tail, as well as on any processes or in any depressions of the body, e. g. in 

 Chihdon (XXIX. 48), Bursaria, Leucophrys, Stentor, &c. Stein represents 

 it as a generic character, that in Paramecium (XXIX. 28) all the cilia are 



