INFUSORIA. 831 



The Infusoria present great varieties of external shape : their body 

 is sometimes eminently contractile, therefore changeable in outline. The 

 Hypotricha, however, have a flattened locomotor or ventral surface, and 

 a convex dorsal ; and the fixed Peritricha are more or less bell-shaped. 

 The surface of the body is rarely completely naked, but is invested with 

 a cuticular layer of varying thickness and consistence 1 . It is elastic, 

 generally hyaline, seldom granular, or marked by vertical lines (? rods or 

 spaces) ; and is often scarcely distinguishable from the underlying proto- 

 plasm. There is sometimes, however, a very distinct cuticle, either a 

 dorsal thickened cuirass (Eiiplotidae among Hypotrichd), a bivalved cuirass 

 (some Dysteridae in the same order), or rings of plates, as in the Holo- 

 trichan Coleps hirtus. The coat of needles in Tiarina ^Coleps fusus), 

 or the similar structures covering the pedicles of some Vorticellae, are 

 probably cuticular : so too the simple or denticulate ring which supports 

 the sucker of all the Peritrichan Urceolarina save Licnophora and the 

 corneous band or teeth of the Opalinid Hoplitophrya. A number of 

 Infusoria secrete a distinct tube or lorica, usually fixed, sometimes free 

 as in Tintinnodea y which may be, as in some species of Stentor, only a 

 temporary structure. It may be gelatinous, or though at first soft and 

 colourless, becomes hardened, dark in hue, and resistent to reagents. In 

 the Tintinnodean Codonella, particles of silica are imbedded in its sub- 

 stance, and in its congener Dictyocysta, it is perforate and Radiolarian in 

 appearance. Its chemical composition is unknown, save in the case of 

 the hyaline investment of the colonial Ophrydium versatile, where it is 

 composed of a cellulose akin to Tunicin (p. 107), and though apparently 

 of a uniform structure, is really divisible into a number of coherent sheaths, 

 one to each individual. 



The cilia, which are so characteristic of the class, are extensions of 

 the outer layer of protoplasm, and are retractile under certain conditions. 

 They are sometimes used as organs of adhesion, and vary in character. 

 The cilia proper are slender, of the same size from base to apex, vibratile 

 throughout : the cirri, the elongated triangular membranellae s. pectinellae, 

 the membranes, are composed of agglutinated cilia, and may be wholly 

 or partially vibratile ; the setae are slender, rarely compound, non-vibratile 2 . 

 The modes in which the cilia are disposed afford a basis for classification. 

 In the Holotricha they are arranged in parallel lines and are of uniform 



1 See Maupas, A. Z. Expt. (2), i. p. 574. Opalina is said by Zeller to be naked, but the 

 superficial bands, supposed by him to be muscular, are probably cuticular. The chemical nature of 

 the plastic cuticle is unknown : it is more resistent than protoplasm, but decomposes sooner or later 

 in water. 



2 For cilia as organs of adhesion, see Maupas, A. Z. Expt. (2), i. pp. 629-30. Note that mem- 

 branellae are said to appear in Stentor as a continuous membrane subsequently broken up. A 

 temporary fusion of cilia into vibratile bundles has been sometimes observed. See Geza Entz, 

 Mitth. Zool. Stat. Naples, v. p. 332. 



