838 THE ANIMAL KINGDOM. 



fragmentation of the nuclei and the formation of a new nucleus. Lateral 

 union between two or three ordinary individuals has been witnessed in 

 Vorticella microstoma, Epistylis brevipcs and Carchesium polypinum. The 

 united zooids develope posterior ciliary wreaths, and are detached from 

 their peduncles. Similar union may take place between zooids of V. cam- 

 panula, already free. The united animals may either settle down again 

 or encyst, the latter being perhaps the normal result 1 . 



All Infusoria appear to encyst ; and the Hypotrichan Gastrostyla 

 vorax has been preserved alive in this condition for two years. Cilia, 

 peristome, all organs save the nucleus and contractile vacuole are lost. 

 The cyst is spheroidal, as a rule smooth, occasionally ridged or papillate ; 

 flask-shaped in Stentor coeruleus, and closed by an operculum. Its mem- 

 brane may be single or double. The formation of a gelatinous envelope 

 has been observed in Zoothamnium mucedo inclosing 1-9 individuals in 

 situ on their stalks 2 . 



The Infusoria are for the most part microscopic in size ; some how- 

 ever are visible to the naked eye, e.g. Stentor polymorphus, which attains 

 a length of -%-$ in. ; so too the colonies of some Vorticellina, especially the 

 arborescent colonies of Zoothamnium arbuscula ; and Ophryditim versatile 

 gives rise to gelatinous hollow masses 5 in. across when full grown. In- 

 fusorians inhabit fresh, salt and brackish waters ; some species, indeed, e. g. 

 Chilodon cuctdlulus, Zoothamnium arbuscula are common to both fresh and 

 salt. The faunae of different seas are not more dissimilar than are the 

 faunae of different freshwaters, and many littoral or brackish-water forms 

 are met with in salt lakes (Geza Entz). A few Infusorians are parasitic ; 

 some ectoparasitic, e. g. Ichthyophthirius on freshwater fish, others endo- 

 parasitic in Vertebrata and Non-vertebrates alike, e. g. the Opalinidae. A 

 Heterotrichan Balantidium coli is found in the large intestine of the 

 human subject and in the rectum of swine. Dimorphism is restricted to 

 certain colonial forms (p. 837) or products of fission or gemmation 

 (p. 837 and note i). 



The Infusoria are classified as follows : 



1 . Holotricha : free-swimming, more or less completely ciliate ; cilia similar or 

 slightly dissimilar, especially the adoral cilia ; an extensile or undulatory membrane 

 sometimes present ; trichocysts not uncommon : Paramerium, Prorodon, Nassula, 

 Coleps, Trachelius, Ichthyophthirius, &c. ; Opalinidae, e. g. Opalina, Anoplophrya, 

 Benedenia. 



2. Heterotricha : free or attached either at will, or rarely permanently ; naked or 

 tubicolous ; cilia of small size covering the body, sometimes supplemented by non- 



1 Claparede and Lachmann, Etudes sur les Infusoires, etc. ii. p. 229 et seqq. ; Stein, Organis- 

 mus, etc. (infra), ii. pp. TO, 113. 



a Gruber, Nova Acta, xlvi. pp. 520-1 ; cf. Geza Entz, Mitth. Zool. Stat. Naples, v. p. 418; 

 Nachtrag, p. 439. Species of Amphikptus prey upon the colonial Vorticdlidae, and encyst on the 

 ends of the branches after a meal. Cf. Entz, op. cit. p. 420. 



