836 THE ANIMAL KINGDOM. 



which is sometimes permanent if the cuticle is thick, e. g. in Paramecium 

 Aurelia, or in the Vorticellina by a canal leading into the oral vestibule. 

 The new vacuole may arise as a simple expanding drop, by the coalescence 

 of a number of droplets disposed irregularly or in a rosette, sometimes, 

 however, taking the shape of converging linear or branched canals. 



Reproduction is by fission, gemmation, and spore-formation. Fission 

 is usually binary and transverse ; oblique in Stentor, Lagenophrys, and 

 Vaginicola ; longitudinal in Vorticella and most of its allies. The ciliary 

 disc and peristome of Vorticella are retracted or obliterated during the 

 process to be reproduced at its close 1 . One of the two individuals retracts 

 its disc, developes a posterior girdle of cilia on the ciliary ring (p. 834, 

 ante), breaks away and swims off to affix itself elsewhere. So too in 

 Spirochona tintinnabulum the funnel and peristome may be retracted, a 

 median ciliary wreath be evolved, and transverse fission then takes place. 

 The anterior part swims away, the posterior developes a new funnel and 

 peristome. In the Vorticellids Epistylis, Zoothamnium, and Carchesium (?) 

 repeated fission gives rise to 4-8-10 microzooids disposed in a rosette 2 . 

 They are detached. Each individual has a rudimentary ciliary disc, and 

 a well developed posterior ciliary circlet : it is probably always destined 

 to conjugate with an ordinary individual or macrozooid (p. 837, infra]. In 

 the Opalinidae variations occur. Haptophrya gigantea is resolved by re- 

 peated binary fission into a chain of eight individuals. Two small posterior 

 segments are produced simultaneously in Anoplophrya nodulata : a posterior 

 region is marked off in A. prolifera and Benedenia, and is then divided 

 into a chain of segments. Opalina itself breaks up into a number of 

 portions by repeated transverse or oblique divisions : and the fragments 

 thus formed encyst and are devoured by the tadpole of the Frog with its 

 food 3 . The peristome of the new individual in Stentor and some other 

 Heterotricha, and probably in most Hypotricha begins to develope before 

 separation is completed. The products of fission may grow in size 

 before the process is repeated ; if they do not do so, the result is a 

 rapid diminution in size of the individual remedied eventually by con- 

 jugation. The products of fission may remain living in societies (p. 830) : 

 or if they are organically connected by a stalk or pedicle they constitute 

 colonies, as in the Vorticellina. New colonies are founded by individuals 

 which retract or lose the peristome, develope a posterior ciliary wreath, are 

 detached and swim away to settle in a new spot. In some species of 

 Zoothamnium, e.g. Z. arbuscula, this duty falls upon special large spheroidal 



1 BUtschli says the cilia persist, M. J. xi. p. 561. 



1 Binary fission of one of two individuals just produced by fission has been seen in Vorticella 

 microstoma, and an instance of the formation of microzooids recorded; see D'Udekem, Mem. Acad. 

 Roy. Belg. xxxiv. p. 8, PI. I. fig. 50. In the genus Vorticella microzooids are perhaps formed as a 

 rule by gemmation. 



3 See on this subject, Gruber, Biol. Centralblatt, pp. iv. 712-13, 715-16. 



