THE INFUSORIA 413 



Family ASPIDISCINA, Stein. Aspidisca, Ehrb. The peristome is 

 entirely on the left side. Marginal cirri completely reduced. Mega- 

 nucleus band-shaped. 0'07. Marine and freshwater. 



ORDER Peritricha, Stein. 



In this order the cilia are generally confined to a single spiral girdle 

 situated at the margin of the adoral disc, and the vestibule. In Trichodina, 

 Cyclochaeta, and Licnophora there is a second girdle of cilia at the aboral 

 end. In some cases the adoral girdle of cilia is surrounded by a ridge or 

 collar of the pellicle which is not ciliated, and in some species of Varticella, 

 etc., this can be constricted above the adoral disc during retraction in a 

 similar manner to the constriction of the margin of the disc of a sea- 

 anemone. In Spirochona there is a delicate spiral membrane at the adoral 

 end of the body (Figs. 22 and 23), but it is difficult to determine whether 

 this membrane should be regarded as due to a fusion of the cilia of a 

 spiral girdle or to an exaggeration of a spiral collar. The general surface 

 of the body of the Peritricha is naked. Nearly all the Peritricha are 

 sedentary in habit during the greater part of their existence. The 

 LICNOPHORINA readily leave their host and swim away. Many of the 

 VORTICELLINA break away from their peduncles and form a new one 

 when another suitable situation for attachment is found. The tubicolous 

 forms leave their shelter if the food supply fails and seek another locality. 

 It is probable, indeed, that none of the PERITRICHA are absolutely sedentary. 



In the more primitive forms the attachment is made by the aboral 

 disc, which acts like a sucker and can readily be released. The disc may 

 be provided not only with a peripheral girdle of cilia, but also with an 

 armament of hooks (Trichodina) or cirri (Cyclochaeta). 



In SpirocJiona and Kentrochona (Figs. 3 and 22) the adhesive disc is a 

 simple expansion of the body-wall, sometimes exhibiting pseudopodial lobes. 



The arrangement of the spiral girdle or collar may be either left- 

 handed (scaiotrichous) as in the LICNOPHORINA and SPIROCHONINA, or 

 right-handed (dexiotrichous) as in the other Peritricha with a spiral girdle. 



In Epistylis and Opercularia the body is provided with a long rigid 

 stalk or peduncle, and in Carchesium and Vorticella with a peduncle that 

 is capable of very rapid spiral contraction. The genera Cothurnia (Fig. 

 81), Vayinicola, etc., secrete a shell or tube which is attached to some 

 animal or plant, and in Ophrydium a colony of stalked individuals (Fig. 2) 

 secretes a common mucilaginous investment. 



The Peritricha may be either solitary in habit (Vorticella, Spirochona, 

 Cothurnia) or associated together in colonies (Epistylis, Fig. 78, Carchesium, 

 Ophrydium, Fig. 2). 



The mouth is usually situated at the bottom of a deep, ciliated, funnel- 

 shaped vestibule (see Fig. 27), and may open into a globular pharyngeal 

 vacuole. The anus (cytopyge) usually opens near the mouth of the 

 vestibule (Fig. 27), but is, with rare exceptions, only temporarily open. 

 In Epistylis umbellaria large nematocysts occur (Fig. 75), but these organs 

 are absent in other Peritricha. 



The meganucleus of the Peritricha is usually a long, bent, horse- 



