SPECIAL MORPHOLOGY AND TAXONOMY OF INFUSORIA 395 



on the ventral surface whereby a slioiilder-hke ledge bounds the 

 cirri-forming area on the right side. In the genus Onychaspis (Fig. 

 80, p. 152), the cirri are brush-like aggregates of loosely associated 

 cilia directed at right angles to the long axis of the body and the 

 peristome begins at the middle of the left margin. In the genus 

 Aspidisca, the cirri are directed parallel with the long axis; the 

 anals are numerous (from 5 to 12) and the peristome begins at the 

 anterior end. 



Order V. PERITRICHIDA, Stein. 



The adoral zone of membranelles of a heterotrich or of a hypo- 

 trich from the gullet, turns to the left if viewed from the ventral 

 or peristomial side but in the Peritrichidse, with few exceptions, 

 the adoral zone if viewed from the A'entral side, turns to the right. 

 How this peculiar reversal came about is a matter of speculation. 

 Biitschli and Faure-Fremiet have attempted to explain it on a 

 phylogenetic basis and at the same time to account for the apparent 

 longitudinal di\'ision of forms like the Vorticellida^. The former 

 interpreted the ^e^'ersal as an adaptation of a flattened hypotrichous 

 form in which the ventral surface ser\'es for attachment while the 

 peripheral region of the adoral zone becomes turned over on the 

 dorsal side. The functional ventral surface would thus be the 

 morphological dorsal surface and the attaching surface the morpho- 

 logical ventral surface {Trichodina for example). In the Vorti- 

 cellidse the ventral surface becomes drawn out into an attaching 

 stalk and the body becomes elongated in the dorso-ventral plane. 

 Division therefore in a morphological sense, would be transverse 

 rather than longitudinal. Faure-Fremiet's (1905) explanation is 

 based on forms like Ancistnnn, Ilemispira and other holotrichs 

 w^ith an area of attaching cilia (thigmotactic area) and with the 

 tendency of mouth and peristome to turn upward. 



As a result of an attached mode of life, colony formation, unique 

 amongst ciliates, is characteristic of the Vorticellida^. Here, also, 

 under conditions as yet unknown, the indi\idual cells may leave 

 their stalks after developing a girdle of posterior cilia and swim away 

 as solitary individuals. Except for this temporary ciliated girdle, 

 body cilia are rarely present but are characteristic of Trichodinopsls 

 and Ilemispira. Houses or tests are present in all species of Cothur- 

 nina, but are generally absent in other groujjs. 



In their sexual phenomena the Peritrichida differ in some import- 

 tant respects from other ciliates. In the majority of cases in which 

 fertilization processes have been worked out, dimorphic gametes are 

 formed and fusion is complete and permanent. Mutual fertilization 

 is thus absent and corresponding changes in the maturation phenom- 

 ena are introduced (see Chapter XII). 



The great majority of Peritrichida are placed in the famih' Vorti- 



