406 Ciliophora 



than the other. The plane of fission in Vorticella passes from near the 

 center of the peristome to a point just to one side of the stalk. One 

 daughter organism thus retains the parental stalk. The other develops 

 an aboral band of locomotor cilia and becomes a telotroch (Fig. 7. 47, 

 C) which swims actively for a short period. Metamorphosis into the adult 

 follows adhesion of the scopula to a suitable surface. In this attached 

 stage, the young peritrich resembles Scyphidia which lacks a stalk. How- 

 ever, the Scyphidi a-st3.ge of Vorticella lasts for only a short time. Soon 

 after attachment, secretion of a stalk begins and the aboral cilia disap- 

 pear within a few minutes (186). In fission during development of a 

 colonial type, such as Zoothamnium (200), the daughter organism not 

 retaining the old stalk secretes a new stalk which becomes a branch of 

 the original one. 



The telotroch is a common stage in the life-cycle and is not limited to 

 stalked species, since it occurs in such forms as Scyphidia (Fig. 7. 44, H). 

 Direct transformation of a stalked form into a telotroch sometimes occurs 

 in Vorticella (Fig. 7. 47, D-F) and is a normal means of asexual propaga- 

 tion in such genera as Zoothamnium (200). After liberation from the 

 parental colony, the migratory stage swims away and then settles down 

 on some object to develop into a new colony. The Urceolariidae are 

 sometimes considered highly specialized permanent telotrochs. In this 

 connection, the telotroch of Epistylis liorizontalis (Fig. 7. 43, A) is in- 

 teresting in its superficial resemblance to the Urceolariidae and in its 

 comparable ability to glide over surfaces (20). Telotrochidiiim (Fig. 7. 

 43, E) also seems to represent a permanent telotroch, although it is not 

 impossible that this genus contains telotrochs whose sessile stages have 

 not been recognized. 



The life-cycle of the commensal Ellohiophrya donacis (25) is more com- 

 plicated than that of most Peritrichida. The adult lives in a lamelli- 

 branch, Donax vittatns, attached to a gill-filament (Fig. 7. 45, A). At the 

 completion of fission one daughter organism (the future telotroch) re- 

 mains attached to the larger by a narrow isthmus of cytoplasm (Fig. 7. 

 45, B). A scopula soon develops in the embryonic telotroch and secretes 

 a stalk wdiich extends for some distance into the attached sister organism 

 (Fig. 7. 45, D). Elongation of the developing telotroch and differentiation 

 of the aboral cilia occur next (Fig. 7. 45, C). The telotroch then becomes 

 free-swimming (Fig. 7. 45, E), usually breaking away at the junction of 

 stalk and scopula, but sometimes carrying its stalk along to be discarded 

 later. After reaching a gill-filament, this migratory stage becomes attached 

 by its aboral end and develops the protoplasmic arms which anchor the 

 organism to its host (Fig. 7. 45, F). 



Axial homologies between the Peritrichida and other ciliates are some- 

 what uncertain. Although the peristome is usually considered anterior, 



