SPECIAL MORPHOLOGY AND TAXONOMY OF INFUSORIA 397 



rated according to the presence (Contractilia) and the absence 

 (AcontractiHa) of contractile elements in the colonial or individual 

 stalks. 



Tribe 1. Cothvniina, Biitschli.— Forms with tests or houses which 

 may be sessile or borne on stalks and which are always erect, genus 

 Cothurnia. 



Forms with tests or houses which are recumbent or attached by 

 the entire side, never erect. Two genera Lagenojjhrys and Vagini- 

 colla. In the former the peristomial disc is raised on a neck-like 

 base and may be lowered as a lid or operculum. In the latter the 

 peristomial area is not thus separated from the body by a neck. 



Tribe 2. Contractilia, Biitschli.— Here the stalks contain highly 

 contractile myonemes which, in colonial forms may form a con- 

 nected system of contractile threads throughout the colony (Zoo- 

 thamniwn) so that the entire colony contracts; or the contractile 

 elements may be confined to the stalks of the individuals of the 

 colony so that the colony does not contract but the individuals 

 composing it do (Carchesium). In the third genus, VorticeUa, the 

 individuals are not united in colonies, each is solitary and contracts 

 on its own attached stalk. 



Tribe 3. AcontractiHa, Biitschli.— This group is the richest in 

 number of genera but more limited in number of species than the 

 Contractilia. The stalks when present are never provided with 

 myonemes and are correspondingly rigid. Colonial aggregates 

 are frequent. 



The following genera are colonial: 



1. O-plirydium , individuals are green through the presence of 

 symbiotic forms, and embedded in jelly; the colonies are spheroidal 

 and vary in size from | inch or less, to 3 feet in diameter. Fresh 

 water. 



2. Opercnlaria, colorless colonies branching in one plane; the 

 individuals have a peristomial lid or operculum as in Lagenoijlirys 

 borne on a neck. 



3. Epistylis, similar to Opercularia but the individuals lack the 

 neck-like constriction, the peristomial region is similar to that of 

 VorticeUa (Fig. 210, p. 502). 



The following genera are solitary : 



4. Astylozomi, free-swimming forms without stalk; with two pos- 

 terior bristles. 



5. Gerda, creeping forms, broadest at the posterior end and ringed. 



6. Scyphidia, cylindrical and cross-ringed forms with attaching 

 disc. 



7. Rhahdostyla, with short stalk, Vorticella-like, but non-con- 

 tractile. 



8. Glossatella, similar to Rhahdostyla but with enormously devel- 

 oped undulating membrane. 



