CHAPTER XXXI 



ORDER 4 HYPOTRICHIDA STEIN 



THE MEMBERS of this Order are usually dorso-ventrally flat- 

 tened and cilia or cirri are restricted to the ventral surface. 

 In some forms there occur dorsal processes which apparently are 

 used as tactile organelles. Peristome possesses an adoral zone 

 which is a left-handed spiral. The undulating membrane often 

 occurs. According to their location, the cirri may be called 

 frontals, ventrals, laterals or marginals, anals, and caudals 

 (Fig. 10, b). There are usually two macronuclei and two micro- 

 nuclei. Contractile vacuoles vary in number and do not possess 

 radiating canals. 



Asexual reproduction is by binary fission and sexual repro- 

 duction through isogamous conjugation is common, Encyst- 

 ment often takes place. Mostly free-living in fresh or salt water, 

 undergoing creeping movement. A few are parasitic. 



The order is divided into the following five families: 



With ventral cilia; adoral zone conspicuous Family 1 Peritromidae 



With cilia and cirri Family 2 Oxytrichidae 



With cirri only 



With frontals, ventrals, anals, marginals Family 3 Euplotidae 



With frontals, ventrals, anals; without laterals Family 4 Aspidiscidae 



Frontals and ventrals much reduced Family 5 Psilotrichidae 



Family 1 Peritromidae Stein 



Genus Peritromus Stein. Simplest form of the Hypotrichida. 

 Cilia present uniformly and densely on the ventral surface. No 

 stouter cilia nor cirri. Marine. 



Peritromus emmae Stein (Fig. 165, a, 6). In salt water. 

 Body about 90 microns long. 



Family 2 Oxytrichidae Kent 



Genus Oxytricha Ehrenberg. Flexible body ellipsoid, 

 rounded at the extremities. Ventral surface flat; dorsal convex. 

 8 frontals, 5 ventrals, 5 anals, and short caudals. Numerous 

 species. 



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