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190 REPRESENTATIVE SINGLE-CELLED ANIMALS 



contrast to the well-developed cilia of the oral 

 region. Spirostomum, Stentor, and Halteria are 

 common genera (c/. Fig. 94). 



3. Order, Hypotricha. — Typically creeping forms 

 with a marked dorso- ventral differentiation. Cilia 

 of ventral surface modified to form large leg-Uke 

 cirri. Oxytricha, Stylonichia, and Euplotes are the 

 most common genera (cf. Fig. 94). 



4. Order, Peritricha. — Typically attached forms. 

 Oral cilia are continued into a depression in which 

 open the cytopyge and the contractile vacuole, and 

 at the base of which is the mouth. Locomotor 

 ciUa are present only during certain phases of the 

 life cycle. Vorticella, and the colonial forms, 



Carchesium (Fig. 103 G), Epistylis, and Zodtham- 

 nium, are the most common genera in fresh water. 



The Suctoria, or Tentaculifera, which constitute the other 

 subdivision of the Infusoria standing on a par with the Ciliata, 

 may be mentioned here. Few of these occur in fresh water, but 

 they are not uncommon at the seashore where they are found 

 attached to various objects. In the adult phase of their Hfe cycle 

 the Suctoria are attached and capture food by means of tentacles 

 to which the prey becomes attached distally and through which 

 its contents are slowly sucked into the body of the suctorian. 

 The fresh-water Podophrya is an example (Fig. 84 D). Suctoria 

 are classed with the Ciliata as Infusoria, because they have a cili- 

 ated phase in their life cycle. This leads us to beheve that in their 

 evolutionary history they have arisen by modification of ciliated 

 forms. 



The Sporozoa 



Monocystis. — The Sporozoa are a large group of Protozoa, all 

 of which are parasitic. Like other parasites, they show a degener- 

 ation of the structures necessary for free life, and a specialization 

 of structure and function and of the hfe history, wherever neces- 

 sary to meet the demands of parasitic existence. The genus 

 Monocystis, which inhabits the seminal vesicles of earthworms, is a 

 sporozoan that is easily obtainable, and one that exhibits within 

 its host all the important stages of its life cycle. In structure, the 

 adult monocystis (Fig. 101) is a simple elongated cell with a single 



