THE INFUSORIA 401 



0'15. Marine. Dysteria, Huxley (Fig. 51). Body somewhat resembling 

 a mussel in shape, with a very restricted ventral side which alone bears 

 cilia. There is a remarkable caudal appendage on the ventral side. 

 The dorsal side is smooth and ribbed. 0*15. Marine and freshwater. 

 Trochilia, Duj. 0'035. Freshwater and marine. Dysteropsis, Eoux. 

 Lacustrine. 



Family ONYCHODACTYLINA, Entz. Onychodactylus, Entz. The body 

 bears at the posterior end a little conical appendage in the form of a tail, 

 at the extremity of which is the anus. The colour is yellow or blue. 

 0'2. Marine. 



SOB-OKDER HYMENOSTOMATA, Delage. 



The HYMENOSTOMATA include a large number of forms that occur 

 in infusions such as Loxocephalus, Colpidium, and Colpoda, from which the 

 name of the Class Infusoria was derived. Some of them are internal 

 parasites such as the OPALININA and ISOTRICHINA. Others are free- 

 swimming in pure water. They vary in size from the minute Cyclidium, 

 03 mm., and Loxocephalus, '06 mm., to the elongated parasite Discophrya, 

 which is sometimes 2 mm. in length. 



The mouth is in some cases at the anterior extremity of the body, 

 but more usually it is situated near the middle of one side. In such 

 forms as Isotricha it is doubtful which end of the body is correctly called 

 anterior. According to Biitschli and others the mouth is situated at the 

 posterior end, and the animal swims with the anterior end foremost. 

 Others consider that the mouth is at the anterior end, and that the animal 

 habitually swims backwards. 



The mouth is usually situated at the bottom of an elongated, gutter- 

 like peristomial depression, and opens into a short oesophageal tube. 

 This tube, however, is never supported by a palisade of rods. In many 

 forms, and perhaps in all of them, there is a small undulating membrane 

 at the margin of the mouth hence the name Hymenostomata. Some- 

 times there is in addition to this one or two very delicate membranes 

 (Pleuronema) at the margins of the peristomium which it is convenient to 

 distinguish as the lips (Fig. 52). Trichocysts are usually present. There 

 is generally a single spherical or oval meganucleus accompanied by one 

 or two micronuclei. In Frontonia there are several micronuclei. In 

 Anoplophrya (Fig. 30) there is an elongated meganucleus and in Opalinopsis 

 (Fig. 55) an irregular band-like meganucleus at first, which in the older 

 forms breaks up into a number of fragments. In Opalina there are several 

 spherical meganuclei, and probably a greater number of micronuclei 

 (Fig. 25). 



Reproduction is effected by simple transverse fission. In Leucophrys 

 there is a resting stage during which the animal becomes spherical in 

 shape, but does not secrete an envelope. During this stage the body 

 divides into thirty -two small individuals. In Oj)hryoglena, Colpoda, 

 and Glaucoma cysts are formed. The kidney -shaped cysts of Para- 

 moecium have recently been figured by Lindner. The body of Opalina, 

 after it has reached a certain stage of growth, divides by fission several 

 times, until the fragments contain only two or three nuclei. Each 



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