39 8 



THE INFUSORIA 



In the remarkable genus Actinobolus there are a number of retractile 

 processes called the tentacles projecting in a radiating manner from the 

 body-wall, which give the animal a superficial resemblance to a Heliozoon 

 (Fig 48). 



In Ileonema there is a single flagellate process at the anterior end of 

 the body which is capable of being completely withdrawn (Fig. 49). 



Nearly all the Gymnostomata are provided with trichocysts. They 

 are situated either in special aggregations round the mouth, in which case 

 they are regarded as weapons of offence ; or scattered over the general 

 surface of the body, in which case they are regarded as weapons of 

 defence. 



The meganucleus is frequently spherical or oval in shape, but it is 

 sometimes elongated, horse-shoe-shaped, moniliform, jointed, segmented, or 

 fragmented. The micronucleus of a considerable number of genera has not 

 yet been discovered, but in others there can be no doubt of the existence 

 of one (Chilodori) or more (Dileptus) micronuclei. 



The genera of Gymnostomata are divided into families by 

 Schewiakoff as follows : 



Family HOLOPHRYINA, Perty. Holophrya, Ehrb. Of very simple 

 structure, with a terminal mouth provided with only a very rudimentary 

 pharyngeal apparatus. They form spherical cysts surrounded by a 

 gelatinous case in which an enormous number of young are produced by 

 rapid and repeated fissions (Figs. 36 and 47). 0-4. 1 

 Freshwater and marine, sometimes parasitic on fresh- 

 water fish. Urotricha, Clap, and L. Exceedingly 

 minute forms distinguished by the presence of a single 

 long straight bristle at the posterior end. 0'04. 

 Freshwater. Enchelys, HilL Anterior and posterior 

 ends somewhat attenuated, but in other respects 

 similar to preceding genera. Well - developed 

 trichocysts. -02-*2. Freshwater and marine. 

 Spathidium, Duj. Body flask -shaped. Mouth 

 ventral at the anterior end, long and slit- shaped. 

 0-4. Freshwater. (Fig. 45.) Cranotheridium, Schew. 

 With an elaborate pharyngeal armature. Numerous 

 micronuclei and a single meganucleus. 0*17. 

 Freshwater. Layynus, Quenn. Bottle-shaped, with 

 a pharynx surrounded by numerous trichocysts. 

 0'18. Marine and freshwater. Trachelophyllum, 

 Similar to the preceding genus, but 

 with the body a little more flattened. 0'2. Marine 

 and freshwater. Lacrymaria, Ehrb. Elongated, 

 extremely contractile forms. Anterior end shaped 

 like a bottle cork and surrounded by four or five bands of long cilia. 0*8. 

 Freshwater and marine. Trachelocerca, Ehrb. Very elongated and 

 contractile forms. Mouth surrounded by four lappets or lobes. Some- 



1 These figures refer to the average length of the genera in millimetres. 



Holophrya muUiJttiis,, 

 Pouquet. 1, free -swim- 

 ming condition. 2, en- 

 cysted condition with pro- Clap, and L. 

 toplasm undergoing its 

 first transverse fission, o, 

 meganucleus ; b, b, con- 

 tractile racuoles. x 120. 



