100 THE INVERTEBRATA 



two kinds of chromosomes, which are held to represent the chromatin 

 of the mega- and micronuclei of other ciliates. The life history differs 

 from that of other members of the class in that syngamy is of the 

 normal type. The agamont, parasitic in the rectum of a frog or toad, 

 reproduces by binary plasmotomy. In the spring the plasmotomy 

 outruns the nuclear divisions so that there arise small individuals 

 with few nuclei. These encyst and pass out of the host. Swallowed 

 by a tadpole, they hatch, and give rise to uninucleate gametes, of two 

 sizes (anisogamous). After fusion of the gametes the zygote encysts 

 for a while, issues, and by nuclear division becomes the adult 

 agamont. 



Suborder ASTOMATA 

 Holotricha without mouth; but with mega- and micronuclei. 



Unlike Opalina^ the members of this group are probably not 

 primitive but degenerate through parasitism. 



Collinia. Parasitic in the blood-spaces of the gills of Gammarus and 

 other crustaceans. 



Anoplophrya (Fig. 86 A). Reproduction by repeated budding at 

 one end of the elongate body, forming a chain. Parasitic in the 

 branchial limbs of Asellus. 



Suborder GYMNOSTOMATA 



Holotricha with a mouth, whose gullet, if any, is without ciliary 

 apparatus (i.e. an oesophagus); and with mega- and micronuclei. 



Ichthyophthirius. Subspherical, with a mouth at one pole and short 

 gullet ; numerous contractile vacuoles near the surface of the body ; 

 and a number of meganuclei, but no micronuclei visible in the adult. 

 Parasitic in various freshwater fishes, where it lies in blisters in the 

 skin. When it is full-grown, it falls out of the host, encysts, and forms 

 by repeated fission a number of small ciliospores, each of which has 

 a mega- and a micronucleus, the latter having appeared during the 

 process, perhaps from within the meganucleus. The spores infect new 

 hosts. A sexual process of the nature of autogamy has been described, 

 but is very doubtful. 



Prorodon (Fig. 88 A). Ovoidal, with mouth at one pole, a deep 

 gullet which is supported by skeletal rods and is capable of opening 

 and closing; one mega- and one micronucleus. In fresh waters. 



Loxodes. Compressed, with mouth as a mere slit in the pellicle on 

 the ventral edge of the body, overhung by the beak-like anterior end ; 

 numerous mega- and micronuclei; a row of vacuoles containing ex- 

 creta along the dorsal border, and a contractile vacuole at the hinder 

 end. In fresh waters. 



