PLATYHELMINTHES 



larval stage, for the short period when they have left 

 parental host and are seeking another (Figs. 26, 27, 42). 



Some Turbellaria (Ehabdocoelida) resemble Infusoria in their 

 minute size, shape, and movements. Nevertheless they possess 

 an organisation of considerable complexity. The fresh -water 

 Planarians (Fig. 14), abounding in ponds and streams, vary from 

 a quarter to half an inch in length, and are elongated and 

 flattened. Their body is soft, and progresses by a characteristic, 

 even, gliding motion like a snail. The marine Planarians or 

 Polyclads (Fig. 8) are usually broad and leaf- like, sometimes 

 attaining a length of six inches, and swim or creep in a most 

 graceful way. Land Planarians occur in this country (Fig. 15), 

 but far more abundantly in tropical and sub-tropical districts, in 

 moist places, venturing abroad at night in pursuit of prey. They 

 are elongated and cylindrical, in some cases measuring, when 

 fully extended, a foot or more in length, and are often ornamented 

 with brilliantly coloured, longitudinal bands. 



Turbellaria are carnivorous, overpowering their prey by peculiar 

 cutaneous offensive weapons, and then sucking out the contents of 

 the victim by the "pharynx." Land Planarians feed on earth- 

 worms, molluscs, and wood-lice ; fresh-water Planarians on Oligo- 

 chaet worms, water-snails, and water-beetles ; marine forms devour 

 Polychaet worms and molluscs. Some Turbellaria seem to prefer 

 freshly-killed or weakly examples of animals too large to be over- 

 powered when fully active. Certain Ehabdocoelida are messmates 

 of Molluscs and Echinoderms, and a few others are truly parasitic 

 a mode of life adopted by all Trematodes save Temnocephala. 



The Trematodes 1 may be divided into those living on the 

 outer surface of various aquatic animals, usually fish (Ectopara- 

 sites) ; and those which penetrate more or less deeply into the 

 alimentary canal or the associated organs of the host (Endo- 

 parasites). They are oval, flattened Platyhelminthes ranging from 

 a microscopic size to a length of three feet (Nematohothrium, Fig. 

 22), and are provided with organs of adhesion by which they cling 

 to the outer surface, or to the interior, of the animals they inhabit. 

 Trematodes occur parasitically in all groups' of Vertebrates, but, 

 with the exception of the liver-flukes of the sheep (JDistomum hepa- 

 ticum and D. magnum), and of Bilharzia haematobia found in man 

 (in the blood-vessels of the urinary bladder) over the greater part 



J Tpijfia, a hole ; referring to the orifices of the suckers. 



