94 PLATYHELMINTHES [CH. 



the thin wall of the tube by osmosis and dissolves the excreta 

 which the cells forming the wall of the tube cast out into it. Such 

 a branched tube ending in flame-cells is known as a nephridium 

 (Gr. v<pi8toj/, little kidney). 



ClaSS I. TURBELLARIA. 



The Turbellaria are free-living animals, and as a rule swim about 

 in the sea or in fresh- water ponds or streams. A few, however, have 

 taken to living amongst moist earth, and some species, e.g., Bipalium 

 ke^vense, are occasionally met with in hot-houses all over the world, 

 being probably imported with the roots of some tropical orchid or 

 fern. Other species of land Turbellaria are common in the Tropics. 



Turbellaria are all very soft animals and capable of considerable 

 change of outline. In their native habitat they are not easy to see, 

 many of them having colours which imitate the sea-weeds, etc., 

 amongst which they live, and many appear only at night from their 

 hiding-places. If, however, a bunch of red sea-weed be shaken out 

 in some clean sea- water in a white china dish, as a rule many of 

 these animals can be seen swimming with an undulating motion 

 like a Sole or clinging to the sides of the dish. 



One of the commonest species in the fresh-water ponds of Great 

 Britain is Mesostoma ehrenbergii, a flat leaf-like organism, perhaps 

 half an inch long, the transparency of whose tissues permits at 

 times the examination of some of the internal organs. The whole 

 of the outer layer of cells the ectoderm bears innumerable cilia, 

 by the action of which the animal glides slowly along when it does not 

 swim by the undulations of its whole body. Within this ectoderm 

 are certain circular and longitudinal muscle-fibres, and these 

 surround a mass of cells called the parenchyma. As just ex- 

 plained the muscular fibres are only specialised outgrowths of 

 certain of the parenchyma cells. 



The ectoderm cells secrete a great deal of mucus, mingled with 

 which are a number of little rod-like bodies called rhabdites. 

 The exact use of these is not clearly known ; in their formation 

 they recall the nematocysts of the Coelenterata. Like the nemato- 

 cysts they are extruded on irritation. The mucus forms a bed 

 over which the animal moves and in which the cilia work, so as to 

 propel the animal. 



The mouth of the Mesostoma is, as its name indicates, near the 

 centre of the body, on the ventral surface. It leads at once into a 

 pharynx with very muscular walls which can act like a sucker. 



