PROTOZOA TO WORMS 47 



processes which urge the animal forward like a myriad 

 of microscopic oars. In our bodies they are sometimes 

 used to keep up a current, e.g., to remove foreign par- 

 ticles from the lungs. The turbellaria is still covered 

 with cilia, probably an inheritance from the gastraea; 

 for, while in smaller forms they may still be the prin- 

 cipal means of locomotion, in larger ones the muscles 

 are beginning to assume this function and the animal 

 moves by writhing. The bilateral symmetry has 

 arisen in connection with this mode of locomotion and 

 is thus a mark of important progress. 



In the turbellaria we find for the first time a true 

 body-wall distinct from underlying organs. The outer 

 layer of this is a ciliated epithelium or layer of cells. 

 Under this an elastic membrane may occur. Then 

 come true body muscles, running transversely, longi- 

 tudinally and dorso-ventrally. Between the external 

 transverse and the internal longitudinal layers we often 

 find two muscular layers whose fibres run diagonally. 

 The body is well provided with muscles, but their ar- 

 rangement is still far from economical or effective. 



Within the body-wall is the parenchym. This is a 

 spongy mass of connectile tissue in which the other 

 organs are embedded. The mouth lies in the middle, 

 or near the front of the ventral surface. The intestine 

 varies in form, but is provided with its own layers of 

 longitudinal and transverse muscles, and usually has 

 paired pouches extending out from it into the body 

 parenchym. These seem to distribute the dissolved 

 nutriment ; hence the whole cavity is still often called 

 a gastro-vascular cavity as serving both digestion and 

 circulation. There is no anal opening, but indigestible 

 material is still cast out through the mouth. 



