202 THE INVERTEBRATA 



The tentacles are projections of the body wall near the anterior end. 

 They are found in the Turbellaria only, but are not present in all 

 these. When present they are quite distinct and have very long cilia 

 which, by their motion, set up currents which pass the water over 

 special sensory areas and so lead us to suppose that their use is for 

 water-testing, or searching for food. Occasionally these tentacles may 

 be sunk into pits. 



A statocyst occurs in primitive forms of the Turbellaria. It is 

 situated above the brain and suggests a connection with the Coe- 

 lenterata where such sense organs are common, but as we know 

 nothing of its nervous supply it is difficult to make a proper com- 

 parison. 



An excretory system exists in nearly all Platyhelminthes. In the 

 Acoela, however, it is absent. The excretory system usually consists 

 of main canals, running down either side of the body (Fig. 148). 

 The position of the openings of these main canals to the exterior varies. 

 The main canals are fed by smaller branches which are ciliated, while 

 the main canals are not. These smaller branches again branch many 

 times and finally end in an organ known as aflame cell (Fig. 149). The 

 large canals are often quite easily visible in living specimens, but the 

 flame cell is exceedingly small and can only be seen in transparent 

 forms as in the cercaria larvae of the Trematoda. The flame cell itself 

 consists of a cell with branched processes extending amongst the 

 parenchyma cells. Attached to the cell are a number of cilia which 

 move together in the lumen of the canal with a flickering movement. 

 It is from this flickering motion that the cell derives its name. It is 

 generally believed that excretion of substances into the lumen of the 

 tube is performed by the cells forming the wall of the tube itself. 

 The flame cells represent concentrations of the originally complete 

 ciliary lining of the canal and their function is to maintain a hydro- 

 static pressure which will cause the excreted substances to move 

 down the lumen of the tube to the exterior (see also p. 197). 



Movement in the Platyhelminthes is effected in two ways. The 

 animal may creep over a surface by the motion of the ectodermal cilia, 

 the surface being freely lubricated when necessary, as is the case 

 in land forms, by the discharge of slime from the ectodermal slime 

 glands. More rapid movement is effected by the general musculature 

 of the body which causes a series of undulations to pass backwards 

 along the flat body and urges it forward (Fig. 150). The musculature 

 of a platyhelminth consists of a covering of muscle lying just below 

 the ectoderm and composed of two layers, an outer circular and an 

 inner longitudinal layer, except in the Cestoda and in the pharynx 

 of the Turbellaria where the outer muscles are the longitudinal and 

 the inner the circular. 



