PLATYHELMINTHES 177 



but as we know nothing of its nervous supply it is difficult to make 

 a proper comparison. 



Taste cells occur in the Rhabdocoela (Turbellaria). They are spread 

 over the whole surface of the body, but as they are more common near 

 the mouth it is supposed that they function as simple organs of taste. 



An excretory system exists in nearly all Platyhelminthes. In the 

 Acoela, however, it is absent, and it has not been observed in the 

 Polycladida owing to the fact that the dense pigmentation found in 

 this group precludes us from tracing it. The excretory system usually 

 consists of two main canals, one running down either side of the body 

 (Fig. 143). The position of the opening of these 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 a. flame cell (Fig. 144). 

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

 the flame cell is exceedingly small and can only be seen in trans- 

 parent forms as in the cercaria larvae of the Trematoda. 



The flame cell itself consists of a cell with branched processes ex- 

 tending 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. 



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. 145). The muscu- 

 lature 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 where the outer 

 muscles are the longitudinal and the inner the circular. 



Passing through the parenchyma and running dorsoventrally are 

 strands of muscle which are attached at either end to the dorsal and 

 the ventral muscle layers. The muscles themselves consist of fibres 

 formed of a homogeneous transparent material that shows no trace 

 of any structure. These fibres are produced by a special cell, the 

 myoblast, which is often to be seen lying alongside the fibre it has 

 produced. 



The outer covering of a platyhelminth differs according to the 

 group to which it belongs. In the Turbellaria the outer covering is 

 formed of ectodermal cells. These are usually large and flat, sometimes 

 with peculiar branched nuclei as in Mesostomum, or smaller and with 



