PLATYHELMINTHES l8l 



and the Cestoda, the ectoderm cells have all sunk into the parenchyma, 

 and the body is covered by a thick cuticle secreted by the ectoderm 

 cells. 



The parenchyma (also called the mesenchyme), which fills the interior 

 of the body, is of very different structure in different Platy- 

 helminthes. It is generally formed of cells with long irregular pro- 

 cesses and much intercellular space. Within these cells are small 

 granules and particles, which stain readily. Their appearance and 

 number vary according to the state of health of the animal, whether 

 it is starved or fed, and they are probably, therefore, products of 

 secretory activity formed after the assimilation of food and destined 

 eventually to be converted into rhabdites or the slime which flows 

 from the slime glands. The parenchyma is no mere padding tissue. It 

 probably serves for the transport of food materials, and it is the paren- 

 chyma that provides for the repair of lost parts of the body. It is in 

 fact a tissue which is almost undifferentiated and serves as the general 

 handy man of the platyhelminth body. 



The digestive system of the platyhelminth differs entirely from that 

 of the higher animals in that it is a sac with one opening only, which 

 serves both for the entry of the food and the exit of the faeces, and 

 not a tube with a mouth and anus serving separately for the entry and 

 exit of food. In the simplest forms, in many of the Rhabdocoela, the 

 sac is a straight wide tube with no diverticula (Fig. 147), while in 

 others the gut is branched. In the Tricladida the gut has three main 

 branches. A muscular structure lined by an inturning of the ecto- 

 derm surrounding the mouth forms the pharynx. The pharynx itself 

 may lie in a pit of the ventral body wall, called the pharynx pouch, 

 from which it can be protruded or withdrawn. The epithelial lining 

 of the gut cavity consists of large cells without cilia, the cell walls 

 of which are often difficult to distinguish. A muscular wall to the 

 gut is present, but is so exiguous as to avoid identification in many 

 forms, and it appears therefore as if nothing separates the cells of 

 the gut from the parenchyma. 



The Turbellaria are carnivorous and will eat small living Crustacea 

 or worms which are caught by the protrusion of the pharynx. A 

 sticky secretion, derived from the slime glands and perhaps the rhab- 

 dites, is immediately poured over the prey, which is thus wrapped up 

 in slime. If the object is small enough it is ingested whole into the 

 gut. Here digestion proceeds. Fat is digested in the lumen of the gut, 

 but the digestion of other substances takes place in vacuoles in the 

 cells of the gut wall. Animals which have recently died are also eaten 

 by turbellaria, and an effective trap can be made by placing a freshly 

 killed worm or a Gammarus or two in a jampot and lowering it to 

 the bottom of the stream or pond. The Turbellaria are able to " scent 



