2o6 THE INVERTEBRATA 



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 certain cells in it provide for the 

 repair of lost parts of the body. These free cells of the parenchyma 

 retain their embryonic condition and do not become vacuolated or 

 branched. They are smaller than the branched cells of the paren- 

 chyma and scattered among them in normal circumstances, but when 

 an injury occurs they migrate to the cut surface, where they collect 

 in large numbers and proceed to regenerate the tissues lost by injury. 



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. 152), 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 pouchy 

 from which it can be protruded or withdrawn.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. It is possible for food substances to 

 pass not only from the lumen of the gut into the cells lining it, but 

 also from the parenchyma. Thus when Turbellaria are starved they 

 can consume certain organs lying in the parenchyma (ovaries, testes, 

 etc.) by passing these into the gut cells or into the lumen of the gut 

 for digestion. 



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 sHme. 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 

 out" the food, and all those within a wide area collect in the pot for 



