162 



THE RISE OF ANIMAL LIFE 



^%7 m2 



Fig. 9-2. Planaria stained so as to differentiate the 

 digestive tract. Note its ramifications into every part 

 of the animal. The protrusible pharynx is clearly 

 shovt^n in this photo. 



of which are drawn into the mouth by a 

 sucking action. 



The digestive system of planaria is sac- 



opening for the entrance of food and the 

 exit of waste materials (Fig. 9-2). In some 

 of the platyhelminthes, particularly the par- 

 asitic members, the sac is merely a straight 

 and unbranched tube, but in planaria it 

 branches into three distinct parts to form a 

 tri-clad intestine. Each part, in turn, rami- 

 fies into many smaller branches which sup- 

 ply food directly to the various regions of 

 the body (Fig. 9-3). Large thin-walled, 

 unciliated cells line the gut and secrete di- 

 gestive juices which carry on extracellular 

 digestion. In addition, the cells lining the 

 intestine are able to ingest solid food by 

 means of pseudopods and digest it intra- 

 cellularly, as in the case of hydra. 



Between the ectoderm and the endoderm 

 is a mass of large star-shaped mesodermal 

 cells, the parenchyma. It is possible for 

 food substances to pass not only from the 

 gut into the linins; cells, but also from the 

 parenchyma into the lining cells. Thus, 

 when planaria cannot find food, it may con- 

 sume certain organs in the parenchyma by 

 passing them into tlie intestinal cells where 

 they are digested. This enables planaria to 

 go without food for quite a long time, 

 through gradual reduction in size. In one 

 species of planaria it was found that dur- 

 ing starvation the absorption and digestion 

 of its internal organs occurred in regular 

 order. First the reproductive organs disap- 

 peared, leaving the animal reduced to sex- 

 ual immaturity; next the parenchyma, the 

 2ut, and the muscles were consumed in that 

 order. The nervous system remained es- 

 sentially intact so that the animal appeared 

 as a weird form with the bulk of its re- 

 maining body in the head region. On feed- 

 ing these starved forms, all the lost parts 

 regenerated to normal size. 



Special excretory organs appear for the 

 first time in a metazoan among the flat- 

 worms. In planaria this system consists of 

 a pair of branching tubes running down 

 each side of the body. The main tubes or 

 canals divide into small branches, each of 



like, similar to hydra, with but a single which finally ends blindly in a single flame 



