130 BIOLOGY: GENERAL AND MEDICAL 



connected, are too independent to feel this need; but 

 in the sponges and hydras the differentiation of the cells 

 becomes sufficient to give the entoderraal cells an advan- 

 tage over others unless some means for transporting 

 the products of digestion can be found. In all prob- 

 ability the primitive means is similar to that seen among 

 plants where material of various kinds is passed directly 

 from cell to cell. Such primitive methods cannot suffice, 

 however, except in cases in which the cell groups are 

 small. 



FIG. 45. A medusa of Obelia. Seen from the oral surface, magnified (ad not.) 

 (After Master-man.) 



Plants soon outgrow the direct transfer and provide 

 themselves with "vascular tubules" through which the 

 sap flows in a continuous current from the roots to supply 

 the evaporation in the leaves. 



The lower ccelenterates among animals, by contracting 

 the body, force its contained fluid rich in nourishment 

 into every part including the hollow tentacles. 



Ascending a little higher among the hydroids we find 

 some of them Coryne, Obelia giving off budding off- 

 spring minute in size but resembling a medusa or jelly 

 fish in shape. From the centre of each of these embryos 



