22 



A PRIMER OF BIOLOGY 



with them in allied organisms such as some of 

 the common jellyfish. The cells of the inner layer, 

 on the other hand, are devoted to the 

 absorption and mgestion of the organic 

 materials which enter the opening at 

 the top of the tubular body. At certain 

 times some of the cells of the outer 

 layer produce bodies which are presently 

 recognisable as reproductive organs. In 

 I short, now that the organism has become 



* multicellular, some cells 

 take on one function, 

 others another, while still 

 retaining certain general 

 powers, e.g., that of self- 

 nourishment. Again, it is 

 obvious that as the duties 

 of any particular cell be- 

 come circumscribed and 

 specialised, its form and 

 structure become modified 

 also, in order that it may 

 carry out the work allotted 

 to it in a more efficient 

 manner. In just so far, 

 however, does the cell in 

 question become more de- 

 ' pendent on its neighbours 

 in other respects. Thus, a 



FIG. 12. Enteromorpha: a, cell which devotes itself 

 portion of plant (natural solely to the formation of re- 

 size) ; b, body wall in section ; productive elements must 

 c, in surface view. be fed by other cells which 



are specialised for carrying out nutritive duties and 

 protected by others adapted to perform that function. 



