34 THE STORY OF ANIMAL LIFE 



Within recent years an" attempt has been made 

 to express the relationship these groups bear to 

 one another, by placing them in separate divisions 

 or grades. The first grade includes only the Pro- 

 tozoa, or unicellular animals. The position of 

 second grade has been assigned to the Coelente- 

 rata or diploblastic animals, whose bodies consist 

 typically of two layers of cells. A third grade 

 includes only a few groups of the lower worms, 

 among which three body-layers may be distin- 

 guished, but no body-cavity is present. While 

 the fourth grade, including practically the rest of 

 the animal kingdom, have three body-layers (see 

 p. 38), and a body-cavity surrounding the internal 

 organs (see p. 38). 



This arrangement of groups is an extremely 

 convenient one; all the more convenient because 

 it easily admits of modification. Already, indeed, 

 we might find room for a grade intermediate 

 between I. and II., consisting of what might be 

 termed monoblastic animals, namely, animals con- 

 sisting of a single layer of cells. For the fre- 

 quent occurrence of Larvae of this kind, consist- 

 ing of a hollow ball of cells, renders zoologists on 

 the alert to find a grown-up organism built in 

 the same way. It is doubtful whether any of the 

 forms that have been supposed to answer to this 

 description really do so. Certain forms of these 

 often claimed as plants by the botanists are, how- 

 ever, in the meanwhile, invited in to fill the blank. 



There are also animals in which the internal 

 layer of the body is very much reduced, consist- 

 ing sometimes in fact of one cell only. Those 

 are the Dicyemidae and Orthonectidae, both of 

 them parasitic forms. They differ so completely 

 from all other forms that it has been proposed 



