536 



THE QUARTERLY REVIEW OF BIOLOGY 



the change may be, such a branch never 

 assumes features characteristic of other 

 types of animals. 



We cannot take refuge, therefore, in 

 the supposition that the peculiarities of 

 the tapeworms and the flukes are due to 

 parasitism. They are instead features 

 which are peculiarly adapted to a parasitic 

 mode of life. 



It cannot be assumed that these four 

 types in exactly the form in which we 

 know them now are the connecting links 

 between the radial and the bilateral 

 animals, but we can assume that they 

 represent the only possible intergrading 

 types. 



COMBINATIONS OF THE FOUR FUNDAMENTAL 

 TYPES 



How can the other types of animals be 

 derived from forms similar to the tape- 

 worms, the flukes, the turbellarians and 

 Microstomum? 



We have drawn a developmental line 

 from a single cell to a radially symmetrical 

 animal. From the radially symmetrical 

 animal we have drawn four radiating 

 lines each ending in an animal type 

 fundamentally different in structure from 

 the other three. 



One of these types is regularly seg- 

 mented. The second shows internal 

 differentiation suggesting the beginnings 

 of ccelomic structures. The third is 

 simple and solitary. The fourth is simple 

 and colonial. 



All four of these types had their origin 

 in the same type, the radially symmetrical 

 animal. They may therefore be presumed 

 to be in a state of unstable equilibrium. 

 The term unstable equilibrium here is 

 used to mean a condition in which an 

 animal type exhibits only a small range 

 of the features found in the assumed 

 ancestral form, the missing features being 

 in abeyance, or existing in a latent form 



and prone to reappear if conditions render 

 reappearance possible. 



Unstable equilibrium predicates the 

 existence of various stresses tending to 

 cause the animal type to retrogress toward 

 the original form through the reassump- 

 tion of one or more of the missing features. 



Unstable equilibrium therefore carries 

 with it a constant tendency to readjust- 

 ment, more or less complete, of the normal 

 balance. So we are not surprised to find 

 four more animal types each of which 

 combines the characters of two of the four 

 types just considered. 



Thus there are CO animals which are 

 segmented like the jointed tapeworms, 

 but possess the ccelomic structures fore- 

 shadowed in the flukes; (z) animals 

 which are unsegmented like the turbel- 

 larians, but possess the ccelomic structures 

 foreshadowed in the flukes; (3) solitary 

 unsegmented animals like the turbella- 

 rians, with no ccelome, but with abundant 

 asexual reproduction like Microstomum; 

 and (4) segmented animals without a 

 ccelome like the tapeworms, but less 

 unified and with the continuous loss of 

 the units as in Microstomum. 



The segmented animals with a ccelome 

 CO are the annelidan or segmented worms; 

 the unsegmented animals with a ccelome 

 CO are the priapulids and sipunculids; 

 the solitary animals with no ccelome but 

 with abundant asexual reproduction C3) 

 are the rotifers; and the animals without 

 a ccelome forming colonies of separate 

 individuals CO are undoubtedly the 

 graptolites. 



Through this readjustment each of the 

 new animal types has combined within 

 itself the features of two of the original 

 types. But since in each of these new 

 types two of the four chief features still 

 are absent, there continues to exist a 

 condition of unstable equilibrium as com- 

 pared with the radial ancestor. 



