THE DEVELOPMENT OF ASTERINA GIBBOSA. 393 
fact that their arrangement is in both cases dominated by the 
prevailing pentamerous symmetry of the adult. 
The reason why the change in the position of the mouth 
takes place in Antedon is that this animal, like the others in 
which a similar change occurs, feeds on swimming or floating 
prey, and, so to speak, turns the mouth upwards to receive it. 
Asterids and their allies, on the other hand, find their food on 
the substratum, and therefore we must suppose that in the fixed 
ancestor of Asterids the body was flexed downwards so as to 
bring the substratum within reach of the tentacles. The 
difficulty suggests itself that a fixed form finding its food on 
the substratum might very soon devour all within its reach ; 
and the suggestion may be made that perhaps the ancestor of 
Asterids never was fixed, but that the divergence from Crinoids 
took place when the common ancestor was a creeping form, 
since we may reasonably conclude that creeping habits formed 
the transition stage between a free-swimming and a fixed mode 
of life. In this case, however, the difficulty meets us of 
accounting for that radial symmetry which is so deeply impressed 
on the organisation of Asterids and other forms. It would 
be rash to say that fixed life is the direct cause of radial 
symmetry when we consider the case of Brachiopods, Cirripedes, 
&c., but this symmetry is only, so far as our knowledge goes, 
developed in connection with a fixed life. 
The proximate cause of the radial symmetry of Asterids is 
the immense preponderance of the organs of the left side, and 
it is difficult to see how this could have gone on to the extent 
it has done in an animal which moved about with a definite 
part directed forwards. The motion of the Asterid when 
metamorphosed is vague,—that is, any part is directed forwards; 
and it seems to me that a fixed stage must intervene between 
this and the mode of motion in which the head went first. 
1 Some might object that Ctenophores and Meduse are radially sym- 
metrical, but the first are not truly so; and as to the second, I hold very 
strongly the view that the Medusa is only a specialised bud, which has secon- 
darily acquired locomotive powers in order to disperse the ova. Its radial 
symmetry has been inherited from fixed ancestors. 
