THE DEVELOPMENT OE ASTEEINA QIBBOSA. 275 



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 difiiculty 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 Medusae 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. 



