116 PHYLUM ECHINODERMATA. 



has until quite recently survived in the juxtaposition of the two 

 phyla which used frequently to be found in works on Zoology 

 and Comparative Anatomy. But of late years it has been 

 recognized more and more clearly that this juxtaposition is not 

 warranted by the facts and that the affinities of the Echino- 

 dermata, in so far as any can be traced, are rather with the 

 higher phyla of the Metazoa, than with the lower. Expression 

 was given to this view in 1877 by Huxley in his Anatomy of 

 Invertebrata and in 1880 by F. M. Balfour, who in his Comparative 

 Embryology placed the Echinodermata at the end of the volume 

 dealing with the invertebrate groups, in the neighbourhood of 

 the Enteropneusta and Chordata. This example was followed 

 in 1890 by A. Lang in his textbook of Comparative Anatomy, 

 and now we, in the light of the most recent work on the subject, 

 have thought it right to take the same course. 



The Echinodermata are radiately symmetrical animals (see 

 p. 117) in which the number of radii is nearly always five or some 

 multiple of five. This symmetry is, however, characteristic of the 

 adult only, for in the youngest state all members of the group are 

 bilaterally symmetrical and in nearly all there is a free-swimming 

 bilateral larva. The view usually taken and adopted by us 

 when referring to .the matter in the chapter on Mollusca in the 

 first volume of this work (p. 317) is that the bilateral symmetry 

 is the primitive symmetry possessed by some adult ancestor 

 and that the radial arrangement is to be regarded as a distortion 

 from the original condition. Now, however, after a more com- 

 plete study of the group, we see reason to suspend our judgment 

 on this matter, and though we should hesitate to adopt the 

 view that the Echinodermata have been derived from asym- 

 metrical or from radiately symmetrical forms, and that the 

 larva has been especially produced and modified for a free- 

 swimming life, we are of opinion that there is at least as much 

 to be said for it as for the older and more usually adopted view 

 that the ancestral form was a form in which bilateral symmetry 

 had been completely evolved.* 



The radial symmetry is expressed not only in the external 

 appearance, but also in the arrangement of most of the internal 

 organs. It is, however, never completely carried out, and in 



* For a discussion of this question, see the section on " Affinities," 

 p. 160. 



