AFFINITIES 617 



the coelom are retained, and it can be proved that the pore- 

 canal, like the madreporite of Echinodermata, is used for taking 

 in water. Further, there are two aberrant sessile members of 

 the group (Cephalodiscus and Rhabdopleura), in which the middle 

 divisions of the coelom which would correspond to the hydro- 

 coels are produced into long arms, each with a double row of 

 ciliated tentacles, which strikingly recall the radial canals and 

 podia of the Pelmatozoa. Taking all these facts into considera- 

 tion, it seems probable that Vertebrata and Echinodermata both 

 arose from Protocoelomata. 



When we turn to the developmental history of Echino- 

 dermata for light on the question as to how the bilaterally 

 symmetrical ancestor became converted into the radially 

 symmetrical Echinoderm, it seems probable that only in the de- 

 velopment of the Asteroidea can we hope to find the solution of 

 the problem. The abrupt changes of habits shown in the meta- 

 morphoses of the other types are clearly secondary phenomena. 

 No species of animal could suddenly change its habits from 

 swimming by means of cilia to walking with tube-feet. In the 

 development, however, of Asterina gibbosa we get a hint of the 

 way in which a free- swimming life could alternate with periods 

 of temporary fixation, gradually passing into a condition in 

 which the fixation was permanent. This period in the history 

 of the race when ancestral Echinodermata were sessile would 

 mark the point at which Eleutherozoa diverged from Pelmatozoa, 

 and the former existence of a fixed ancestor explains the 

 tendency first to asymmetry and later to radial symmetry. 

 Bilateral symmetry is characteristic of most free -swimming 

 animals which have to pursue a straight course through the 

 water, but in fixed forms no disadvantage arises from want of 

 symmetry. A radial disposition of organs is, however, valuable 

 to them, since food must be sought and danger avoided from all 

 points of the compass ; and hence we can understand, when 

 fixation became permanent, how one hydrocoel could grow 

 larger than the other, and finally assume the form of a ring. 



The last question which arises is the vexed one of the mutual 

 relationships of the various Classes constituting the Phylum. 

 Before attempting to seek for light on this problem from develop- 

 ment, it will be necessary to sketch the life-history of Antedon 

 rosacea, the only Pelmatozoon whose development is known. 



