THE SCOLECIDA. 47 



which is exhibited by a Medusa, the tendency towards that kind 

 of symmetry being always disturbed, either by the disposition of 

 the alimentary canal, or by that of some part of the ambulacral 

 apparatus. Very often, as in the Spatangoid sea-urchins, and in 

 many Holothuridea, the ambulacral and nervous systems alone 

 exhibit traces of a radial arrangement ; and in the larval state, 

 as we have seen, radial symmetry is totally absent, the young 

 Echinoderm exhibiting as complete a bilateral symmetry as 

 Annelids, or Insects. 



XVII. THE SCOLECIDA. 



Nothing can be more definite, it appears to me, than the 

 class Echinodermata, the leading characteristics of which have 

 just been enumerated ; but it is a very difficult matter to say 

 whether the seven groups, some of considerable extent, which 

 are massed under the present head, are rightly associated into 

 one class, or should be divided into several. The seven groups 

 to which I refer are the Rotifera (or Wheel-animalcules), the 

 Turbellaria, the Trematoda (or flukes), the Tseniada (or tape- 

 worms), the Nematoidea (or threadworms), the Acanihocephcda, 

 and the Gordiacea. Of these, five are composed of animals 

 parasitic upon others; and exhibiting the anomalies of struc- 

 ture and of development which might be expected from crea- 

 tures living under such exceptional conditions. 



There is one peculiarity of organic structure which the first 

 four of these groups certainly have in common ; they all present 

 what is termed the " water-vascular system," a remarkable set 

 of vessels which communicate with the exterior by means of 

 one, or more, apertures situated upon the surface of the body, 

 and branch out, more or less extensively, into its substance. 



In the Rotifera the external aperture of the water-vascular 

 system is single, and situated at the hinder end of the body ; 

 it communicates with a large, rhythmically contractile, sac, 

 whence two trunks proceed, which usually give off short lateral 

 branches, and terminate in the ciliated " trochal disk " of the 

 Kotifer, in the middle of which its mouth is placed. Both the 

 lateral offshoots and the terminal branches contain vibratile 



