THE SCOLECIJ'A. 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 ambulacra! 

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

 many Holothuriclea, the ambulacra! 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. 



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

 clas.s 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 next head, that of Scolecida, are rightly 

 associated into one class, or should be divided into several. 

 The seven groups to which I refer are the Piotifera (or Wheel- 

 animalcules), the Turbellaria, the Trematoda (or flukes), the 

 Tseniada (or tapeworms), the Nemcdoidea (or threadworms), the 

 Acantliocephala, and the Gordiacea. Of these, five are com- 

 posed of animals parasitic upon others; and exhibiting the 

 anomalies of structure and of development which might be ex- 

 pected from creatures 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 'Rotifer a 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 

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

 lateral offshoots and the terminal branches contain vibratile 

 cilia. The Trematode and Tsenioid worms have a similar, but 

 usually much more ramified apparatus ; and it is interesting to 

 observe that, in these animals, as in the Asjpidogaster conchicola 



