THE HIGHER GROUPS. 575 



another very natural assemblage. But it must be admitted 

 that the highest forms of this division are separated by no 

 very sharp line of demarcation from the Annelida y while 

 the simplest Turbellaria are almost on a level with the Phy- 

 semaria and the lower Hydrozoa. Even a Planaria is com- 

 parable to a free zoophyte ; its proboscis may be likened to 

 the hydranth of a Medusa, the prolongation of the alimen- 

 tary sac to the gastro-vascular canals, the central nervous sys- 

 tem, with its lateral prolongations, to the marginal ganglia 

 and nerves. The water-vascular system and the complication 

 of the reproductive organs, indeed, afford clear marks of dis- 

 tinction ; but both of these systems vary indefinitely in the 

 degree of their development within the limits of the Turbel- 

 laria. 



On the other hand, the connection of the Hirudinea by 

 such forms as Malacobdella with the Turbellaria and Trema- 

 toda is very close ; Polygordius appears to be a transitional 

 form between the Turbellaria and the Polychatta y while the 

 Potifera, in many respects, represent larval forms of the 

 Polychaita and of the Gephyrea. 



The Cestoidea are usually regarded as anenterous Trema- 

 toda, in which case, of course, they must be associated with 

 the latter. 



I propose to establish a division of Trichoscolices for 

 the natural orders now enumerated, in order to discriminate 

 the morphological type which they exemplify from that of 

 the Nematoscolices, containing the Nematoidea, which are 

 as remarkable for the universal absence of cilia as the former 

 are for their presence ; and which are further so clearly dis- 

 tinguished by the arrangement of their nervous and muscu- 

 lar systems and of their water-vessels ; and by their ecdysis. 



The connection between the two divisions by way of the 

 Nematorhyncha and the Potifera is undoubtedly very inti- 

 mate, and there is almost as much reason to arrange the Ne- 

 matorhyncha with the Trichoscolices, as with the Nematosco- 

 lices. On the whole, however, I think that, notwithstanding 

 the cilia of the Gastrotricha, the closest affinities of the 

 Nematorhyncha are with the JVematoidea, and I therefore 

 place them among the Nematoscolices. 



But I may remark, once for all, that the attempt to estab- 

 lish sharply-defined, large divisions of the animal kingdom is 

 futile. The progress of knowledge every day renders it 

 more and more clear that morphological groups are compara- 

 ble to distributional provinces ; each, however well marked 



