RECENT WORKS ON THE ENTOZOA. 331 



to go SO far as our esteemed colleague, when he objects to the reten- 

 tion of these classes within the limits of the same sub-kingdom. The 

 existence of such forms as Sagitta, the Eotifers, and the Leeches, 

 militates, in some degree, against this opinion. The Leeches, for 

 example, so obviously resemble the Trematoda, that Diesing* has 

 placed both groups in one order, Myzlielmintha. And yet the 

 Leeches present a nervous system agreeing in its plan of structure 

 with that of the higher Annelida. If, in this portion of their 

 organisation, the latter approximate to the Arthropoda, not the 

 less, in their pseudo-vascular system, do they find their nearest 

 allies among the Annuloida. The affinities of the Scolecida and 

 JEchinodermata are not to be ignored. Neither must we lose sight of 

 the relationship between the Scolecida, Botifera and Annelida. 



'* Our classifications," writes Mr. Darwin,! '' are often plainly 

 influenced by chains of affinities." Such chains of affinities connect, 

 it may be said, the Artliropoda with the higher Annelida, the 

 higher Annelida with the lower, the lower Annelida with the 

 Scolecida, the Scolecida with the Echinodermatay and all these forms 

 with the Rotifera. The series, though a broken one, is never 

 completely sundered, and each class has its own characteristic type 

 of structure. 



If, however, the argument from " chains of affinities " be rejected, 

 then no choice is left us but to accept, for the time being, the 

 classification suggested by Mr. Huxley. For though, as Linnaeus 

 long ago observed, a group is not constituted because we are able to 

 define it; still, in systematic zoology, all groups of which we cannot 

 offer exact definitions must be suspected. We fear the Anartliropoda 

 are somew.at in this predicament. 



Eetaining, for the present, the sub-kingdom Annulosa, we may 

 arrange its anarthropod forms under three distinct provinces, the 

 Annulata, Annuloida, and J£chinodermata. This we have done in 

 the accompanying table, v» herein the classes and orders of ' Vermes^ 

 are enumerated. The arrangement therein submitted is thoroughly 

 eclectic, however much it may seem to agree, in several of its aspects, 

 with the views advocated by certain helminthologists. In its prepa- 

 ration we have been careful to consult all the classic memoirs on the 

 structure and affinities of Worms, and to avoid, as far as possible, 



* See his * Eevision der Myzhelminthcii,' Wieu Sitz., 1858. Also Note *, p. 26. 

 t Origiu of Species, 1859, (p. 419). 



