78 INTRODUCTION TO CLASSIFICATION. 



and none possess a head composed of even four modified 

 somites. 



Most Annelids are provided with that peculiar system of 

 vessels termed " pseudo-hsemal ;" but, in some, that system has 

 not yet been discovered. 



In endeavouring to separate from among invertebrated 

 animals a first large group, comparable to the Vertebrata, it 

 appears to me that the resemblances between the Annelida, the 

 Chcetoynatha, and the Artliropoda outweigh the differences ; and 

 that the characters of the nervous system and the frequently 

 segmented body, with imperfect lateral appendages, of the former, 

 necessitate their assemblage with the Arthropoda into one great 

 division, or " sub-kingdom," of ANNULOSA. 



But what of the Echinodermata and the Scolecida ? Should 

 both these great classes be also ranged under the Annulosa ; 

 or do they belong to different sub-kingdoms ; or, if they belong 

 to the same, should they constitute a sub-kingdom of their 

 own? 



I will endeavour to reply to these questions in succession. 

 Whether these two groups belong to the Annulosa or not, must 

 depend upon whether they possess any characters in common 

 with the Arthropoda and Annelida other than those which they 

 have in common with all animals. I can find none of any 

 great moment. No Echiiioderm, or Scolecid, has a definitely 

 segmented body or bilaterally disposed successive pairs of 

 appendages. None of these animals has a longitudinal chain 

 of ganglia. 



On the other hand, there is much resemblance between the 

 ciliated larvae of some Scolecids and Echinoderms, and those of 

 Annelids ; and the form of the body of many Scolecids is so 

 similar to that of one of the most familiar of Annelids, as to 

 have earned for both them and the Annelids the common title 

 of " worms." Nor must it be forgotten that, in the Annelids, 

 there seem to be representatives of that singular system of 

 vessels which attains so large a development as the "water- 

 vascular " apparatus in many Scolecids. 



Whatever value may be attached to these resemblances, it 

 must, I think, be admitted that, in the present state of our 



