THK IIICIIKI; .I:OUPS. 



There i> \i<> in\ . rtebrated animal at present known which 

 cannot at once be referred to one or other of the natural 

 <lris which have been discussed in the preceding pages. The 

 next question which arises is, How far are these groups sus- 

 ceptible of arrangement into assemblages of a higher order, 

 distinguished from all others by certain common characters? 



It is universally admitted tint the Insecta, M;/r!<ipoda, 

 Arac/i"><fit, Crustacea, Pycnogonida, and Tardigrade, form 

 such an assemblage, termed the AKTHROPODA, and character- 

 ized by the segmentation of the body ; the clutinous cuticula ; 

 the absence of cilia upon, or in, the body at any period of life ; 

 the segmentation of the central nervous system, and its per- 

 foration by the gullet ; and the presence (with the possible 

 exception of the Irilobita) of limbs, which, almost always, 

 are themselves subdivided into joints. The reasons for in- 

 cluding the Peripatidea in this division have been irivn. in 

 Chapter XT. ; and, though the Pentastomida must be regarded 

 as hardly within the limits of the definition, I think that, tak- 

 ing into account the strange modifications which are under- 

 gone by the parasitic Crustacea and Arachnida, it is not 

 needful to depart from the ordinary practice of associating 

 them witli the Arthropoda. 



The L'imellibranchiata and the Odontophora constitute 

 another very well marked division, the MOLLUSCA, the char- 

 acters of which have been discussed in Chapter VIII. 



The proposal to separate the Polyplacophora from the 

 Molhtsca, to which I have already referred, appears to me to 

 be devoid of any justification. The resemblances between 

 certain Gephyrea, such as Ch(jetod<-r)i><i and Neo)nenia, and 

 the Polyplacophora, are accompanied by wide differences ; 

 and even if these resemblances are to be regarded as evi- 

 dences of affinity, some considerations, such as the restriction 

 of the branchiae to the hinder part of the body, and the reduc- 

 tion of the foot in Chitonelbis, rather lead to the suggestion 

 that Chcetoderma and Neomenia may be extremely modified 

 Mollusks, allied to the Polyplacophora. 



As to the supposition that the resemblances between the 

 N'udibranchiata and the Tarbellar'ut indicate a direct affin- 

 ity between these groups, it seems to be forgot ten that the 

 Nudibranchiata are all, when young, unmistakable Gastero- 

 pods provided with mantle and shell. Their adult structure 

 is as little evidence of any Turbellarian affinities as that of 

 Lemvea is proof of its being allied to the worms rather than 

 to the Crustacea. 



