THE HIGHER GROUPS. 573 



There is no invertebrated animal at present known which 

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

 ders 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, 

 distinofuished from all others by certain common characters ? 



It is universally admitted that the Insecta, Mijriapoda^ 

 Arachnida^ Crustacea, Pycnogonida, and Tardigrada, form 

 such an assemblage, termed the Arthropoda, and character- 

 ized by the segmentation of the body ; the chitinous 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 Irlloblta) of limbs, which, almoit always, 

 are themselves subdivided into joints. The reasons for in- 

 cluding the Peripatidea in this division have been given in 

 Chapter XL; and, though the Pentastomida must be regarded 

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

 ing: into account the strans-e modifications which are under- 

 gone by the parasitic Crustacea and Arachnida, it is not 

 needful to depart from the ordinary practice of associating 

 them with the Arthropoda. 



The L'xmellibranchiata 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 

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

 be devoid of any justification. The resemblances between 

 certain Gephyrea, such as Choetoderma and Neoinenia^ 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 Chitonellus, rather lead to the suggestion 

 that ChcBtoderma and JSTeonienia may be extremely modified 

 Mollusks, allied to the Polyplacophora. 



As to the supposition that the resemblances between the 

 Nudihranchiata and the Turhellaria indicate a direct affin- 

 ity between these groups, it seems to be forgotten that the 

 Nudihranchiata 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 

 Lern'Ba is proof of its being allied to the worms rather than 

 to the Crustacea, 



